All About K&E Rules

Sidebar: The Problems with K&E Rules

In my own exposure to the vast array of K&E slide rules, I've noticed some problems that pop up consistently throughout my inventory of rules. While these aspects don't detract from the overall wonderful collectability of their products, there are some annoyances you need to be aware of when you seek to collect them yourselves.

The most common problem is the "K&E Rotting Cursor Syndrome," known as "KERCS." (You know it's a problem if there's an acronym for it.) Many rules made prior to ~1950 had celluloid cursor rails that didn't hold up well over time and would eventually crumble to pieces. This begins with the outgassing of the nitric acid from the celluloid, which rusts the cursor spring and compromises the spring's anchor going through the rail. Eventually, the spring breaks away and the celluloid rails become brittle as the camphor plasticizer is also forced out of the celluloid.*

Thus, if you are looking at K&E slide rules on eBay, inspect the pictures carefully as a majority of the older rules will have cracked or crumbling cursors. Once compromised, any disassembly of the cursor for cleaning runs the high risk of watching the cursor rails crumble into your hands. If cracks in the rails are not obvious in the pictures, then note any rust stains either at the cursor or along the slide rule rail itself. If this is noted, avoid purchasing the rule.

K&E seemed to rectify this condition after 1950 with a more durable plastic that also held up better against the typical yellowing of old plastics.

Less known about K&E wooden rules, especially when compared with the Japanese bamboo-constructed rules, is that while mahogany is a generally stable wood (often used in the making of guitar necks), it isn't as stable as bamboo. Some of the rules in my collection show slight signs of cupping across the face of the slide and stator (base) rails, the result of changes in humidity over the life-time of the rule. I've noticed that it can affect the appearance of a rule, as well as its functionality if the cursor becomes too tight to the rails due to the swelling. This is especially true with all-glass cursors from 1916 to around 1930, whose cursor rails thickness to the body has a very small tolerance. In such cases, I've had cursor glass break just because the glass is too tight to the rule when using it, all because of the slight swelling of the mahogany and/or a loosening of the celluloid over time. If this is noticed, it is beneficial to shim between the cursor rails and the glass.

In a similar way, because the slide is designed to glide into the stator rails with a tongue and groove design, the mahogany of the slide's tongue is prone to chipping off at the ends, and in many cases breaking off along the wood grain much further down the slide than many would like. So, an inspection of the slide before purchase is a good idea. That said, with a donor slide rule of matching mahogany wood, you can typically repair broken slides with some wood glue or super glue.

Many of the single-sided Mannheim types have a stator rail screwed onto and through the rules backing. This is not a problem in general, but because the backing is thin, overtightening these screws can wear out the countersunk shoulder in which the screw rests, eventually widening and no longer holding the rail firm to the backing plate. This can be revealed often times in pictures when there's larger gaps than you'd expect when the slide is in place.

And because the celluloid plastic is bonded to the wood with glue, separation can occur over time. While this is often repairable with some super glue and a clamp, neglecting the separation can lead to broken-off sections of the plastic on many samples. The quality of the adhesives did improve overtime, so older rules are more susceptible to delamination, which is why companies like Nestler and AW Faber actually used screws to reinforce the celluloid laminations of earlier slide rules.

Finally, the smaller, single-sided plastic rules of the 4150-1 and Doric types of pocket rules are a little too flimsy in my evaluation. The plastic backing has easily cracked in my own hands when cleaning them, or can peel away from the stator rails. Again, a little super glue can fix this, but I find the overall quality in those rules disappointing. In a similar way, the older Ever-There 4097 and 4098 rules made of "Xylonite" are very brittle now, the toll of 70 to 80 plus years of existence, many of which having warped or curled over those years. I snapped a slide in two when trying to dislodge a stuck and warped sample myself, so condition is crucial when considering a purchase. Likewise, these rules have yellowed much more than any other K&E slide rule you will find. In most cases, the aged yellowing of celluloid can actually be cleaned or sanded away from the surface of the rule without affecting any of the engine-divided scales/fonts. But I've been unable to make much progress with restoring the Xylonite in a few of my own Ever-There slide rules.

Having those concerns mentioned, the great thing about many of the K&E rules is that they can be brought back to life, remarkably so, with a few repairing skills. You need a completely intact rule (especially the cursor), but cosmetic issues can be corrected indeed. I've taken a 20" 68-1200 (4081 type) duplex rule that was covered in severe rust-colored stains, and then I used sandpaper, razor blades, and Scotch-Brite pads to bring the celluloid back to it's old glory without affecting the scales themselves...or at least not in a way that hasn't been worth the effort of restoration. As such, I've bought some pretty cheap, cosmetically-damaged K&E slide rules and turned them back into a display-worthy sample. Likewise, with use of a cheap donor slide rule, a 3D printer, or via metal fabrication, you can replace missing components, especially cursor rails, on an otherwise pristine slide rule.

* Note: There is some debate about what causes the rusting of the spring in these cursors. While camphor does sublimate out from old celluloid eventually, it is a well-known rust inhibitor, and as such would not be the cause of a compromised cursor spring. Of note though is that the camphor is the plasticizer of the nitrocellulose structure, and as such the leaching of camphor would certainly make for brittle celluloid. Some feel that "celluloid rot" can cause other rot in other pieces of celluloid, but I find this untrue in observation...the rule itself, also covered in celluloid, never seems to suffer from the breakdown of the cursor rails beyond some rust marks from the cursor spring.

A second opinion regarding KERCs comes from the Autumn 2011 issue of the Slide Rule Gazette, where the author, through personal correspondence with K&E expert collector Clark McCoy, says that McCoy feels that the rotting cursors appear on rule made between 1940 and 1945, which involved only rules made between serial numbers 750,XXX (in 1940) and the first "roll-over" back to 000,XXX (in 1945). This can be seen in McCoy's Serial Numbers chart at right. McCoy posits that K&E changed the recipe of the celluloid at the government's request to preserve nitrates needed in the production of explosives. (John Hunt Sr., Slide Rule Gazette, Issue 12, Autumn 2011, p. 37.)

All About Keuffel & Esser Rules

Author's note: This, and most articles I write, are living documents. This article (or book) about K&E represents my research and understanding at the time of writing. There are no targets with this writing, no thesis; no other purpose except to work my way through my own process of learning. The text will grow, change, and evolve slowly until, some day, a reader finds value in it. If that is you, and if that is today, thanks for taking the time to read it. Please forgive some of the "under construction" aspects of this writing, as things like the appendices and descriptions of items in the latter chapters might be incomplete. That will change the closer to "completeness" it gets.

This article was last updated September 21, 2024. Many changes are being made to reflect a 1968 K&E Catalog Price List that was stumbled upon!

To click to specific Chapters on this page, use the Table of Contents at left.

Introduction

If you are a lover of slide rules, or merely a connoisseur of trinkets, then it would be difficult to find something more collectible than slide rules made by Keuffel and Esser. Not only is this because of the enormous variety of rules to be found in circulation today, but because of the vast number of model types and styles of rules manufactured by K&E over the 100 plus years of its existence.

Speaking as a slide rule collector myself who, like most, begin the hobby not as a respecter of anything particular, swallowing up any rules we can get our hands on, it's not until we begin to accumulate a few K&E rules that we realize that we could spend most of our lives acquiring, researching, cataloging, and writing ONLY about K&E. The history is that extensive. I have found myself charmed by their rules; enthralled with understanding K&E product evolution and design decisions; curious as to the nature of their consumer demographics and preferences; and wondering about K&E marketing, company leadership, and the business successes and failures surrounding these slide rules.

For those who live in the United States, count yourself fortunate that K&E slide rules, among other collectible K&E products, are easily and quickly found. For the most part, particular K&E models dominated sales for them. Such models, like the 4081 Log Log Deci-Trig and the 4053 Polyphase Mannheim are in no way rare, nor particularly valuable. But because each of those models have existed for 70 years or more, and because K&E always felt that their rules could be improved, a collector like me might end up with 20 to 30 of EACH of these models, realizing that no two of them will be exactly the same. And because of this, the evolution of changes can be mapped in a way that would please Charles Darwin.

I am thankful that K&E was never satisfied with the status quo, because I can collect an enormous number of meaningful slide rules without needing to pay exorbitant prices for them. That said, I could do that if I wanted as well! Historically, there are rare slide rules of substantial value. Such can be the result of either extremely limited production or historical significance. And when these slide rules are unearthed, there is a large group of dedicated K&E collectors who want to acquire them, not only the rules themselves, but the story they tell.

And I believe it's a exciting time to become a K&E devotee, as we are now two or three generations removed from people who actually know what slide rules are. There is the opportunity for even ultra-rare K&E slide rules to come out of hiding, as family heirlooms turn into dispensable curiosities of "what is this thing and why do we still have it?"

So unlike any other makers, including Pickett, Hemmi, Faber-Castell, and Aristo, Keuffel and Esser - both the slide rules and the company - sparks curiosity in me. So much so that what follows here will be a written love-affair of not only the slide rules in my collection, but information and understanding about every other slide-rule-related fact to be unearthed about K&E.

Chapter 1: Understandings and Conventions

Keuffel and Esser is the oldest major US slide rule maker, going back to 1867. Beginning as a supply house for anything engineering related, they are widely known for drawing supplies, surveying equipment, and of course, their long history and diverse models of slide rules. Famously, in 1891, they originated the "duplex" design of having scales on both sides of the rule with a dual indicator, beginning 70+ years of innovation and production in the slide rule business.

The key to understanding K&E slide rule production is to pay attention to both their model naming convention as well as their cycles of serial numbers. I will attempt a healthy description of the histories here, which will also reveal much about the company on the whole. Likewise, we'll discuss slide rule construction, the major families of rules and their evolutions across model lines, and we'll wrap up with my own portfolio of K&E rules, which are the most numerous in my collection. The goal is to get the would-be collector up-to-speed on the K&E world of slide rules.

Model Number Naming Conventions

Model numbers typically take the form of 4XXX-L, where "L" is the approximate length of the slide rule in decimeters (1/10th of a meter). As an example, a popular K&E slide rule historically is their 4053 "Polyphase" series of Mannheim-style slide rules. This family of rules was in continuous production from 1909 until just before the end of the slide rule era in 1970. These rules came in 4053-1 (5-inch), 4053-2 (8-inch), 4053-3 (12-inch), and 4053-5 (20 inch) variations. This "dash" suffix was introduced in 1911 to signify slide rule length (in decimeters), seemingly a lovely way to communicate model variants within the model line.

K&E also used an "N" in front of some model numbers, as in N4XXX, to denote a major change in the slide rule for a new production year. Presumably, this means "new" to distinguish it from the old version. (Pickett would later do something similar). They did this to a majority of the model lines in 1925, the first instance of the "N" designation occurring in 1922 with their N4135 "Power Computing" Duplex rule. And seemingly arbitrarily, they would revert back the original designation in some cases. For example, the 4080-3 "Log Log Duplex Trig" replaced the 4090-3 model in 1937, became the N4080-3 in 1948, only to switch back to the 4080-3 in 1954. The switch to the "N" in 1948 reflected a major change in the scales for that rule. Another major change in scales occurred in 1954, whereas K&E just dropped the "N" from the model name.

I said earlier that K&E models are typically a 4XXX type of number. There are notable exceptions. Prior to the year 1900, all of their slide rules (and other items like Thacher and Fuller Calculators and their sector rules) used a 17XX scheme. Likewise, the earliest K&E products made before 1887 used a 4XX three digit scheme.

Going forward in time, Models with a 9XXX designation started to appear in the late 40s and early 50s, which denoted the Doric series of plastic slide rules. The Doric slide rules that remained were reclassified into the 4XXX naming scheme by the 1955 Product Catalog.

And, finally, in 1962, K&E shifted away from the 4XXX numbering scheme and began using model numbers of some 68-1XXX variation. "68" was an indicator of "slide rule," so likely the company shifted all company products to this number format for accounting reasons.

Thus, you will see something like their 4081-3 "Deci-Trig" slide rule become known as the 68-1210 "Deci-Trig" in 1962. Same rule; different number, of which, by the way, they often didn't stamp on the slide rule itself. This includes two of their best slide rules, the 68-1261 "Jetlog" and the second variant of the powerful 68-1100 Deci-Lon 10 rule. It did indicate the model number on the box, however. Furthermore, there are known instances when a post-1962 slide rule in a 68-1XXX labelled box still contained a slide rule with the old 4XXX model number, likely in the early 60s as K&E was still unloading their inventory of older rules.

The move away from 4XXX series numbers with the "dash" suffix is frustrating for collectors, as Indicating the length of the rule within the model number with the "dash" is very helpful. Plus, the 68-1XXX scheme is very difficult to remember. I do question the changing of 62 years worth of model familiarity among their consumers. This couldn't have helped sell more slide rules.

Albeit a decade later it would hardly matter anyway. In a way, the 68-1XXX scheme began the countdown towards the hand-held calculator age and the collapse of the company.

Serial Number Conventions

As for serial numbers, K&E tried to make it difficult for us in that they used a sequential numbering of their rules regardless of the model. So, a 4080-3 model could have a serial number of 123456 while a 4070-3 might be SN #123457. These six digit serial numbers, which K&E didn't use at all prior to 1922, would run from 000000 to 999999 and restart to zeros, starting the cycle over again. There were 4 cycles and 3 "roll-overs" back to zero, occurring around 1944, 1956, and 1967. Therefore, dating a slide rule will reveal three date possibilities, but usually only one logical choice based on the type of scales, model type, and construction used. You will discover that within a typical model type, there will be subtle changes from year to year, thus creating a number of "variants" with each rule. Production rates were fairly linear, with around 66,000 units made prior to World War II, 77,000 per year post-war, and then ramping up to nearly 100,000 per year during the late 50s and 60s until, in 1965, they tapered production back down to around 70,000 units per year. It is thought by many that very few rules were made after 1968 and that sales into the 1970s were of slide rules K&E still had in stock.

There is also an extra wild-card where serial numbering is concerned. K&E used specific serial numbers for certain models as separate from the usual SN# chain, which was reserved for mostly their wood-constructed rules. As of 1947, any of the plastic rules are not included within the serial number line-up as they had their own series for numbering. There has never been an attempt to rationalize K&E serial numbers to dates of manufacture among the plastic K&E rules, at least not to my knowledge. And, as stated, slide rules made prior to 1922 did not have serial numbers at all. Thus, it's typically the post-1922 wooden rules that are more easily dated (not including the Beginner's 4058 rules that never had serial numbers).

This chart shows the pattern of serial numbers of most K&E slide rules on a year-by-year basis. Note the "rollover" of the serial number as they manufacture each one-million slide rules. - courtesy of https://www.mccoys-kecatalogs.com/

This shows a variety of K&E single-sided rules and the way conversion and formula charts can be laid out on the back of the rule, usually borrowed from the US Bureau of Standards, Circular 47 (1914). Such charts were typically pasted onto the back of their nicer wooden rules and screen printed to the back of their budget and plastic rules.

In dating my own slide rules, I typically use the chart above to give me a idea of possible dates, but then I peruse Paul Tarantolo's massive collection of K&E (and other) rules archived over at the Oughtred Society. This is an amazing collection of models collected and categorized for us to see. Compare your slide rule with one of his and you'll know what you have. Proudly, I have a few K&E rules that he does not have!

Paul's site is testimony to the way slide rules within a model line evolve, and specific dates can be tracked where evolutions were common across most K&E models. For example, when K&E advanced their cursor design, it was typically implemented across ALL of their rules in a given production year. As such, these changes can help pinpoint exact production dates within a model series, as long as your slide rule sample does not have a replacement cursor. This adds to the confusion, at first, but in time the K&E aficionado will be able to detect an old slide rule with a newer cursor. Some of those highlighted production changes by significant date are also shown in the chart above.

Slide Rule Construction

Let's now establish some of the construction jargon that we will use. Slide rule bodies had very similar, common elements, those being two fixed, grooved rails (or stators) and a tongued slide, free to glide between the rails. The way the rails are fixed together determines the platform for the slide rule, either single-sided or double-sided.

Single-sided rules will have their rails affixed to a backing plate. The back side will typically have formula charts and conversion tables. For most wooden rules, this information was printed to a paper label, affixed to the rule with adhesive, and further protected with a lacquer finish. Less expensive wooden and plastic models would have screen printed (painted) the information to the rule's back. The data on these are generally consistent for a model over time, though small changes in some of the text from year to year can help to date the rule. The slide is free to glide between the rails. The slide is removable, typically with additional scales on the back of the slide. Often, there will be windows on the edges of the rule's backside to input values for those scales, reading the outputs off of the front side. This will be the case for the Mannheim types of rules made from wood. Other rules might allow the slide to be flipped over and used entirely on the front side. This will be the case for Mannheim rules made of plastic, especially the shorter pocket models. The void between rails is called the slide well. The slide well itself can have maker's marks and patent information, as well as a centimeter extension scale to extend the length of the slide rule for linear metric measurements. Other rules can integrate scales onto the slide well itself.

Doubled-sided rules, also known as duplex rules, have their rails affixed with end brackets (or sometimes known as braces) in either metal or plastic, with the slide free to glide in between.  This allows removal of the slide so it can be flipped to access scales on either side of the rule, and in some cases with early K&E rules, they could have shipped with multiple slides to increase a rule's capabilities. A double-sided, duplex platform rule that does not utilize the back side for additional scales is known as a simplex design.

The scales themselves can be engine-divided (etched), painted-on, or cast, depending on the material. Their spacing is established by a scale length which determines the overall length of the rule's platform. As such, there will be scales with total lengths of 12.5mm, 20mm, 25mm, 40mm and 50mm. The scales themselves might be linear, as such for inch and centimeter rules typically on the edges of the rule, or in the base 10 scale for solving logarithms, which K&E refers to in its early literature as a "scale of equal parts (or measures)." For the most part, scales will be logarithmic which, for collectors, is what defines a "slide rule" in the first place. All computations involving the two basic operations (multiplication and division),and the functions that arise from them, are made possible by using logarithmic scales. Because K&E is in the United States, classifying the slide rule lengths by their metric scale lengths would have been a disaster. Therefore, the above mentioned metric scale lengths are traditionally known throughout all K&E literature as 5", 8", 10", 16", and 20" respectively.

The last common element of the slide rule is known as the runner or indicator in the earlier literature, shifting to be known as the cursor in the later era, a term which we will use mostly in this writing. The earliest rule, known as a Gunter style, came without a cursor. But all other models used some kind of indicator. This element wraps around the rule's edges and glides in grooves on single-sided rules. The earliest cursors will be "metal" or "brass" in the literature, a style which today is referred to as a chisel style because of the chisel-like fingers that project over the scales. These chisels are typically on the left side of the indicator but evolved in the later models to be placed on the right side as well, something known as the double-chisel design. By the turn of the century, window cursors began to replace all-metal versions. Such a cursor will have two cursor rails made either of metal or plastic attached to a window etched with a visible hair-line. This window could have been made of glass, clear celluloid plastic in early rules, or modern "acrylic" plastic in later rules. These cursors were adaptable to double-sided, duplex rules simply by adding a second window to the back side, completely wrapping the cursor to the rule. These will affix a spring to one of the rails to give tension to the cursor (also in single-sided cursors as well). Adjusting tension of the spring is accomplished by oversized holes in the edge of the glass (or a glass frame in the evolved versions) whereby screws can be tightened at the proper tension to hold the cursor in place, yet provide a nice glide across the slide rule. Alignment of the hairline on the front and back windows is accomplished carefully by the same means.  Cursor design would evolve over the 100 year history of K&E slide rules. Where that occurs, it will be noted in the individual descriptions of all K&E slide rules.

On the whole, most traditional K&E rules - meaning those after 1906 - are of wooden mahogany construction covered in celluloid, which is an ivory-colored organic plastic displaying the engine-divided (etched) scales. These are the slide rules for which K&E became known and highly regarded, helping them gain strong foothold in the U.S. market. But in the first three decades of the company's existence, slide rules were almost entirely single-sided boxwood rules, with the exception of the 1890s which began to incorporate not only the mahogany laminations, but two sided, "duplex" rules based on the William Cox patent licensed exclusively to K&E. There is inconsistency prior to circa 1900 when K&E mixed their in-house rules with those supplied from two European companies (or perhaps more). Once mahogany rules with celluloid laminations began to dominate the K&E product line, the only exceptions were their "beginner's" type of consumer rules that were made of cheaper "hardwood," painted white with black-painted scales; and their first line of plastic rules, starting in 1931 with their "Ever-There" family of pocket slide rules. Certainly as low-cost plastic construction became more practical as a material, it would begin to see progressively more use in the slide rule.

By the 1950s, their "Doric" type of mostly experimental plastic rules, sold alongside their flagship mahogany rules, showed K&E how well both construction types could fill out a product lineup and fulfill the needs of their market. And as the Doric rule evolved over a decade, it gave rise to their more "modern" slide rules like the Jet-Log and Deci-Lon series rules, both of which are highly collectible and well-regarded slide rules made of "Ivorite."   This plastic was consequential to most all of their rules, incorporated into the cursors or the rules themselves, regardless of price, and not merely used in their budget rules. As such, most of the traditional wooden construction techniques evolved to incorporate more plastic features. Some wooden models would give way to plastic rules entirely, becoming the spiritual successor of the older model lines. By the end of the slide rule era, most all K&E rules were constructed entirely of "Ivorite," with only a few flagship, mahogany slide rules remaining because, after all, that is what a hundred years of customers had grown to expect. So by the end of the slide rule era, K&E had managed to preserve their long-standing heritage of powerful wooden slide rules while also supplying users with even more powerful, all-plastic models.

Regardless of construction, K&E made it a point of pride to produce high quality rules, and for the most part during their history they absolutely succeeded. Their slide rules even came with a life-time warranty, which is unheard of today (likely for good reason as we will discuss later). But as a collector, so many years after the fact and when judged over the course of time, there can be a variety of issues that make your sample less than mint. For more, please see "Sidebar: The Problem with K&E Rules..." above.

The K&E Model Map - A timeline and categorization of K&E slide rule models over time - courtesy of Michael Frey of the International Slide Rule Museum.

A slide rule in my collection, the Model 4082-3 "Radio Special," is a Specialty Rule made by K&E. But it shouldn't be dismissed as "miscellaneous" in our thoughts. Rules like this had a long history in the K&E product line. I will be equally excited to talk about the long history of Merchant's, Stadia, and Radio/Electrical rules later in the article, even if this author wouldn't necessarily see use in them compared to a general arithmetic slide rule.

Slide Rule Families vs. Slide Rule Series

Understanding slide rule construction, which will be further discussed when talking about the individual slide rules, we now want to consider how K&E would organize their rules throughout their long history and the way they would market to consumers. What we will see is somewhat of an evolution in this regard.

For the most part, from the inception of the company, slide rules were categorized according to their construction, whereby single-sided rules and double-sided rules served as the major differentiation, and within those extremely broad categories, they would be categorized according to purpose (a model line or a scale set). These are known as slide rule "families." This is very predictable in the first 50 years of K&E product catalogs. But around the 1930s, we begin to see certain models of rules sold in "series," a collection of rules with similar build and technology, yet with a diverse range of uses.

The way K&E thought about "pocket rules" is a good example of this. During the first 50 years, the company would focus first on a Model line within a particular family of rules, like the 4041 or 4053, and then decide if they wanted to make a pocket version of that rule. In doing so, they would essentially cut the 10" wooden slide rule stock in half and build the shorter rule in exactly the same way as the longer rule. Labor costs in doing so remained high, since production time isn't decreased just because there is only half of the materials. And as we will see in our discussions of the rules, K&E did NOT make pocket rules any less expensive than their full-sized rules. And in many cases, they cost more.

Around 1930, with the introduction of plastics beyond celluloid/wood lamination, we see that K&E could (and did) change production techniques to produce their pocket rules, whereas any cost-savings could be passed onto the consumer. And because of this, product models that traditionally were NOT manufactured with a "pocket" version, could now be made at low production cost for a demanding consumer. As such, K&E would birth the "Ever-There" rules, a series of pocket models intended to supply users of their traditional model lines a more portable option of their favorite slide rules.

Similarly, K&E produced several other "series" of rules once the technology became available to them, and this does not make it easy to sort individual rules into the traditional "family" of K&E slide rules in which they belong. Rules in a series are often the completion of an existing model line, or at least their spiritual successor. And this obfuscates how we could view the entire ecosystem of K&E rules.

Article Conventions

As such, attempts to organize K&E rules from today's collector's perspective, or to write a comprehensive article about K&E slide rules, is, therefore, complicated. But previous efforts are substantial and very much appreciated. For example, Michael Frey of the International Slide Rule Museum (ISRM) did a wonderful job of mapping K&E slide rules over history in his K&E Model Map (see image above). Here, we see used the three broad categories of Mannheim, Duplex, and Specialty. It's logical and, for the most part, mirrors the way K&E would have organed the rules themselves. It does take some interpretation however, which we will do shortly. But for me, a good discussion about K&E slide rules would not be possible just by moving straight down the K&E Model Map, even though I will stick mostly to that organizational structure until I feel it no longer suits me.

In my writing, it should be noted that my thoughts about K&E and their rules comes primarily from firsthand experience, from my own use of the rules, gleaning of the available product catalogs, price lists, and supplementary product brochures, mostly as made available by Clark McCoy at his important resource website here. This, when coupled with a study of my own collection of K&E items, including slide rules, product manuals, and catalogs, yields a tremendous amount of understanding, especially in terms of product time-lines, product "families," and the evolution of the slide rules within individual model lines and series.   This provides the initial framework of my writing, whereas I might conjecture about items within the broader context of what I've already gleaned. At that point, especially where many of the more rare models are concerned, extra research is conducted to fill in gaps of understanding or specifics about a product. This research begins first at the aforementioned ISRM, as well as the rich resource of materials made available by the Oughtred Society, in which I am a member. There are also many websites from fellow collectors available on the internet from which extra information can be gained. And finally, where applicable and available, any books, journals, or other published works are consulted. Most of these will appear as works cited within the body of this writing, though an Appendix (see the end of the article) eventually will list all support materials used.

My approach in this article will be to itemize rules according to construction, followed by purpose, in the traditional sense. As such, major sections of the writing will be divided by "family" of rules. But where a "series" of rules could belong to multiple families, I will discuss the "collection" itself to gain a better understanding of that collection as a series. And then, if those rules are also the evolution of a traditional model line, I will discuss those also within that family context. For example, while the Model 4097C of the Ever-There is important to that "series" of rules, it functionally works as the fulfillment of a pocket rule for the Merchant's "family" of slide rules, and as such, will be highlighted in those conversations as well.

In my mind, this is the best way to focus on the evolution or continuity of a product line, and perhaps give some insight into what K&E was thinking in this regard. Doing so in any other way would be too granular in terms of their slide rules, which weren't isolated from other rules, but rather part of an eco-system of K&E products. As we will see, this approach to describing their slide rules works well, even if we run the risk to talking about it in multiple places in this article. I would recommend getting in-depth looks at a specific rule from many of the wonderful resources on the Internet, especially the ISRM, the Slide Rule Universe, or the Oughtred Society. While you will get some individual product depth here, I want to make some sense of what K&E was thinking with the production of their rules, and not just make it about the rules themselves.

Therefore, it does make sense to divide the discussion of K&E rules into their major families, similar to the K&E Model Map. From there, I will place my focus on individual model lines within those families, how they evolve, and how K&E likely thought about their overall placement within their slide rule ecosystem. And within that framework will come specifics about a particular slide rules.

Where scales are concerned, the following convention will be used, using the original single-sided Polyphase Mannheim Model 4053 as an example:

Front Side: Inches // A [B, CI, C] D \\ K
Back Side: [S, L, T]

Reading across the front side, we have an inch ruler on the top edge of the rule, A on the top rail, B CI and C scales on the slide, D on the bottom rail, and a K scale on the bottom edge of the rule. On the back side, the bracket indicates that the back of the slide has S, L, and T scales. While single-sided in build, the "back side" would consider the scales on the usable part of the rule, which is the back of the slide only in this case. Where applicable, the physical backside of a singled-sided rule would typically have formulas, unit conversions, or even instructions for a rule's use. That will be noted where it happens. For duplex rules, you could expect something more like the post-1947 Model N4080/4081 Log Log Duplex scale description here:

Front Side: LL02, LL03, DF [CF, CIF, CI, C] D, LL3, LL2
Back Side: LL01, K, A [B, T, ST, S] D, L, LL1

So what follows in the article will be the five major categorizations organized in separate chapters: Single-Sided/Mannheim-Type, Double-Sided/Duplex-Type, Specialty Rules, Miscellaneous Rules, and Out of Catalog/Custom Rules.

The chapter covering Single-Sided/Mannheim-Type rules will transition across the families, Mannheim >> Polyphase Mannheim >> Modern Mannheim.  "Mannheim" implies a purposed scale-set; however for K&E rules, especially as listed in the K&E Model Map, it's broadly implied to mean "single-sided" construction as well.  This is why a series of rules like the "Ever-There" would be categorized as "Mannheim," on the model map, even though only one such rule had the Mannheim scale set (as differentiated from Polyphase Mannheim). All Ever-There rules were single-sided. So, in some cases, it will be necessary to talk about a slide rule under multiple family designations.

Similarly, in our chapter covering Double-Sided/Duplex-Type, "Duplex" is logically about the two-sided design with a cursor/indicator capable of reading both sides of the rule with one setting. And certainly the duplex design is organized amongst purpose-built slide rules based on scale set, a spectrum moving from basic Duplex >> Polyphase Duplex >>Log Log Duplex >> Vector >> Modern Duplex. However, as we get to the Modern rules, we begin to see that while many rules are built with duplex construction, they have some traits akin to the single-sided rule, most notably the GP-12 and Analon slide rules.  Likewise, a "duplex" format rule like the "K-12 Prep," being only printed on one side and replacing all the Model 4058 "Beginners" rule in the evolution of that product line, should be talked about naturally in the context of the single-sided Mannheim type of rules - even if it isn't a Mannheim scale set - as well as in the section of Modern Duplex rules in which it should be fairly categorized.

Thus, to eliminate some confusion in the K&E Model Map, I add single-sided and double-sided distinctions to these two chapter labels, which becomes the way I think about these general mathematics types of slide rules.

The chapter covering Specialty Rules can be thought of as rules that are "everything else" if you'd like, but this would be inaccurate for two reasons. First, that would not be fair to some important families of rules in the specialty class. The Merchant, Stadia, Radio/Electrical, and Demonstration families are every bit as important throughout our history here and are a fundamental part of the eco-system from the very beginning of the company. Secondly, there are many specialty rules that are not associated thematically with a broader family of rules, and so those should be talked about separately. Either way, rules in this chapter, within these families, didn't always care about using the same construction or form-factor. For example, there are both singled-sided Mannheim-style (the Model 4100) and doubled-sized duplex (the Model 4102) "Stadia Family" slide rules. So, for this chapter, it's important to note that the Specialty rules are not miscellaneous K&E rules, but rather those of interesting distinction that still reside within broader family types.

After that, there remains many specialty rules that are not specific to a "family" of rules, and thus these rules get their own "Miscellaneous" chapter. These are those items listed in a K&E Product Catalog to be discussed individually, out of the context of a more broad categorization. This is where most of the non-linear production slide rules can be found, such as the pocket-watch rules, cylindrical rules, and one-off linear rules that were promoted in K&E literature.  Some of these rules are historically significant and are some of the more exciting products in a K&E collection. As such, the dismissive "miscellaneous" moniker should in no way be interpreted by the reader as being slide rules that are not worthy of discussion.

And finally, we have a chapter of rules, which here we call the "Out of Catalog/Custom Rules," that consists of items not found within a K&E Catalog, but yet still existing "in the wild." They could have been custom rules built for industry or they might have been a short-term slide rule that K&E hoped would catch on, yet was introduced and then discontinued between two issues of their product catalog. We will see that such catalogs, and other K&E price lists and brochures, often went many years between publication. These represent the most mysterious of K&E products with less of a story historically shared, yet lends itself to a plethora of historical questions. Such questions are the subject matter of many advanced collectors of K&E rules and provides some of the more exciting discussions about K&E rules in the present day.

And now, the K&E slide rules...

Chapter 2: Rules of the Single-Sided, Mannheim-Type

As I mentioned earlier, it's easy to equate the "Mannheim" rule with "single-sided," since historically speaking they go together. But not all single-sided rules are Mannheim. In Europe, the Rietz and Darmstadt rules dominated historically, yet the Mannheim originated over there. We just know that K&E decided to place its focus on the Mannheim (and it's improvements) when producing a general arithmetic rule for its customers.

Likewise, the Mannheim scale set can have interpretations on double-sided rules. As examples, the "Duplex Family" was largely based on a classic Mannheim scale set and the Polyphase Duplex was just a re-imagined Polyphase Mannheim slide rule.

Some of the single-sided rules near the end of the slide rule era were duplex-designed rules that simply chose to use only ONE side of the rule. The other side, like the traditional Mannheims, would contain a variety of conversion scales and useful information. So whereas you might regard a slide rule like the 68-1400 Analon as a duplex rule (or even a specialty rule), the choice to utilize only one-side of that rule for computations could very easily require me to talk about it in the "single-sided" section of slide rules. But because those are actually duplex in construction, we will talk about them in the next chapter.

So, what follows will be those historical K&E slide rules that are foundationally "Mannheim" in its scale set. These rules fall into three broad categories. The first are those rules with the traditional Mannheim scale set of 7 scales (A, B, C, D, S, L, & T) and as such many K&E rules regardless of construction type are considered to be in the "Mannheim Family" of rules. Secondly, we have budget options, the "Beginner's Family," that thematic needs to be discussed because the purpose of these slide rules becomes more important than what scales they have; but for the most part are still a Mannheim originated slide rule. Third, we will look at the "Polyphase Mannheim Family," an evolution of the Mannheim scale set, yet also formatted with same type of non-duplex form of construction. After that, we will look at the game changing "Ever-There Family," the first all-plastic K&E pocket rules that solidified their product line across all possible markets. And finally, we must look at the "Modern Polyphase Family" of rules, the all-plastic rules of the non-duplex type, also still founded on the traditional Mannheim scale set of slide rules.

The Mannheim Family

General-purpose rules obviously range in capabilities and were classified into several slide rule families. K&E's longest running family of rules, the basic Mannheim family, utilized the front side only with A, B, C, and D scales, and with formulas filling the back. S, L, and T were placed on the back of the slide and could be utilized with a small window indicator on the back of the rule. Pre-1900 models in this family were named "Engineers Slide Rules" within catalogs (shown below). But at the turn of the century, it's clear that K&E shifted the marketing of these rules toward the "everyman," showing that anybody could benefit from doing the basic arithmetic and trig operations made possible with the Mannheim rules. In 1901, K&E revamped their entire product line-up and marketing campaigns, pushing toward broader markets, including the in-house production of slide rules. They managed to hit every price point with their catalog of new slide rules, ranging from $1 for their budget "Beginner's rules," to as much as $12.50 for their new 20" Mannheim model. In 2022 money, that's $35 for the basic rules and as much as $420 for their flagship Mannheim model.

Note that early on, K&E rolled out a revamped series of the Duplex and Specialty rules as well, which we will talk about in later sections.

Earlier models in the Mannheim family of rules pose extraordinary degree of difficulty to describe. Mostly, we can thank the lack of surviving samples of these rules for this, as well as the absence of good descriptions or any firsthand, anecdotal, historical reports. But there will be a recurring theme until we get to the "flagship" Mannheim model in this family, the Model 4041 Series: up until the turn of the century, K&E seemed content to provide slide rules to customers using whatever was readily available to them, including from multiple suppliers, without too much rigid adherence to what the catalog stated was available.

"Early models in the Mannheim family of rules pose extraordinary degree of difficulty to describe...K&E seemed content to provide slide rules to customers using whatever was readily available to them...without too much rigid adherence to what the catalog stated was available."

As we will see, the development of a standard celluloid-covered mahogany Mannheim side rule with the ever-so-familiar "K&E look" was most definitely a process!

Ordered historically, the major slide rule models in the Mannheim family include:

The Model 479 Series

Let's start with a description of these first slide rules offered by K&E before we get into the real issues of the rules' manufacture.

We see the catalog illustration for the Model 479, K&E's first slide rule produced from 1881 to 1886 here (above).

The original, cursor-less 479 model, offered in 1881, was 10" long and 7/8" wide, made of boxwood and a real ivory slide. Known as the "Engineer's Slide Rule," this 479 was later classed as a "Gunter" version of the Mannheim, which featured only the A, B, C, and D scales and no indicator cursor (it was used with calipers). This now rare - seemingly mythical - rule was introduced at a cost of $3.50, which I am certain was an absolute fortune back then.

In 1883, the 479 Gunter model became known as the 479-1,ditching the ivory slide. K&E also added two others to the line-up, the 479-2 and 479-5 rules. It's a curiosity, but the dash did not represent the length of the rule (a convention first appearing in 1911). Instead, all K&E slide rules were a Model 479, with the dash indicating a different product offered. The 479-2 and 479-5 were 10" and 8" models respectively that included trig functions on the back of the slide and a brass bracket indicator, also known informally today as a "chisel cursor." These were true Mannheim rules and grandparents of the future 4041 models that would be produced in-house by K&E after the turn of the century.

Assuming their first Model 479 was sold in 1881, in keeping with the first product catalog, then there would be 20 years of slide rules sold by K&E that were either outsourced from a manufacturer and branded by K&E or re-sold by K&E with their own catalog designation. The distinction is important. At this point in history, K&E production capacity was on drafting tools and surveying equipment, filling out their catalog with other items while acting as a distributor. For these 20 years, most would agree that K&E functioned as a re-seller where slide rules were concerned, first gaining the notion of designing and manufacturing their own slide rules in the 1890s, but not gaining capacity to do so until the end of the century. While we will see throughout this writing that K&E might have begun in-house production of some models even as early as the Cox Duplex rule in 1894 or 1895, it wouldn't be until 1901 when full production capacity of most all slide rules was reached.

It seems to be the general consensus that two companies were mostly responsible for providing slide rules to K&E during this time, French maker Tavernier-Gravet, during the Model 479 years, followed by German maker Dennert & Pape for most of the Model 17XX years. Working under the assumption that the shift to the newer models in 1887 also signified their shift in manufacturer, we should realize that their is evidence is to the contrary (please see Sidebar: Solving the Early Maker Mystery at right).

If I had to hazard a guess who K&E chose to manufacture these early Model 479 Mannheim rules, then I would suggest Tavernier-Gravet.  Comparisons of known samples of T-G rules with model illustrations in the K&E product catalogs should be all we need to be rather definitive about the provenance of these rules. There are enough samples of T-G rules around today, including one in my own collection, pictured below, that gives us a strong indication of what these Model 479 slide rules might have been.

The 10" Tavernier-Gravet Mannheim rule in my collection is likely what we should expect the Model 479-2 slide rule to look like. This rule could have been produced as early as 1883, matching well K&E's catalog descriptions for the rule. The fact that no Model 479 rules have resurfaced today indicates that K&E may not have stamped their brand physically on the rule for the early models, and thus such rules are out there, but in hiding.

However, renowned collector Bob Otnes conjectured that this rule could have been produced by John Rabone and Sons in England. In the Otnes' article, "Keuffel & Esser - 1880 to 1899" published in the Journal of the Oughtred Society, Spring 2001, p. 18, Otnes bases this thought on similarities between a Rabone 8" rule in his collection and the 10" Model 479. In an earlier JOS article (Oct. 1995, p. 15), he mentions that a regular scale rule (not a slide rule) in his possession in the 1883 catalog has both Rabone and K&E labels. He also states that he possessed a "gunter" style rule from Rabone, with no K&E markings, but that he felt could have been a 479 slide rule.

I credit Taverner-Gravet with the majority of the production of the Model 479, conceding that Rabone could have made the earliest version of this rule prior to T-G. And that tends to conform well to the way K&E did business during this time, as it seems that K&E would have sold anything to the public if there was money to be made. Because the catalog descriptions and illustrations are wildly generic, the actual rule being sold could have been anything that fit in a K&E wrapper.

So it should be noted that the Tavernier-Gravet rule in my collection could very well have been the same rule sold by K&E during this era, despite not having a K&E maker's mark on the rule. I feel this might be especially true in the early years of the company when K&E seemed urgent to increase their catalog offerings as rapidly as possible. This would also explain the absence today of these older K&E slide rules. I do find it difficult to believe that these rules are so rare, as known rules from other manufacturers of the era are not. Rather, today, I believe these older Model 479 slide rules might have gone "incognito," bouncing around today in a different guise.

From the collection of the late Bob Otnes, this rule, coined as the "Rosetta Rule," connected Dennert & Pape definitively with K&E during this era. With celluloid-facings, this would most certainly be a candidate (there are others) for the Model 1746 rule (sans cursor) in the new Mannheim series beginning in 1887, yet celluloid-laminations are not mentioned in the catalog until 1890. The Otnes collection also has a 50cm version - K&E model 1748 (?) - yet with the D&P maker's mark in the slide well ground away.

The Model 17XX Series

Officially replacing the Model 479 series in 1887, the new model numbers were necessitated by a better way to catalog K&E product offerings (see the Sidebar at right). So for a time, there is no evidence to suggest that these new models would be any different than those that preceded it. However, by 1890, these slide rules would include the likelihood of a shift to a new supplier; namely, the German maker Dennert & Pape (known later as Aristo).

The first two rules in the series were the Model 1745 and Model 1746 rules, both 10 inches. The Model 1745 retained the basic Gunter design, made of boxwood with engine-divided scales, and no indicator cursor. Similar in construction, the Model 1746, with brass cursor, was a true Mannheim rule, adding S, L, and T scales on the reverse of the slide, as well as inch and centimeter scales on the edges. It also had a centimeter extension rule in the slide well.

Subtly important is the catalog description which lacks indication of the wood type. A pattern develops in these descriptions - when characteristics of the rule they will sell are certain, K&E will say it; but when uncertain, they leave the description intentionally vague. The wood type is suspiciously missing in the 1746 Mannheim descriptions all the way through the 1899 catalog. This should be an indication that K&E left open the possibility of a variety of slide rules that could meet the description, some boxwood and some mahogany.

These rules, like all T-G and D&P rules known to this point, had horizontal "embellishment" lines, known by most today as "railroad" scales. By the end of the century, both of the rules had transitioned to mahogany construction. A 20" version of this rule called the 1748 was introduced in 1890, and a 5" version called the 1747 arrived in 1899. Of the 20" rule, K&E seems to be rather apologetic. Their catalog description in 1890, and as printed below in the 1895 catalog, states...

Sidebar: Solving the Early Maker Mystery

Until recently, and rather myopically, I made the assumption that K&E switched from the Model 479 series to the newer Model 17XX names in 1887 because of a change in suppliers. In self-defense, it is common among collectors to believe that Tavernier-Gravet was responsible for the early rules and Dennert & Pape supplied the later rules of the 19th century. Yet, we have evidence that the shifting of model designations was not because of a change of supplier, but rather that the sheer number of products sold by K&E necessitated a better system for cataloging their offerings. We note that below in the Preface to the 1892 catalog below...

Please note that 1887 is the "twentieth edition" of their catalog.

Most certainly, Tavernier-Gravet remained the chief supplier for the K&E Mannheim rules well past the 1887 date. This seems reflected in catalog descriptions, which are largely unchanged until the 1890 year, when celluloid-faced rules are described. It's also important to note that there is no provision for a non-celluloid, boxwood slide rule, with cursor, to be sold after 1890.

The timing seems about right. According to Hans Dennert, grandson of the founder of Dennert & Pape, it wouldn't be until 1888 until they had fully shifted to making slide rules with celluloid-laminations on mahogany. By 1890, we can see that the new slide rule designations and descriptions would have been easily justified. And given that K&E likely had extra inventory of the old Tavernier-Gravet Mannheims, it makes sense that they would continue to sell these old rules with the new model numbers until 1890, when the new D&P rules were stated to be available.

Yet, the slide rule pictured above gives me reason to think that Tavernier-Gravet slide rules were sold by K&E past the 1890 catalog date, and perhaps well past that date. This 8" slide rule, listed in the Rarities Gallery of the Oughtred Society, estimates a date of manufacture of 1885. This rule bears the name of both Tavernier-Gravet and Keuffel & Esser. An assumption of this date would indicate the rule as likely the Model 479-5, which was the 8" rule made prior to 1887. I believe it is more likely to be true that the date is approximately correct, but that the same model rule would have been sold for many years at a time without any evolutionary changes. The real point is not the manufacturing date, but rather the date that K&E would have sold it.

Two markings on this rule pinpoint the sale date to much later than 1885. First, on the rule's back is stamped "Medailles d'or 1878 et 1889," indicating the gold medals their product won in the World's Fair in Paris those years, the later year famously marking the completion of the Eiffel Tower. While the slide rule was certainly produced early, Tavernier-Gravet likely added this stamp to their existing inventory and new slide rules after this 1889 date. Second, also notice the reference to "Chicago" on the rule's front. Knowing that K&E did not have an office in Chicago until 1891, as also expressed in their 1892 Product Catalog imaged below, this is perplexing. K&E would have to be offering this rule for sale after 1891, despite not having a matching boxwood model in their catalog.

Was there a line of boxwood only, T-G slide rules offered by K&E during the last decade of the century?

I would date this boxwood rule to the mid 1880s, as did the Oughtred Society, which also falls in line with my Tavernier-Gravet sample pictured at left. The indication of "Chicago" on front, identifying the rule as post-1891, could have been stamped after the fact, akin to the additional stamping on the back of the rule by T-G. This might be the typical method of operation for K&E: importing the components of the rule unassembled, stamping on all the numbers and labels, and then building the rule in-house with their own end brackets and cursors. (See also the Cox Original Duplex rule in the next chapter).

Likewise, it should be mentioned that early products offered by K&E were not required to have "Keuffel & Esser" stamped somewhere on the actual product. Often enough, K&E served as a distributor only. There is a fine line between products that K&E sold versus those that K&E wanted to be identified strongly with; yet, catalog entries for a product did not necessarily have to identify branded-products as from another maker. Likewise, it would have been sufficient enough if K&E wanted to "brand" such products, to personalize only the product packaging with their own "Keuffel & Esser" identifier. So, we shouldn't assume all samples sold by K&E to be clearly marked as such.

At some point, K&E certainly understood the concept of product branding, whereas by the turn of the century there would no longer be slide rules and accessories that did NOT have the company name emblazoned upon their surfaces. And we see indication of this earlier, with samples where the old maker's mark is scratched away in favor of the K&E branding.

Therefore, matching the catalog descriptions prior to 1890, Tavernier-Gravet most certainly supplied this boxwood Mannheim rule (shown in these pictures) as late Model 479 and early Model 1746 slide rules, with no real change in the rules appearance. The K&E stamped versions of the rule were undoubtedly offered after the catalogs stated that they were no longer available.

To further complicate the scene, we have the slide rule shown below:

This is a Tavernier-Gravet slide rule with celluloid-facings of unknown wood choice. It has been assumed that Dennert & Pape was responsible for these early celluloid-faced rules since it is they who held the patent for the technology, but we see here that T-G also produced such rules for K&E, even into the 1890s as seen by the "Chicago" stamp also on this sample.

We have evidence that K&E was offering celluloid-laminated slide rules as early as 1888, according to E.A. Gieseler in a March 9, 1888 article within "Railroad Gazette" (Volume 20, p. 149) where the author mentions these rules by name as current offerings.

This matches well the 1890 catalog description and the three year gap between catalogs (from 1887 to 1890) whereas K&E could have been receiving rules from D&P. And without doubt, Dennert & Pape was around in 1891 for the production of the early duplex rules, particularly the Wm. Cox rule designed in 1891.

A wildcard in need of further research regards the use of the horizontal longitudinal lines (railroad scales) on samples from both T-G and D&P. This feature seems consistent with T-G rules of the era. But it is noted by a few sources that D&P no longer used such scales on rules after 1888, particularly in the U.S. markets. However many samples, including the Otnes' "Rosetta Rule," are in conflict with this generalization as there is no question that D&P produced rules with that feature during the 1890s.

So what can be said about this early maker mystery?

It is reasonable to believe that Tavernier-Gravet was responsible for the production of most boxwood rules prior to circa 1901, including those with celluloid-laminations. The exception would be the potential for the earliest rules to be made by Rabone and Sons, as Bob Otnes opined, and those possibly made in-house by K&E later in the century. This aligns well with some of the "Transitional models" and the "Favorite" rules to be discussed later in the chapter. Dennert & Pape, owner of the celluloid-on-wood patent in 1886, had shifted entirely to mahogany by 1888 according to Hans Dennert, so it is likely that all mahogany rules with celluloid after that date were made by D&P. Whether they also made boxwood rules special to K&E during the 1890s, or whether the T-G rules with celluloid were anything other than boxwood, is unknown; however, I would suggest that this could be a reasonable theory.

It is also reasonable to believe that catalog offerings of K&E products prior to the early part of the 20th century should be taken with a measure of uncertainty. There are samples of slide rules that do not seem to conform to catalog descriptions. And the era was such that consumers would not have required the level of product certainty that we demand today. A box labelled "Mannheim rule with celluloid-facings" could have been filled with any number of products.

In conclusion, a picture of what was going on in the these early days at Keuffel & Esser is incomplete today. But there is enough circumstantial evidence to see that they, in addition to building some rules in-house prior to the turn of the 20th century, sourced their slide rules simultaneously from both Tavernier-Gravet and Dennert & Pape, and perhaps even others if the opportunity availed itself.

Interesting that this statement is made a decade prior to 1901, when they would roll out their patent for the "adjustable slide rule."

Two more rules in the series arrived in 1897, including a glass cursor version of the 1746 called the 1746-1/2 and the first of the "Beginner's Rules" known as the 1749-1, strangely also a 10" model. I say strangely because K&E used the 1749 model number for a wide variety of products by applying a dash. For example, Colby's and Crane's Sewer rules (see Specialty Rules in a later section) were numbered 1749-2 and 1749-3 respectively, and their 12" ivory and boxwood sector rules were numbered as 1749-7 and 1749-8.

A 1744 duplex 10" rule (at a retail cost of $6.50) was also produced in this series (see The Duplex Family in a later chapter). This, known as the "Cox Patent Rule," was an important slide rule to the history of K&E, so it will be handled in the later context.

Sidebar: Mahogany vs. Boxwood

As early as 1900, we see Keuffel & Esser shift the majority of its slide rule construction from boxwood to mahogany wood. This, following on the heels of their major supplier, Dennert & Pape, having done the same. Likewise, American competitor Dietzgen also followed suit.

The exception to this great "mahogany migration" were K&E's Beginner's rules which I believe were some other wood, as I will contend later in our discussion of them. Likewise, the Favorite series would continue to utilize boxwood. Even so, within a short amount of time, every wooden slide rule manufactured by K&E would be made of mahogany.

So, clearly, there is an aspect of mahogany that makes it the preferred wood for slide rules. I believe if we hope to understand K&E's mind on this, a deeper dive into the nature of these woods is required. As a woodworker myself, I think that perhaps I can bring some clarity.

Boxwood is more of a shrub than it is a tree. While it can grow to 20 feet or more, it yields a trunk no more than about 6 feet on average, and is seldom straight. This means you could never really supply longer projects with it as it could never produce conventional "lumber" in lengths any more than a foot or two. Instead, any of the three European species that historically supplied rulers and slide rule makers in the 17th and 18th centuries - England was almost entirely the center of manufacturing during that era - would have been cut into logs like firewood and then milled up longitudinally to form short "blanks."

These boxwood blanks would have been hard, much harder than mahogany, making the milling process a challenge. And while the wood has small pores, the grain of the wood does not always run straight. This causes something referred to as "grain run-out" which affects ones ability to smooth the blanks, as the boards tend to "tear out" if planed (smoothed by knifes) in the wrong direction. Additionally, multi-directional grain tends to lend to more seasonal wood movement along the entirety of the wood's surface, not only at the blank's end grain. This means the blanks will be prone to warping, bowing, or swelling if they take on increased moisture from the air, as well as end-grain "checking" or cracking during the drying/curing process.

Wood with multi-directional grain may also hold tension, meaning that once milled, it will have a natural tendency to bend to another shape. So the procedure when working such wood is to cut pieces over-sized, wait a day or two, and then mill the wood to final dimensions after the board relaxes. Not only does this property require extra time in milling, but it can also make the boards bind against the blade during cutting operations as the wood releases its tension through the cut. This, coupled with its extreme hardness (there is no harder wood in Europe), makes working boxwood more risky than woodworking-friendly lumber, like mahogany.

Another important point directly relevant here is that the properties of boxwood would not make it the best candidate to be laminated with celluloid, even if many rules did just that just prior to the turn of the century...but more on that shortly.

But once well-seasoned (dry) and sealed, then boxwood tends to hold its shape. It also can be brought to a very smooth finish after sanding and polishing. For this reason, today, boxwood is considered a lovely wood when "turning" (with a lathe), for making small pens, chess pieces, small wind instruments, croquet mallets, wooden goblets, and bowls. It is seldom used for objects with flat dimensions since boxwood is not very "workable" beyond its use in turning.

In the 19th century, when K&E first began offering slide rules, they would have taken their lead from Tavernier-Gravet, so it makes sense they would have continued with the European tradition of using boxwood until it became impractical to do so.

On the other hand, mahogany is a very versatile and wonderful wood to work with. It too is a hardwood, though only moderately so. It is soft enough to work with relative ease. Orangish-brown in hue with large pores and a glimmering chatoyance in brighter light, this very straight-grained wood could be milled from very large stock as such trees grew quite large. The wood is known to be dimensionally stable, making it perfect for use in products like musical instruments, especially in guitars and for guitar necks. It's a woodworker's dream, as it can be easily milled flat with very little tear-out of the grain upon plaining. And it retains its shape once cut, with very little internal tension.

The only point of curiosity with regard to slide rules is one of supply, particularly to Europe. True mahogany comes primarily from the tropical Americas, major sources being Honduras, Mexico, Cuba, Jamaica, West Indies, and many regions around the Caribbean, including Florida. There are many mahogany "knock-offs" - related species - that are marketed as such, but only African Mahogany (Khaya) would be close enough to have been possibly used in slide rule manufacture. I have heard it stated that D&P and, therefore, K&E received their mahogany supply from the German slide company, Nestler, though it begs the question of how Nestler was supplied?

It's not beyond reason, if this is the case, that Nestler could have imported their lumber from the Americas, as the wood is plentiful enough to be reasonably priced. Likewise, a single board would have produced an enormous number of slide rule blanks without any waste, something that could not have been stated about boxwood. And once K&E began to manufacture their own mahogany slide rules, then finding a supplier in the United States would have been quite easy...and relatively cheap compared to the imported boxwood.

By the turn of the 20th century, the shift from boxwood to mahogany was industry wide. We can speculate the reasons why. First, it is likely that boxwood just became increasingly more difficult to supply. It was most certainly becoming over-harvested. Combining this with its susceptibility to disease and moth infestation, a known problem to world supply over the past century, it would have made boxwood far too in demand...and therefore expensive. So the shift to the better mahogany supply would have been required.

More importantly, since celluloid-lamination of wood was quickly becoming the desired construction for slide rules, then it would have made sense to use a wood-type that paired well with the plastic. As mentioned earlier, boxwood is not really the best candidate for that. We need look no further than with Dennert & Pape for evidence, whose first boxwood slide rule was introduced in 1872, yet a short 16 years later, D&P had shifted production almost exclusively to mahogany, coinciding almost exactly when their 1886 patent for celluloid coverings was granted.

As such, two questions come to mind. First, why did K&E still offer boxwood with some of their rules, even after 1901 when many rules were mostly produced in-house? And, second, once K&E made their own rules, did they pay a royalty to D&P until 1906, when the original 1886 patent would have expired?

To answer the first question, I believe K&E received their boxwood supply of both slide rules and blanks from Tavernier-Gravet. While the writing was on the wall for boxwood, they certainly could have had a back-stock of inventory needing to be sold. Remember that these slide rules were expensive for the time, and there wouldn't have been many people persuaded to need them yet.

Although I have not heard it mentioned anywhere, K&E would have needed to have arrangements with D&P extending to 1906 even if rules were no longer supplied to them. There is no doubt that K&E would have honored the German patent (DRP 34583) and paid them a royalty for using their patented construction.  This would seem to make sense as, next to 1901, 1906 was the next major catalog year, where K&E made a large number of changes to their slide rule lineup, as well as solidifying existing models, including their Mannheim and Duplex Family lineups.

For this reason, D&P would likely be responsible almost entirely for the mahogany-based slide rules approaching the 20th century, with Tavernier-Gravet, certainly paying for that right, to make their own celluloid-laminated, boxwood rules. (I have not been able to confirm if they also produced such rules from mahogany.) So during this era, a picture develops of K&E receiving their supply of boxwood rules from Tavernier-Gravet and their supply of mahogany rules from Dennert & Pape. I could see 1901 as being a target for in-house manufacture for all slide rules, with inventory to burn until 1906, at which point they would no longer be under the contract with D&P and could build most all of their slide rules to a more uniform consistency.

By that point, mahogany would have been the rule of choice for a variety of reasons, most importantly, price.

Today, nothing has changed. Boxwood is very expensive, mostly because by today's standards, lumber mills see no value in putting in the extra work to prepare small blanks of wood for woodturners unless they can extract more value from them in the form of high prices.

Mahogany was not exclusively used for celluloid-faced rules during this time, as the Model 1748-3 was introduced to this series in 1899. This 10" boxwood Mannheim rule would become the Model 4054 "Favorite" rule in 1901; though there is no model number on the rule. However, the 1748-3 is described as "Favorite" in the 1899 catalog. To save time here, you can read about the 1748-3, it's construction and purpose, below within the section on The Favorites, since it essentially is the same rule.

Finally, it is speculated by some that this same Model 1748-3 rule might have first appeared shipped in a box labelled "American," not appearing in the catalogs. This boxwood rule, every bit as similar to the "Favorite" Model 4054 series (described later in this section) and the 1748-3 just mentioned, is thought by some to be K&E's first manufactured rule. At least that is a working theory, with the "American" label being apropos if that's the case. It is mysterious though, since there is no labelling as such on the rule itself, but rather the only indication of its existence comes from actual samples that came in a case so labeled.

Samples of this "American" rule are widely known, but rarely found, and is very desirable among collectors. There is some indication that the American rule might have continued through 1901, as there are samples that are also adjustable. This most certainly is perplexing on top of already being a mystery! There is no known associated model number to go with the American slide rule and no company literature that talks about it historically.

The absolute best source to read with regard to the "American" rule can be found in Part 1 and Part 2 of Mike Sypher's insightful articles found at his terrific www.followingtherules.info website.

It appears that one of the common traits of all rules within this 17XX series is the 26cm long form factor with scales of 25cm length for the 10" rule; and all rules regardless of length did not leave much room at the ends. This 1/2 centimeter on each side of the rule complicated the use of a glass indicator with single hairline, since barely 1/2 of the cursor would be on the slide rule when set at the indices. This is an oversight, but there is some evidence from drawings in the early catalogs (specifically 1899) that a double hair-line cursor did exist, which would be available as an option later for the Model 4041 Series, as well as used on some later specialty rules such as the Model 4133 Roylance Electrical and the Short Base Triangulation rule.

A note regarding wood choice. As we have shown, Dennert & Pape was producing celluloid-faced mahogany rules during this time. But we should likely expect variance in the types of wood in use between models as well as model years. Exact dates of the use of wood within models are unknown, as it is possible that a mix of woods could have been used to supply the same models during this time.

Similarly, as clarified in the Sidebar: Solving the Early Maker Mystery above, there is a sample of a celluloid-covered slide rule made by Tavernier-Gravet that also meets the description of the 1746 model. As such, it becomes difficult to know how K&E incorporated both suppliers into this picture. But most certainly Dennert & Pape was responsible for the early celluloid rules, as they would have been the only maker able to make these rules due to patent rights. Such rules from Tavernier-Gravet would have required some lead-time before D&P licensed (we assume) these rights to other makers. Let's call that logical conjecture.

Because of the variance in both maker and make, it's highly probable that K&E used generic descriptions in the product catalogs of this era so that they could offer slide rules covering multiple construction materials and techniques from multiple suppliers. There is not enough conclusive evidence to say when Tavernier-Gravet and Dennert & Pape supplied rules for K&E. Nor do we know when K&E began producing such rules in-house, as we believe they did with the Model 1748 "American." I could see all three events happening simultaneously, with K&E putting any one of those rules in a box that was labeled "Model 17XX."

Mannheim slide rules of the 17XX series were replaced across the board in 1901 with a complete line-up of "new" slide rules, thought by most to be company-made. But the transition to full-production of all rules would still take some time giving rise to some slide rules referred to in the next section as "Transitional."

Nevertheless, any rules of this era are rare for today's collector.

The Transitional Years and Models

During the last 5 years of the 19th century, it is obvious that K&E was experimenting with what they wanted their Mannheim Family to look like. Some rules, like the American/Favorite, had most certainly been made in-house; but not all, showing that K&E was transitioning between the roles of importer and manufacturer of their rules.

Between 1901 and 1906, despite the new model designations and added slide rules to K&E's product line, a few of these slide rules seem to have an identity crisis, as they would not resemble the rules of the product line a decade later. The rules themselves seemed to shift freely between boxwood and mahogany construction during this era - and I surmise that some rules were made of any wood K&E could find. Others held on to the metal "chisel" indicator when it was clear that glass was largely preferred. Several others retained the "railroad" style of scales, unlike all other 4041 series rules, yet some like the 4030 model dispensed with the horizontal embellishments and took on the cleaner, more modern look of the next series.

In fact, many of these rules could be classified in the "Model 4041 Series," a classification given by collectors, not K&E; however, these rules, which I've classified as "transitional," are different enough despite their spiritual heritage and similar model numbers. I suspect this is mostly because of existing back-inventory from Tavernier-Gravet and Dennert & Pape that K&E did not want to waste, and so they sold them with revised model numbers to match the 4XXX series designations. Therefore, here are some models sold in the first decade of the century that held onto life as remnants from the 17XX model line or remain different enough to deserve separation from the rules talked about in the next section.

These rules are as follows...

Model 4028 Gunter - This 1901 model succeeded the 479, 479-1, and 1745 "Gunter" rules of previous years. Remaining cursor-less, this model was discontinued after 1906, and would have likely never been offered for sale in the 20th century if K&E didn't still have a back-stock of slide rules in which to dispose. Their rarity today is likely an indication of sales; I know of no known samples of this Gunter rule. Or, as mentioned, it could be incognito. Regardless, the rule disappeared after the 1906 catalog.
Model 4030 - This model is the 5" Model 1747 described above, both in form and function. It is non-adjustable boxwood with the basic Mannheim scale set. Like all models for the 1901 years receiving a new 4XXX designation, the 1747 was christened the Model 4030 and sold for the next couple of years. I would speculate that the sole reason it sold after 1901 was because of back-inventory of the 1747 rule, and as such is a likely a Dennert & Pape imported rule - operating on my theory that D&P produced the rules made just prior to 1900. Albeit, it is indeed curious why D&P would make these boxwood rules when it is known they had shifted primarily to mahogany construction during this era? This rule was, however, in the 1901 catalog and listed among the "adjustable" rules. This shouldn't pose any issue, since the 4030, 4031, and 4032 are completely interchangeable at this time other than their cursor options.
The Early Model 4031 - Continuing with 5" slide rules, this rule is essentially a Model 4030 made to be adjustable like the mahogany Model 4041 Series, and is listed as such within the 1901 catalog. But because it is boxwood with the identical 1747 legacy, this production rule is far different than the K&E-built rule to come in 1906 with the same model number. Hard to say why this rule did not change to mahogany with the rest of the 4041 series in 1901, but noting that this is a 5" slide rule, I would surmise that K&E had the most back-stock of this size. As we will see with most all early wooden slide rules, K&E couldn't sell shorter slide rules at a discount to the longer ones and likely didn't sell many 5" rules. So with essentially an over-supply of 4030 slide rules laying around, then adapting them to the new adjustable technology makes sense to me.
Model 4032 - Once again, a 5" rule identical to the 4030 and 4031 models of this time, the Model 4032 came with the earliest of K&E's magnifying cursor. Otherwise, like the 4030 and 4031, I'm guessing it too is a D&P rule. It would translate like the Model 4031 does into a full-fledged member of the Model 4041 Series as far as construction goes, but it would eventually be discontinued by the 1909 catalog.
Model 4040 and Model 4050 - These are 10" and 20" models of the same Model 4041 that would appear in 1901, fully-adjustable and made from mahogany. However, these would be the last K&E rules to offer the chisel-type brass cursor. These, like the Favorite/American just prior to the new century, were most certainly built in-house by K&E. They could just as well be placed with the rules of the 4041 Series, but the chisel cursor banishes it to this list of transitional models. Both models, like the 4030, would disappear from the 1906 catalog anyway.

Beginning with 1901, most collectors sort the Mannheim Family of rules into three broad categories: the flagship Model 4041 series, the middle priced Favorites series, and the budget-minded Student or Beginner's series. So we see that K&E is already showing that market positioning and product placement is essential if they hoped to sell slide rules, particularly in the numbers that they would eventually sell.
Also important for collectors is the idea of standardization within the appearance of K&E rules, and as such, what follows is mostly a strong commonality in the look and feel of K&E's wood rules that would dominate the next half-century. As such, next, I will focus on aspects of the Model 4041 Series that are typical of what we picture when we talk about these rules, as well as what we can realistically expect when we purchase them more than 100 years after they are made.

From the 1906 K&E product catalog, this page defines their Mannheim rules as being "adjustable." This was also true of the 1901 catalog, but this version drops the 4030, 4040, and 4050 models, as the brass chisel type indicators were no longer offered. While the cursor-less 4028 Gunter was still being offered, it is mentioned on the preceding page and not with the adjustable rules.

Model 4041 Series

Once K&E replaced the 1746 and 1748 rules, as well as having moved past their transition period of various and sundried slide rules mentioned above, the Mannheim product lineup solidified and standardized around the Model 4041 series, both in form and function. These rules described below were manufactured in-house by K&E and sold with a sewed leather sheath. Although many of these rules were listed in the 1901 catalog, what we will classify here as the Model 4041 "Series" will be those models listed in the 1906 catalog and moving forward.

The most important aspect of this series that differentiates them from those I mentioned in the previous sections are that 1) all are made of mahogany and 2) all rules of this series are made to be adjustable, based on Keuffel's patent of the "Mannheim Adjustable Slide Rule Frame" in June, 1900.

Regarding the shift to mahogany construction, I explore the reasons surrounding it in the "Sidebar: Mahogany vs. Boxwood" at left. Regarding adjustability, K&E long understood that weather could change the moisture content of the wood, causing seasonal swelling - remember their comment above regarding the tendency of the 20" Model 1748 to warp? So any rule without the patented adjustability, namely those made prior to 1901, would have been hard to use on occasion.

As such, after 1901, all "flagship" models with the Mannheim frame, including the Model 4053 "Polyphase Mannheim" rules in the next section, would boast "adjustability" as a significant improvement, and they could justifiably charge more for it! Conversely, mid-tier and budget rules would not include the extra feature and could be viewed by the consumer as a cost saver.

With the original Model 4041 on the right, we see the 3mm difference with the new 1925 N- version of the model (left).

Despite early questions about what all their Mannheim rules looked like in 1901, the Model 4041 would be the flagship model for this Mannheim series, most certainly in the eyes of modern day collectors. This Mannheim rule ($4.50 in 1901 and 1906) was a lovely 10" mahogany rule covered in celluloid with engine-divided scales and glass cursor (evolving over time). The original 4041 rules would have no model number until around 1912, at which point it would print the model number vertically in red at the end of the slide, as has become familiar with most all K&E rules.

Prior to 1911, when K&E would move to the "dash length indicator" naming convention, K&E used different model numbers for the same rule in multiple lengths; therefore, the same 4041 rule became the Model 4031 in a 5" length and the Model 4051 in a 20" variety. In 1906, they would add the Model 4035 in an 8" length (the first 8" rule since the original Model 479-5) and the Model 4045 in a 16" length. Most models are similar in construction and appearance, though many took on evolutionary changes earlier than others. Formula and conversion charts on the back of the rules pretty much stayed identical across all models and across all years. The exception is the 5" Model 4031, which retained the old chart since it was too small to use those of the larger rules. But recall that these are fundamentally Mannheim rules, and just as a reminder they all consisted of the following scale set:

Front Side: Inches // A [B, C] D \\ centimeters
Rear of Slide: [S, L, T]

The rule would also have a centimeter extension scale in the slide well on laminated celluloid, though it would disappear in 1912 in an evolutionary change.

Speaking of which, let's look at how the 10" 4041 model evolved over time:

The rule retained a similar font and scale design as the earlier 4030 models for the first year or two, but changed between 1902 and 1904, while also adding red ink to the maker's mark.
In 1904, the 4041 added a centimeter to the rule's physical length, making for a 27 cm format. This solved the issue of the indices being too close to the ends of the rule, and therefore fixed the problem with the cursor hanging off the end. Even so, K&E would continue to sell the two-hairline cursor, designated the 4052 D.L until beyond 1916. Note: K&E would also sell the magnifying cursor (4053 M) and a "decimal keeper" cursor (4052 DP) through 1906 (as shown above), being discontinued by 1909.
With the extra space at the ends of the slide, the vertical, bracketed model number <4041> appeared in 1912. The centimeter extension scale in the slide well disappeared as well. This left only the June 9, 1900 patent notice remaining on the celluloid-covered slide well, still in black ink.
In 1914 or 1915, the metal-framed cursor gave way to the "frameless" all-glass indicator, first with metal rails, and then with celluloid plastic rails in 1916.
In 1917, the celluloid-lamination in the slide well disappeared and the patent notice moved to the front of the rule, in red.
In 1921, it is interesting to note that the pricing of the 5" and 8" models was raised to S7.30 and $7.70 respectively, whereas the 10" charged $6.50. This indicates that the shorter rules were more expensive to make, likely because they were deemed to be "divided as finely" as the 10" version. If extra labor or tooling was involved to make that happen, it's curious why it took 20 years before this was reflected in the pricing?
In 1922, like all K&E wood rules, serial numbers appeared on all models of the series.
The 4041 (and 4035) added the "N" designation in 1925 with a slightly wider frame (see right) - growing from 32mm to 35mm wide - as well as scale labels. The 4035 model followed suit two years later.
In 1936, like almost all K&E rules, the improved-glass, metal framed cursor was introduced. The scale designations were shown on the front of the rule.
In 1939, the 16" and 20" versions are discontinued.
In 1941, only the N4041 is left in the catalog, disappearing itself in the 1943 catalog.

Throughout the history of the Model 4041, a few custom versions appeared as special order rules. Interestingly, and mysteriously, a sample of the Model 4041 appeared around 1910 with railroad track scales. This seems to pay homage to the old days of the D&P rules, as it was otherwise similar to the typical K&E-made 4041 being produced at that time. Even so, it's a real curiosity, and should not be a surprise to us given the amount of experimentation going on all throughout the early part of the 20th century. This version is not listed in a catalog. This 4041 rule was also produced between 1906 to 1935 with a finely-divided scale option known as the N4041F (a $3.50 upgrade). These rules included engine-divided inch and centimeter rulers on the sides (and a centimeter extension rule in the slide-well up until 1912). Likewise, during almost the entirety of the 1920s, a version of the rule could be ordered with decimal-reminders; "Quotient +" and "Quotient -" labels on the end of the slide. This option is not listed in any catalog of which I am aware.

As I mention in the Collector's Outlook (see below), the 10" Model 4041 is quite ubiquitous; however, the length variations of the 4041 are quite hard to find, as is the N4041F, "finely-divided" model of this series, as too the railroad track and decimal-reminder variants.

I would conjecture fewer of the variations of the 4041 were sold due to the pricing structure. $4.50 could have bought the consumer any of the 5", 8", or 10" models, as they were all equally priced. One might question why this is the case, but it makes sense when we consider that production time and cost would have been similar regardless of the rules' length. And most consumers of the day would have only been able to choose one such slide rule due to the high cost, not feeling compelled to have, for example, a pocket model to go with their full-scale daily driver. Similarly, the 16" Model 4045 and the 20" Model 4051 were priced $10 and $12.50 respectively. Those prices represent a rather steep requirement for slide rules that wouldn't have been significant functional upgrades to the 10" rule.

A size comparison of rules in the 4041 series, shown here without cursors. The middle rule is the 8" 4133 Roylance Electrical (discussed in Chapter 4) shown for comparison to the Model 4035 above it. Two other items to note. First, the diminutive size of the 5" Model 4031 might surprise you. My sample is missing its cursor, so finding a replacement could be a challenge. Second, the bottom rules show the difference between the "N" and non-N versions of the Model 4041, with the N4041 (bottom) growing 3mm wider in 1925.

Modern day consumerism, whereby products providing 20% performance improvements at double the price is an acceptable trade-off today, would not have been the nature of K&E's market in the earlier part of the century. Thus, to the consumer, it's easy to see how the 10" Model 4041 rule would have represented the very best in value, and as such would have been K&E's primary seller in this model series. Because of this, more than 100 years later, it has made tracking down the entire series of rules quite a challenge since, undoubtedly, relatively few of anything OTHER than the 10" rule was sold.

The "Favorites" - Models 4054, 4055, and 4056

Collectively marketed as "Favorites," these basic 10" Mannheim rules are lower cost varieties of the 4041 series, also part of the huge product roll-out in 1901. The 4054 ($3.00 retail price) and 4056 ($2.75 retail price) ran during the length of the original 4041 production run (which changed to an "N" designation in 1925). Dropping all Favorite models in 1925, they reinstated the line in 1927 in the form of a lone 4055 model. The Model 4054 was identical to the 4041, complete with celluloid scales, except for its less expensive choice of boxwood* and painted-on inch and centimeter rules on the sides. The Model 4056 saved consumers even more money by being a bare "polished" boxwood, completely painted-scale model. Both models lacked adjustability.

Once the 4054 and 4056 were dropped in 1925, the new Model 4055 design featured celluloid-laminated mahogany with engine-divided scales, but it lacked inch and centimeter rules on the sides. It was considered a middle-ground option between the top 4041 Mannheim rule and the budget student/beginner rules (see next model) that K&E always offered. A 10" scale length was the only option for these rules.

Both of the 4054 and 4056 models would be reintroduced at points in the future. In 1936, the year that the "improved-glass" cursor was introduced across many K&E slide rule lines, the 4056 was reintroduced as a budget version of the 4055, the $3.50 latter rule receiving the new cursor and the former getting a standard metal-frame glass cursor, saving consumers a dollar. Otherwise, the rules are identical, including mahogany and celluloid-facing for this new 4056 - a vast improvement over the original, spartan 4056 model.

In 1944, when K&E finally ended the long reign of the 4041 Mannheim, they also discontinued the 4055, leaving the 4056 as the sole "Favorite" model. The 4054 was also reinstated as a nice budget option for the N4053 Polyphase Mannheim, which we will talk about shortly.

The new version of the 4056 would endure through 1952, when it was discontinued. I would speculate consumer tastes were shifting more toward the wide-variety of pocket rules K&E offered at the time, including their variety of Ever-There models (see next section), as well as the new line of modern, all-plastic Polyphase rules being pushed out around 1950, known as the Doric models (please see Modern Polyphase Rules in the next section).

I should note that the original boxwood versions of the 4054 and 4056 Favorite rules are very hard to find. I suspect this is because they weren't as long-lasting as the adjustable mahogany rules that litter eBay. These rules, if found, are easy to mistake as "ordinary," so it's likely that a collector might not have to pay a lot if they stumble across one. Shown below, a 4054 Favorite rule from circa 1915 in my collection was acquired in exactly just this manner.

*Note: I do not feel boxwood was less expensive to K&E, but only that they likely had enough supply to produce rules for the Favorite series.

This is a 4054 Favorite rule in my collection from approximately 1915. Note the plain boxwood on the inch ruler, the celluloid-covered face, the early production number on the ends, and the indices almost at the edge of the rule. It is exceptionally rare. At any given time on eBay, there will be 15 or so 4054 rules, yet all of them will be the later Polyphase model. A keen eye might be able to notice this gem when it arises, and it will likely be priced like all the 4054 Polyphase rules. I paid less than $20 for this sample.

The Beginner's Family

It would become obvious by the turn of the century that the everyday person would not be able to afford a slide rule. With the rising prices of materials and costs of quality craftsmanship, a budget alternative was necessary. In many ways, such rules served as an introduction to slide rules, encouraged by K&E to be a first purchase so as to begin what they hoped was a life-long customer with fierce brand loyalty.

As mentioned earlier when discussing the 17XX series of Mannheim rules, it was apparent that the push toward a more broad customer base was happening, both in the language used within their product catalogs and in the 1901 expansion of product offerings. That said, let's take a look at the next series of single-sided rules, based on the Mannheim type. Whereas later rules in this family will shift away from the pure Mannheim scale set (which could be talked about in the next section), we will continue to discuss them here to better demonstrate the evolution of K&Es least expensive slide rules.

The Model 4058

A "student" model for any budget, made of a less expensive wood, glued-on paper-scales, and a metal-framed, glass cursor, the Model 4058 was the carry-over of the previous model 1749-1 introduced a few years earlier. Reintroduced in 1901, the rule had a rock-bottom price of $1. These rules did not include the inch/centimeter scales on the sides, but otherwise offered the Mannheim scale set. It should not be assumed these were constructed in-house. I've seen rules by Nestler that look suspiciously similar to me.

While all 4058 variants served the purpose of a budget rule, these were not intended to last long, nor be rules to keep for long, as the catalog states, "for the use of beginners to enable them become familiar with the slide rule without incurring the expense of the regular rule..." Apparently, once the user became "familiar," they are recommended to acquire another rule, at which K&E knew a place they could be purchased.

As such, they were likely imported rules up until the 1913 Product Catalog when they are described as printed directly on the rule (without the white paper). There is no indication that the new rules were made in-house, but the cursor was certainly K&E-made, however. W.L.E. Keuffel's patent for a folded-celluloid cursor, issued in 1916, was introduced on these rules at that time. Rules would begin to be stamped with that patent date on the front of the rule around 1920, the same year that K&E added a "Made in USA" on this slide rule. I treat that as significant while others might not.

This model was introduced as the "Student's Slide Rule" from its inception, with that label also getting placed above the formula table on the back of the rule in 1923, this also coinciding with a change in formula charts to the familiar "US Bureau of Standards Circular No. 47" version.

Collector's Outlook: Single-sided K&E rules

Note: Suggested prices are for slide with case on eBay. Any extras, especially those considered complete, with box and documentation, can double these prices.

Single-sided K&E rules are easily collectible, if by "collectible" we mean easily and cheaply found. The basic student rules of the 4058 series are abundant on used markets between $10 and $20 respectively for the full-scale 10" rule. The same can also be said of the 4053 Polyphase series.  The 8" and 20" versions do carry a higher price, but can typically be found in the $30 to $60 range depending on condition and completeness.

The special versions of the Beginner's rule, Models 4059, 4058D, and 8858 Special War Time rule are all very rare, appearing maybe every other year on eBay. Prices for the 4059 and 4058D are as cheap as the normal 4058 rules, likely because nobody recognizes them when they come up and aren't truly collectible. On the other hand, the Model 8858 with its special war time packaging is more collectible, likely going for $50 to $100 if one were to come up again. It has been quite some time that this has happened for any of them.

The 4041 series is also quite common, likely $20 to $30 for a rule in good condition. The variations of this rule are where they become much more desirable. The 5" 4031 and 8" 4035 are difficult to find. The 4031 model comes up on eBay maybe once a month, selling for around $40 on average. The 4035, maybe once per year for a similar price. The original 4040 version in this series, that with the brass cursor, only had a run of 5 production years, and thus it's quite rare. Only two samples have been sold on eBay over the last 23 years, averaging $273. A sample of the 16" 4045 and 20" 4051 will come up for sale every other month or so, with an average price of ~$50. Of course there's often another one posted for maybe 3 times that amount that remains there forever.

Highly desirable is the "F" or "fine-scale" version of the 4041 and 4053 slide rules. They come up maybe once or twice a year with an average selling price of perhaps $100.

Others that are moderately collectible are the 4054/4055/4056 "Favorite" series of rules, which are found easily on eBay for maybe $20. The exception is the original boxwood version of the 4054 and 4056 rules, which are very difficult to find, as most in the wild are the later mahogany versions. Very few have come up on eBay over the last couple of decades, though prices are less than $100 for the 4054 and $20 for the 4056 when they do.

Any of the Ever-There Pocket rules, as well as most of the more modern pocket rules made of the better plastics come up very frequently on eBay and can be had for around $15 to $20 or so.

Anything from the 19th century is basically a unicorn. Only a couple of 1746 samples have come up on eBay over the last 23 years, selling for around $1000 each. The early 1749-1 Beginner's rule has seen one sample sold on eBay for nearly $500. The so called "American" slide rule, in the box, exists in very few collections. This can cost perhaps $300 to $500.

I never seen a Model 479, nor it seems has anybody else, though I would speculate that these rules were those imported from Tavernier-Gravet, lacking the model number which could have been known as the Model 479.

The "Student" label would change to the "Beginner's Slide Rule" in 1925. With the name change, a new version of the rule with a wooden-rail glass cursor was added to the normal 4058. Called the 4058C, it was very much an improvement for a quarter cents more than the base rule.

There is some overlap between the name-change and the new 4058C model, as there are versions of it with each of the "student" and "beginner" labels.

In 1930, another rule was added, giving buyers the option of three grades of the beginner rule as follows:

Model 4058 with Xylonite cursor: $.75
Model 4058C with glass cursor & wooden cursor rails: $1.00
Model 4058W - like the N4058C but with white painted faces: $1.25

The newest model, with white-painted face, also utilized the wooden-rail cursor. In 1937, the new 4058W took on a metal framed indicator lasting until 1950 when it would get an all-plastic cursor. This series would go through a variety of cursor types among its variants. For more specifics, I reference you to Appendix 3: A K&E Cursor Study.

This ad printed in the Univ. of New Mexico student newspaper on September 19, 1941, showing the inability of K&E to meet the demand of students across the nation during war time. The problem would exacerbate after the war, when scores of soldiers returned home to enter engineering college. It is said that when students were forced to purchase cheap slide rules like Lawrences and Charvos-Roos, the slogan, "Don't get stuck with that stick" was born.

As World War 2 neared, the Model 4058 lineup of rules shifted. A new Model 4059 appeared in 1939, which was the first and only 8" version of the 4058 slide rule. With Xylonite cursor, a trimmed down version of the conversion chart on the rule's back, and lacking any scales on the back of the slide (no trig scales), the 4059 sold for only $.50 in 1939. This is a rule I would not mind collecting. But it is rare, being discontinued three years after it was introduced.

In 1942, the Model 4058D arrived, which was merely a relabeling of the base Model 4058 rule with the Xylonite cursor. I suppose it was meant to replace both the 4058 and the 4058C model, which was discontinued that year. Even so, it would disappear the very next year itself, having appeared in only the 1942 catalog.

Another rule with strange product numbering was the Model 8858 Special War Time Issue of the 4058D rule. Produced only in 1942, this slide rule removed the Conversion Tables from the back of the normal version, with a special message printed instead. It read:

"No. 8858 Slide Rule. Loaned on deposit of $1.00 by Keuffel & Esser Company to meet a temporary shortage caused by defense orders. This deposit will be accepted as part payment toward the purchase of a standard K & E slide rule through your regular dealer before July 1, 1942."

In a unique way of supporting customers, the rule functioned as a loaner for those wanting a slide rule that was not available during WW2 due to a lack of production capacity (see flier at below). Apparently the 8858 functioned like a coupon in exchange for a more expensive rule, yet making sure that the customer had something to use in the meantime. The redemption date of July 1, 1942 seems sudden for a rule produced in 1942, but apparently K&E had it under control!

The white 4058W model continued on, but became the Model N4058W in 1944 when its Mannheim scale set shifted entirely to the Polyphase Mannheim. Still a beginner's rule, it would make little sense to talk about it among the next family of rules, but this new version would continue until 1960 when it would be replaced by the all-plastic rules we discuss next. Even so, as a budget rule, there was nothing significantly different with the N-prefix rule, other than the scale set.

Earlier models of the 4058 are reputed to have been quite enduring. While most collectors deem the rules to be made of boxwood throughout the entire 4058 production run, the only real indication of wood type is in the 1897 catalog where the 1749-1 (the original "Student's rule") was stated to be "hardwood." In 1915, K&E changed the catalog description to read, "The graduations are printed on light-colored wood." This indicates a move to not only the printed scales, but likely the type of wood that it would remain for most of the rest of the model's production. There are some suggestions of pearwood from internet research, but that wood too is mostly European, as found in many rules from A.W. Faber (Castell). Like Boxwood, pearwood would be difficult to supply and prohibitively expensive, making it unfeasible for US import.

In fact, if the 4058 were indeed boxwood, then the rule would be remarkably similar to the Model 4056 Favorite rule described in the previous section (absent the paper scales), which carried a $2.75 price tag when it was introduced in 1901. (!) Likewise, as with the 4056, K&E never had a problem telling you that a slide rule was made of boxwood. That term is not used in any of the catalog descriptions for this beginner's series of rules. But more importantly, if K&E desired you to learn on one rule and then upgrade to another, then they simply would NOT make the beginner rule too "good." Student rule owners would have wanted to upgrade. Planned obsolesce isn't reserved for modern home appliances and cellphones!

I would suspect the lack of wood descriptions in the catalog, or using generic language like "light-colored," allowed K&E to utilize whatever wood they had available. Since the early wood rules would be covered over with paper scales, it shouldn't matter much anyway. It also doesn't dismiss the possibility of boxwood or mahogany of lower grade; less desirable wood blanks not suitable for use with their better models. "Waste" wood, or those wood blanks that might not have passed quality controls for the better models, had to be in abundance...a fact that is true whether or the construction of these rules were outsourced.  This would tend to agree with one sample in particular from Paul Tarantolo, a 4058 rule he dates to 1909 that is most certainly mahogany. Plus, many of the post-1913 do seem to be "boxwood-like."

Please see SIDEBAR: Mahogany vs. Boxwood earlier in this writing for more perspective on those woods choices in slide rules.

I will say that I have two 4058C samples from 1930 to 1935 that are actually quite lovely, as they also came in nice boxed cases. These "C" versions are bare wood, with scales printed directly on the wood, yet finished nicely in what is very likely shellac or varnish. 90 years later, they still function very well. These are the versions that feature a glass cursor with wood cursor rails. But comparing them to boxwood rules in my collection from European manufacturers, as well as the Model 4056 boxwood rule in my collection, even these rules are most certainly not boxwood, albeit they are indeed a moderate hardwood and different than the wood used in the 4058W versions. In my mind, this is the only version K&E should have made. It is functional, slides well, and feel very good in the hands.

By the 40s and 50s, judging by several samples of the white-painted, 4058W rule in my own collection, it is obvious that the source of wood was not of the highest quality, nor was it "hardwood," as I can somewhat easily indent the wood itself with my fingernail.  My two 4058C models of the 1930s also fails the fingernail test, just for reference. These too are light-colored rules that could be easily mistaken for boxwood.  Worse, the wood of the write-painted varieties in my possession appears to be quite blonde and quite soft.   In particular, my 4058W rules are quite unusable now, having surrendered their usability to wood movement.

Like the Favorite series discussed above, the 4058 models were NOT adjustable like the flagship Model 4041 series - that feature is naturally priced out of a "budget type" slide rule.

I do not have a large collection of the Model 4058 rules over the timeframe and across the variations in which they were produced, so making a definitive judgment to the wood type and quality in the "Student's rules" is not possible at this time; however, because they are inexpensive, it's a long-term project that I might tackle.

Aside: In the interest of full disclosure, I doubt that I will. Budget rules of this quality of construction do not appeal to me as a collector. This is one of the reasons I have not bothered to collect many of the numerous varieties of Lawrence and Engineering Instruments slide rules, both of which have a similar type of construction and feel.

Overall, some form of these wooden rules ran from 1901 to 1960, but as you might have deduced, they are typically underwhelming from this collector's perspective. None - other than the 4059 and 4058D - are rare, nor are they expensive to acquire today.

Model 4158/68-1892 "K-12 Prep"

In 1961, the Model N4058W gave way to this all-plastic (vinyl laminations with heat-pressed markings) slide rule. The Model 4158 "K-12 Prep", so named to emphasize its intended use in the classroom, was constructed in a duplex form yet printed on one side only - scales on the front and totally blank on the back. It used a duplex-style cursor without a rear glass window, braced at the ends with plastic brackets and coming in a plastic slip-case. It did include more scales than the normal, strictly Mannheim beginner's rules:

K, A [ B, S, CI, T, C] D, L

While this scale set is not specifically Mannheim, and the rule is based on a duplex style, it might just as well be moved to a later chapter. However, its importance is as a continuation of the student's series of rules, as noted by the evolution of the model number, and as such should be cataloged here.

The 4158 designation was short-lived, since all K&E products shifted to the 68-17XX a year later. From that point until the end of the slide rule era, the 4158 was known as the 68-1892 model. A 10" scale length was the only option for this slide rule.

Functionally speaking, it is a fine slide rule, comparable to any of this era's student models from other makers, such as the plastic models from Pickett, namely their 120/121 Trainer and 140 Microline models, as well as something like the Aristo 90X student rules. However, with a lack of anything useful on the back of the K12 Prep rule, I give the Picketts and the Aristos the upper hand.

Even so, the rule served the purpose as a beginner's, low cost rule, at $2.25 in 1962. This was by FAR the least expensive rule in the 1962 line-up, priced $3 less than the next cheapest rule. I find that fact somewhat remarkable.

The Model 4098A

Originally part of the "Ever-There" series of pocket rules discussed later in the chapter, the "A" version of the 4098A became a stand-alone pocket rule in 1936. Priced originally at $1.75 - the least expensive Ever-There Model 4097B model was $3 - this was K&Es lowest priced slide rule other than the Model 4058 Student's rules. Made entirely of Xylonite plastic and using a cheaper frameless Xylonite cursor, this rule was simplified to the same Mannheim scale set used in the 4041 (as well as the 4095 "Favorite" and 4058 "Beginner's" rules). The revised 4098A was narrower with straight sides, dropping the angular sides of the previous version (and the rest of the Ever-There series). After the 4041 was discontinued in 1943 - this slide rule was the only true Mannheim rule remaining in the entire K&E lineup until it would be discontinued in 1953, as even the 4058 had transitioned to a Polyphase model by that point. And by that time, better Polyphase versions in a superior plastic would make the 4098A obsolete.

My sample of the Model 4098 pocket rule, shown below, is in very good condition for what it is; a thin Xylonite construction that typically did not age well, even over moderate amounts of time and exposure. It is, however, slightly bowed, which is unsurprising because the rule is only 1/8" thick. This sample is black-ink only, but earlier samples would have had both black and red ink on the rule. Serial numbers look to be sequential for this rule alone. I have seen numbers ranging from approximately 00000 to 120000, with a delineation for the red/black and black-only versions of the somewhere between 60000 & 70000 range. Pinpointing a date is difficult, but it likely wouldn't be too inaccurate to say that if the first version of the 4098A was produced in 1936, then K&E likely shifted to black-only versions around 1940.

Of another note, the Ever-There rules allowed the slide to be reversed, although they were not intended to be used that way. The 4098A has non-symmetrical tongue & groove construction on the slide, which means that physically reversing of the slide is impossible. Reason for the distinction? Perhaps if the buyer knew the slide was not reversible, then he or she wouldn't attempt to do so and then complain to K&E that the rule was broken and didn't work? It might seem silly, but there are many anecdotes of K&E serving customers who made such complaints. We do see a similar thing occur with the Deci-Lon rules which we discuss in Chapter 3. Those rules too began their run with a physically reversible slide, only to be altered to non-reversible later in the production run.

The Model 4098A, front side, with simple Mannheim scale set. The real leather sheath is embossed with "K&E Co," a label typical of the era.

The back side of the rule, with inch and centimeter rulers. Note the S, L, and T scales on the back of the slide. The trig functions set the angle through the indicator notched on the back and the value is then read off to the B scale on front. The log scale sets the input off of the right index on the D scale and reads the value off of the back.

Once the A suffix was added to the 4098 model number, it ceased being a part of the Ever-There family, as we see with the 1936 instruction manual. This would be the first rule advertised simply as a "pocket slide rule."

The Polyphase Mannheim Family

Note: We should note that at this point in K&E history we can be reasonably assured that full-production capability of all general-purpose math slide rules had been achieved, where all rules, save the student rules, shared the same familiar look and feel.

Introduced in 1909 and pushed out in a variety of scale lengths a year later, the Model 4053 was essentially the big brother of the 4041 model. Almost identical in most ways, it included two more important scales over the regular Mannheim rule. Added were a "K" scale to compute cubes and cubed-roots, and a CI (inverted C) scale to permit easier chaining of division and multiplication operations. As for the name "Polyphase," see SIDEBAR: What's in a Name? below. This Mannheim version of the with the Polyphase scale set was a response by K&E to offer more scales on the same single sided, wooden platform, and to adjust to similar advancements made by their competitors at the time, most notably the Rietz slide rule being produce by Dennert & Pape since 1902. And if you compare the Model 4053 with the Rietz, you'll see striking similarities. So much so, that if you referred the Polyphase Mannheim mistakenly as a "Rietz," I wouldn't try to correct you!

This family of Polyphase Mannheim rules was perhaps the least changed over K&E's production history, often varying from year to year with minor evolutionary changes, but never shifting model names beyond the addition of an "N" designation in 1925 to the 4053 model name. The Polyphase scale set itself would find itself on most of the more basic of K&E rules, eventually superseding the basic Mannheim scale set. It would also be translated for use on various duplex rules throughout K&E slide rule history. Members of this family include the following:

Sidebar: What's in a Name?

You could be a dedicated, well-studied slide rule collector for over a decade and NEVER really gain a grasp what the term "Polyphase" really means. I can find no real source for the name, though I can speculate a bit on where it came from, which I will do shortly.

Certainly, we "kinda" get it anyway, right? We likely took enough language classes to know that poly means "many" and phase comes from the Greek word, "appearance."   And therefore, if we are to do something with a slide rule like the new Polyphase models, then it likely indicates that we can do multiple things with the rule because its multiple appearances (or functions).

So it would make sense for K&E to let us know that any improvement to the Mannheim rules comes with a change in name, and Polyphase is the term they chose.

The defining feature of the Polyphase scale set, regardless of the platform of the rule, is the improvement of the typical Mannheim scales of A, B, C, D, S, L, and T, with the addition of two new, important scales to appear additionally functional. These scales are the K scale and a CI scale, or "inverted" C scale. Doing cubes and cubed roots with the new K scale pretty much speaks for itself. Yet, it's the CI scale that yields most of the power in the new scale set. It does this in two ways:

Because dividing by a number is the same as multiplying by its reciprocal, then the user can opt to perform multiplications in the method of division by using the CI scale rather than the typical C scale.
The flexibility of using the CI scale, in addition the the C and D scales allow them to work together for greater efficiency when doing multiple operations.

It is this second characteristic that defines the "poly" and "phase" meaning of the word, wherein the user can customize their method of operation depending upon the nature of the numbers. The only inflexible aspect of being able, for example, to multiply three factors together with one setting of the slide is the fact that some computations could still be "off-the-scale." In those cases, users will often learn to re-order their computations to assure they remain ON the rule. Though with the addition of a folded scale into that mix, it represents the best of all possible worlds since an index will always be available to the user. The original Polyphase Mannheim rules did not offer this, though the future, "improved" Polyphase rules did.

We will see examples of these rules later in the Modern Polyphase era of single-sided rules, within the Ever-There series of rules, and in several of what will be discussed as "Mystery" rule. Likewise, it should be understood that this improved Polyphase scale set was also found in the Polyphase Duplex Model 4088, which you might be surprised to know appeared only four years after Model 4053, "normal" Polyphase rule.

So, what's in a name?

In 1904, the Dietzgen company produced their Model 1762 Mannheim-type rule as a response to the K&E Cox duplex rule, which boasted the ability to do efficient, chained operations. Dietzgen's model featured a scale innovation that added an extra two-decade B scale, with a single index in the middle, running one decade to the left and one decade to the right. This rule was coined by Dietzgen as the "Multiplex," meaning, quite literally, "many appearances."

You can decide who's name came first. However, not to be outdone, Dietzgen would soon call another type of their slide rules the "Maniphase."

And I'll give you one guess what that word means!

Part of the collection: 8 inch, 10 inch, and 20 inch versions of the N4053 Polyphase Mannheim.

The Model 4053

The original 1909 model of the 10 inch 4053 was similar in construction to its contemporary, the Model 4041, based on celluloid-covered mahogany wood and engine-divided scales. However this rule replaced the centimeter scale on the rule's edge with three-decades worth of "K" scale and squeezed in the "CI" scale in between the B and C on the slide. The cursor had an indicator on the bottom allowing easy reading of the K scale at the edge, a feature that K&E mentioned made the 4053 a "hybrid" of the Mannheim and Duplex style of rules.

Two years after its introduction, the 1911 models came in many lengths using the "dash" system of model labelling, from an 8 inch rule with a 4053-2 designation, the normal 10" rule relabeled as the 4053-3, and a 20" variant known as the 4053-5.   These models would take on an "N" prefix in 1922 when K&E removed the K-scale from the front edge of the rule and squeezed it (without regard to aesthetics) onto the bottom stator rail and gave users back their centimeter scale on the front edge. Interestingly, during this transition year in 1922, the printing of the "N" on the rule itself varied from outside of the bracket (as in N<4053-3>) to being above the bracketed model number. By the next model year, the prefix joined the model number inside the brackets.

A finely-divided scale option of the 4053 was also produced (as they did with the Model 4041 Mannheim), which was designated as the Model 4053-3F. This model was "divided as finely as the 20" rule." As such it provided the precision of a larger rule. K&E produced this rule between 1911 and 1943, but is rather rare. Perhaps this is because the upgrade to finely-divided scales came with a $3.50 premium!

In 1954, all 4053 versions to follow would employ semi-plastic construction - the back of the rule was now laminated with celluloid as well, making printing directly on the rule possible. The name of this model would drop the "N" prefix to reflect the major feature change. More plastic would be added over time, evolving to an all-plastic base with typical celluloid-covered mahogany rails and slide in 1962 and following. K&E also produced a variant of the rule for special use by the U.S. government known as the Model D4053-3 beginning in 1954. This rule, stamped "U.S. Government," came with manual, magnifying cursor, and custom leather case to also accommodate the higher-profile cursor. The "D" designed decimal trig scales, which was new to the Model 4053 rule (see Chapter 6 for more on this slide rule).

There would also be discontinued rules in the series, including the 8" N4053-2, discontinued in 1938, and the aforementioned N4053-3F version in 1943. With the 1962 model year, when K&E changed the model numbering convention for all of their slide rules, the 10" 4053 would become the 68-1617 (with the military version called the D68-1617) and the 20" N4053-5 was issued model number 68-1607. Both rules could be upgraded to a chamois-lined leather case, known as the 68-1622 and 68-1612 respectively, but it did not change the model number on the rule, only on the shipping box and in the catalog.

Cost for the original 4053 was $5.00 in 1909, or $0.50 more than the 4041 Mannheim. The K&E product line included both the 4053 and the 4041 for a surprising 35 years until the 4041 was finally dropped in 1943. I would have expected it sooner, since the 4053 largely replaces the capabilities of the 4041 for only a little more money. Then again in 1921, K&E seemed to realize this when they raised the price of the 4053-3 to $7.60, which was a full $1.10 more than the 4041.

Likewise, they raised the price of the 8" model to $8.80, which put the price of the shorter rule above the 10" model. Recall that they did something similar to the 4041 family of rules that same year. This solidifies the notion I've mentioned previously that shorter rules in the single-sided Mannheim format were not inexpensive to make, and for consumers there was no such thing as a discount just because there was less of the slide rule to buy!

The Model 4053, first introduced in 1909, would continue in production in some variety until the end of the slide rule era in 1975. It was easily K&E's longest, continuously running slide rule model.

Model 4098 and 4097C "Ever-There"

These models of the Ever-There series could be slotted in here because they were Polyphase Mannheim pocket rules, but rather, I'll leave their full descriptions for the next section's complete discussion of the series. But for the long duration of the Model 4053, K&E had never produced a 5" pocket version. Never, as remarkable as that seems. But once new modern plastic rules could be built, it made sense to make a pocket rule based on the 4053 Polyphase Mannheim scale set. The Model 4098 was introduced in 1931, followed by the 4097C in 1936. Importantly, these rules provided the functionality of a rule that was always too costly to sell, which would begin a shifting in the way K&E thought about their products philosophically. It just took them a while to learn that sometimes form should follow function.

Model 4153

K&E worked hard to find new plastic rules to replace the Ever-There series. They were successful Coming a few years after the end of the Model 4097C, in 1957, a new model arrived to take on the role of the pocket Polyphase Mannheim. The rule had two options: either the Model 4051-3 with normal pocket sheath, or a new Model 4153C model featuring a leather slip-case with a leather-covered metal "clip." In 1958, cost for the 4153-1 would be $5.50, with $6.25 for the "C" model with the clip.

Construction of these rules were very similar to many of the pocket rules K&E was producing at the time. In fact, the platform of this particular rules, with "Ivorite" (ABS plastic) and small clip-on "unbreakable" cursor, served duty in other slide rule families, including a "Merchants" pocket rule, Model 4150-1 (see Specialty Rules later in the article), and the Model 4161-1 pocket rule for the "Modern Polyphase" family of rules (see Modern Polyphase Rules in the next section). All three rules together are identical except for their scale sets.

In 1962, the rules would be redesigned as the 68-1648 and 68-1643, for the non-clipped and clipped case versions respectively. They would hang around until the end of the slide rule era.

The Model 4054 Polyphase

Earlier I mentioned that the original Model 4054 "Favorite" Mannheim model was discontinued in 1925. When it was reinstated in 1944, its role shifted to a more advanced model, a sister rule of the N4053-3. As such, this new Model 4054 variant would be marketed as a "Polyphase Slide Rule" and no longer associated with the "Favorite" series of slide rules. With improved glass cursor, the new 4054 rejected its basic Mannheim heritage to sport the same 4053 scale set, missing only the inch and centimeter scales on the sides. Equipped with mahogany instead of the original boxwood, this $5.50 model saved users $2.00 over the N4053-3 flagship Mannheim-type (single-sided) rule in 1944. Even so, that's almost double the price of the original 4054 model that was first introduced in 1901. That's inflation for you!

The Polyphase version of the Model 4054 would last until 1953. It would seem that company and consumer interest was shifting toward the all-plastic rules at this price point, particularly the pocket rules, as K&E would discontinue no fewer than SIX of their traditional Mannheim and Polyphase based models between 1950 and 1954. Despite this, the Polyphase version of the Model 4054 is abundant on eBay. K&E most certainly sold a ton of them.

The Ever-There Series

Named the "Ever-There" series of slide rules, K&E introduced this product line of pocket rules in 1931. K&E's first all-plastic rule, made of a celluloid known as "Xylonite," and with a frameless Xylonite cursor, the two models in this series were meant to give users of K&E single-sided rules a light-weight pocket option for the full-sized rule they already had, namely, for those with either "Polyphase" and "Merchant" slide rules (more on the Merchant Family of rules can be found below among Specialty Rules).

Other than the 4031 Mannheim pocket rule and their pocket duplex models, the intent of the Ever-There series was obvious, to provide a cost-conscious solution to all future pocket-sized rules, regardless of the scale set.

Models 4097 & 4098

Initially, the series began with only two models, the Model 4097 and the Model 4098. These cost the customer $3 and $4 respectively in 1931. When compared to the 5" wooden pocket Mannheim 4031 at $5, the Ever-There's make financial sense for both K&E and the consumer. Just as a way to gauge that, $3 in 1931 is approximately $60 in today's money. It's not a trivial amount, especially at the height of the Great Depression, but just as my daughter might be required to purchase a $100 calculator for her college classes, the Ever-There rules were priced similarly, even if my daughter would think those in my collection are just a slab of plastic. But I digress.

Almost identical in construction, only the scales were different between the 4097 and 4098 models. The Model 4097 mimicked the Model 4094 "Merchant's" rule, a 10" specialty single-sided rule introduced the year before. The Merchant's models will be discussed later (see Specialty Rules in Chapter 4), but the 4097, as did the Merchant's rules, did away with the A & B scales (and the S, L, and T scales on the back of the slide) in favor of folded C & D scales (CF & DF) and an inverted C scale (CI) on the slide. The idea of this was to allow rapid chaining of numbers more efficiently, with fewer moves of the slide and the ability to avoid off-scale computations. The 4097, as with the 4098, did make use of the rule's back side to give 5" and 13mm rulers.

The Model 4098 stepped in as a true Polyphase Mannheim pocket rule, filling the gap of a 5" pocket rule that the 4053 series failed to offer - at that point there were 21 years of 4053 production and the smallest rule offered was the 8" Model 4053-2. In fact, the 4098 not only used the same scale set as the 4053, it put the scales in the same place. Except the Ever-There, as mentioned earlier, put the inch and centimeter scales on the back.

Both rules uses a clear Xylonite cursor, which also appeared on the Model 4058 Beginner's rules. Starting clear, these cursors did not stand up well to UV light radiation, and so turned yellow over time.

Model 4097B, 4097C, and 4097D

The Ever-There series proved to be much thinner, lighter, and budget-friendly than all slide-rule technologies that preceded them. Not stopping there, five years after their introduction, K&E revamped the Ever-There line-up in 1936 to offer more choices to consumers and to increase their profit margins by selling these easy-to-manufacture slide rules.

Dropping the 4098 from the lineup, it would become redesigned as the 4098A (as mentioned earlier) to give the original Mannheim users a cheaper option - it worked, as the $1.75 rule would be competitive enough to make K&E discontinue the 5" 4031 four years later. They would keep the 4097 as the only slide rule model in the Ever-There family, but produced three variants of the 4097 to meet consumer needs in three separate markets. Interestingly, the models would be known as the 4097B, 4097C, and 4097D.

K&E would never offer a "4097A" rule, presumably because the "A" designation went with the 4098, honoring the 4098's legacy in the previous Ever-There lineup. This new 4098A version (discussed fully in our discussion of the Mannheim Family of rules), would be known simply as the "K&E Pocket Slide Rule," no longer under the catalog heading of "Ever-There Pocket Rules."

The new Ever-There rules were as follows:

The 4097B (with a 1936 cost of $3) essentially retained the functionality of the original 4097 slide rule, using the "Merchant's" scale set of their 4094 slide rule.
The 4097C (with a 1936 cost of $3.75) was redesigned to become what the old 4098 was, the true Polyphase Mannheim rule it had been for 5 years.
The 4097D (with a 1936 cost of $4.25) would become, so it seems, a "hybrid" of the other two 4097 rules, merging both the folded scales and the Polyphase scales onto a one-sided slide rule. K&E accomplished this by altering the Polyphase scale set, moving the B-scale to the slide (joining the S, L, and T scales), and replacing it with a CF-scale, and then squeezing in a DF-scale just below the A-scale on the upper stator rail.

To me, the intent of the "D" variant is obvious...to provide a cheap pocket version of the 4088 Polyphase Duplex rule (see Polyphase Duplex Rules below). Comparing the two, the only scale that the 4097D lacks is a CIF scale (folded, inverse C-scale) as found on the 4088 line of rules. And while K&E did manufacturer a pocket version of this Polyphase duplex rule with their 4088-1 model, it was exactly twice as expensive as the new 4097D Ever-There in 1936 prices ($8.50 versus $4.25 respectively). As a result, the 4088-1 would be discontinued within three years of the introduction of the 4097D.

Note: There is much more to be said about this scale set historically, which I do in the Sidebar: The 4088, 4097D, and the Mystery Rules at right.

As far as construction, the new Ever-There series did evolve from their preceding lineup, going to K&E's new-improved glass cursor, which they began using in all their slide rules beginning in 1936. They also tweaked the shape of the rules slightly, enough to appear dimensionally different from original 4097 and 4098. It is not known if K&E repackaged any of their old 4097 and 4098 stock into new 4097B and 4097C boxes, but it's definitely something I would have done. However, this will have to remain an inquiry in need of future research.

Finally, it is important to talk about the Ever-There series in terms of the competition that stood against K&E during this time. German-based Dennert and Pape introduced all-plastic slide rules in 1936 marketed under the name "Aristo." But at that time, U.S. and European markets were completely separate.

The chief competition for K&E state-side came from the Frederick Post Company, who began to compete for market-share with their sale of 4" and 5" Bamboo pocket rules, imported from the Sun-Hemmi company in Japan. Post would attack strongly in that market, producing no fewer than SEVEN pocket models by 1937. And, not surprisingly, Post would market these rules as both "Mannheim" and "Multiphase" rules, the latter a term Dietzgen gave to their rules.

As a slide rule user, I much prefer using the Post/Hemmi bamboo pocket rules, something I freely admit. Though this speaks more to the longer-lasting nature of a bamboo constructed rule as compared to an early plastic rule some 80 years later. (I just love my bamboo Hemmi-built rules!)

Sidebar: The 4088, an Ever-There,
and the Mystery Rules...

During a 20 year period, from 1930 to 1950, K&E produced a surprising amount of rules that didn't have a model number. I will speculate later on what I feel could be the origin of some of these rules, which are known by collectors as K&E Mystery Rules, but there are a few statements we could make about the Mystery Rules in general.

First, these rules aren't necessarily rare, so you would be mistaken to think they were not produced in generous numbers. We can see this today by how often these rules come up for sale, which in my opinion seems to be "not often" rather than "rarely."

Second, K&E did not show these Mystery rules in their catalogs, as they did not have a model number. We can judge from this that these rules were likely developed for a specialty or custom market, where K&E did not want the general public to think these rules were offerings they could purchase.

Third, and more relevant here, is that the Mystery Rule scale set is almost identical to the Model 4088 Polyphase Duplex, which predated the Mystery variants by two decades or more, and the 4097D Ever-There pocket rule which would be introduced a few years after the first known instance of a Mystery rule. This "Improved Polyphase Mannheim" scale set, as it came to be known, is as follows:

Front Side: inches // A DF [CF CI C] D K \\ centimeters
Back of Slide: [S B L T]

Many rules are strikingly similar to the Model 4053, and some are indeed the exact same rule without the model number. But many of what we regard as these "mystery rules" have an unusually longer slide than the actual frame, so when indexed to the left, the slide protrudes perhaps 8mm from the right side of the frame. This was a feature of many Dietzgen models of the era which enables (arguably) easier manipulation of the slide when the rule is in the "closed" position (i.e. left indices are aligned).

A single "K&E Co." maker-mark is placed on the right side of the slide, vertically in the fashion of the normal model number. Other mystery rules after 1947 might have only modern K&E logo. Yet others might contain nothing at all. Serial numbers on the rules match the production of all other wooden rules of the era, so they can be dated just like any other wooden K&E rule.

To compare, collector Mike Syphers has started a detailed list of known "mystery" variants at his amazing website, Following the Rules.

All of this all sounds odd, but we do know that K&E produced MANY custom slide rules for a variety of purposes and industries, a practice that Pickett would pick up on a half-century later with their own specialty types of rules. Many such K&E rules are so rare that their existence is likely lost to antiquity. Those we do know about will be discussed in Chapter 6, as there is a rich history of strange/custom K&E slide rules. But we should not be surprised that K&E would produce slide rules specifically for universities, among other places, perhaps even made to order.

K&E's desire to reach out to the education markets was obvious. Jack Burton, former VP of Sales at K&E, confirms this (J.O.S., Spring 1999) by stating that all regional managers targeted university bookstores as potential distributors. Yet he also refers to the ones sold at these stores as "junky," not in terms of quality, but rather in terms of less functionality, like with the Mannheims and Polyphases.

We also know that K&E began to print Educational Catalogs and price lists in the 30s, seemingly during the early production these Mystery rules. That speaks to K&E strongly focusing toward the educational markets, as I suggested. The real questions become, what type of rules would college students need and what type of rules could the company realistically offer them in terms of functionality and value?

If I am a university professor wanting a powerful slide rule for my students, I'm more inclined to inquire about the Model 4092 Log Log Duplex or 4093 Log Log Vector rules, which were K&E's flagship models for many years.  At least this is what I would have wanted as an engineering student. But that price point would have put them out of reach for most students. Knowing that the 4092 and 4093 have too many scales for a conversion to a one-sided platform, then I can see K&E pushing the less-expensive 4088 instead to the college market. As strange as it does seem that the Mystery rules do not appear in any of the educational catalogs, neither in 1933 nor 1936; however, the Model 4088 is indeed listed, while the 4092 and 4093 are not.

So I could see K&E producing a variant of the 4088 on the less expensive one-sided platform, especially with the knowledge that the 4088 would be discontinued in the late 30s. While this would not give engineering students the power of the flagship rules complete with log-log scales, it most certainly does give more power than the "junky" rules described by Burton. If this happened, then I can see why such a Mystery rule would be excluded from product catalogs...if K&E was providing at-cost rules to engineering students in select engineering programs around the country, then they would not want general education markets (and the general consumer market) to think such slide rules were available to them as well.

We shouldn't be surprised that perhaps the company didn't mind being charitable when it suited them, giving back to an education industry who would produce more engineers, builders, and slide rule buyers of the future.  In fact, we have every indication that K&E understood the importance of quality and brand loyalty in all of their products, so making new loyal customers at the college level, those who would be most likely to buy their flagship slide rules in the future, makes a lot of sense to me.

This theory sounds outlandish, but one of the first Mystery Rules produced - which had the standard Polyphase Mannheim scale set but without the 4053-3 model number - had a customized conversion chart on the back with a University of Washington College of Engineering insignia on it. This was around 1930. Perhaps surprisingly, the rule for Washington students would reappear around 1940, but this one with the "Improved Polyphase Mannheim" scale set.

I have a similar sample, without a university name to it, with a 1937 serial number. This rule has "decimal keeping" instructions on the back of the rule, unique to general purpose rules found on the typical single-sided K&E rules (which focused more on unit conversions, weights, and measures). Several of this type are known as "Decimal Keeping Mystery rules." Yet others mystery rules have formula charts containing more classroom specific geometric formulae and calculus derivative/integral rules.

Following through with the conjecture, it would make sense that the early 30s began with custom-built rules for a specific university book store and labeled as such; and yet, they also produced a more generic version for any university to sell. Such rules without the Washington connection do appear as early as 1931. The fact that there is also a Washington rule as late as 1940 should not be surprising, since K&E could always customize a rule for a university upon request.

Additionally, there would appear 20" versions of a similar rule, but missing some of the newer Polyphase scales. These likely do not have a connection to education, though could have very well been to custom clients, or a niche group of consumers.

As for the Ever-There 4097D, when K&E decided to merge their B and C variants of the 4097 into their "D" version of the rule in 1936, we had already seen this combination of scales in not only the 4088-3 Polyphase duplex rule that had been in regular production since before the 1st World War, but also in those Mystery rules produced for universities. Thus, with the 4097D, it is apparent that K&E was providing a budget-friendly, single-sided version of the 4088 for the regular consumer. At half the price of the duplex rule in 1936, this certainly had market demand.

You might think that this rule would have stopped the manufacture of the "Mystery rule" based on the same scale set, but serial numbers indicate that those were likely made up until ~1950. This would seem to indicate that K&E was indeed comfortable providing even lower cost wooden rules to select clientele, as a $4.25 Ever-There was still a hard ask for a college-going student.

And speaking as a collector, the Ever-There rules are somewhat hard to date. Their serial numbers seem to fall in line with the serial number scheme of the wooden rules, but McCoy indicates that the Ever-There's, as well as all-plastic rules later, had their own serial numbers sequences. Further research is required.

But at the time, the Ever-There's were innovative rules, gaining a solid foothold in the pocket rule market in 1931, and redesigned in 1936 to match the diversity of the 1936 Post offerings. The 4098A matched the lowest price of the Post offerings at $1.75. And the 4097s sold well enough that K&E was not compelled to match price with the imported Post/Hemmi pocket rules, none of which cost buyers more than $2.70 according to the Post 1937 Catalog. K&E simply knew that their $4.25 4097D was going to sell a lot slide rules, especially since it was half the price of their 4088-1 duplex rule that pretty much offered the same capabilities.

However, if there were ever a K&E slide rule that could be considered of questionable quality, especially in the longer-term, it would have to be the Ever-Theres (see the earlier SIDEBAR: The Problems with K&E Rules). The Xylonite plastic was not long-lasting. It became brittle over time, prone to yellowing (as the clear Xylonite cursors did) and warping. K&E knew the plastic was not ideal and would seek alternatives to replace it after 15 years. This is a historically important distinction, as it's the impetus for some of the corporate decisions made after World War II in the late 1940s.

Therefore, the Ever-There series, beginning in 1931 and revamped in 1936, would hang on until the early 1950s, when at that time newer plastic versions of these rules were finally introduced. Those models would be classified simply as "Pocket Slide Rules." But the Ever-There's had served their purpose of supplying all K&E family model lines an affordable, pocket version of their popular slide rules.

Sidebar: The Doric Family of Rules

Around 1948, when there hadn't been a K&E Catalog since war-time 1944, several all-plastic rules began to be offered by the company with a 9XXX model number designation. These were different than the Ever-There series, with a different type of plastic altogether. They were labeled "DORIC" on the rule, on their boxes, and in their documentation.

It's difficult to find good research on what the Doric rules are and why they were introduced as a series. The 1949 catalog classifies the "Doric Family" with only three rules, while we know that many others existed around the period of 1950. Moreover, this 9XXX series rules would be converted to 4XXX rules by 1952, only some of which would persist in carrying the Doric moniker on the rule itself, and none of which are identified as a Doric Family rule in 1952 catalog. It's confusing, to say the least.

But in total, there are 7 known rules planned by K&E to carry the Doric designation in production and 2 others that were described and barely exist in reality, likely as prototype rules. A summary list of these rules is listed here, with deeper descriptions of many of them found with the main body of the article.

9068 - 5" Polyphase Duplex (1948): Described in the 1949 catalog; became the 4168 in 1951 and the 68-1555 in 1962; labelled as Doric on all versions of the rule up until 1975.
N9081-3 - 10" Log Log Duplex (1948): Described in the 1949 catalog; became the Model 4181-3 in 1952, while dropping the Doric label
N9081-1 - 5" Log Log Duplex (1949): Described in the 1949 catalog; never known to exist as an actual rule; perhaps the forerunner of the 4181-1 "Jet-Log Jr."
9061-1 - 5" Polyphase Mannheim (1950): Became the 4161-1 in 1951, dropping the Doric label.
9050-1 - 5" Merchant's Rule (1950): Very few known samples. Became the Model 4150-1 in 1951, carrying the Doric label for maybe one year.
9071-3 - 10" Polyphase Duplex (1950): A plastic version of the 4070 with a Doric label, with its only catalog appearance in a part's list; disappeared entirely by 1952.
4168 - 5" Celanese Celcon (1968): A custom rule made of a special green resin, produced only that year, based on the 9068/4168 platform; used Doric name on the rule.
10000 - Prototype 5" 9068 rule (1946): Thought to exist prior to the 1948 build of the 9068; appeared only in a catalog part's list. A 10000/10002 manual exists in large number, but a small number of known samples of the actual rule exists.
10002 - Prototype 10" 9068 rule (1947?): See Model 10000 description. Very rare, with two samples known to exist.

Pictured top to bottom, the N9081-3, 4168 Celanese Celcon (originally the 9068), the 4168 Polyphase Duplex (also the 9068), and the 4150-1 Merchant's Pocket Rule (formerly the 9050-1). I expect to find the 9071-3 and 9061-1 at some point to complete the collection of known, actual Doric rules.

"Doric," in classical architecture/design meaning "simple of form" or "non-ornate," clearly indicated that K&E's new line of all-plastic Doric rules were intended to be nothing fancy. In fact, every major family of rules by the late 40s had a Doric representative slide rule and all of those had the same, basic black font, different from K&E's standard font. Interestingly, the term "Doric" would also be used with other products offered by K&E, most notably their drafting tools and lettering kits, with Doric indicating their most basic kits. However, there is no mention of Doric "anything" in their 1944 catalog, so it seems to be a name that K&E was determined to associate with a variety of their products by the next catalog 5 years later.

As such, It is difficult to know if the Doric rules were introduced as a cost savings option, or as a "budget rule," mostly because the quality of the rules were quite high. Many assume so, though I am not so sure.

The common element of the Doric rules is their construction of what has been called by many sources online as a "white Xylonite" plastic, which would be the same description as traditionally given to the Ever-There series by K&E in their catalogs since the beginning of their production and continuing through this time. But these rules are clearly not the same plastic as the brittle, yellowing, and somewhat sticky Ever-Theres. Possessing many of these rules, my hands would declare them a different formulation - the Dorics are of much better quality plastic; far more durable, resembling more the plastic construction in their later molded ABS plastic rules known as "Ivorite," albeit notably heavier.

According to Joe Soper (JOS, Vol. 11, No. 2, Fall 2002, p. 17), the Doric rules, made in Hoboken, were not molded, but rather made from plastic sheet. Certainly, these rules are built of a very different and superior plastic to the Xylonite, and in the only K&E catalog that describes the Doric rules, they are called "plastic" as opposed the "white Xylonite." Certainly this is not a mistake, as K&E wants to draw a distinction between these rules and the Ever-There rules.

And I believe this might give a hint to the purpose of the Doric series in the historical product line of K&E rules - to be a transitional, short-term model between the older "Xylonite" plastics traditionally used, and the newer ABS-based "Ivorite" plastics that would dominate the later years of the K&E product models.

As such, I do see some inconsistencies in the rules carrying Doric labels. Not all of them feel the same. While the 9081-3 feels like the newer, modern rules made of "Ivorite," particularly close in feel to its 4181-3 successor (I note that the 4181-3 is a little more light in weight), the 9068, on the other hand, feels very similar to all successor rules, so much so that I couldn't tell them apart if I couldn't see the model numbers.

I would speculate that when the Dorics were first produced around 1947, they were still a couple of years short of the "right formula" for the newer products to be injection-molded, so the Dorics were individually made from any number of suppliers and plastic recipes. While they were a wild improvement over the Ever-Theres, it would seem that 1950 was when K&E felt like they got it right and thus began changing the 9XXX models into 4XXX models, signaling a change in construction technique and plastic composition. As such, construction of these new molded rules would begin in Hoboken around 1950, shifting to a new plant in Salisbury, CT., beginning around 1957, according to Soper.

In total, the Dorics lasted from 3 to 5 years, where it would seem 1947 marked the earliest of the Doric rules, and 1952, the latest. And I do NOT believe it's a coincidence that the older Ever-There rules were discontinued completely sometime around 1953 and 1954. Out with the old brittle plastic, and in with the new.

So why not keep the "Doric" label? I think that's quite simple. If the Dorics were intended to become the pocket versions of so many of their other families and models of slide rules, then they should be modeled after those rules aesthetically. Meaning the "Doric" name would no longer be appropriate if future rules needs red scales again, or if they would intend the rules to look like others in those families of rules.

To add to the conjecture, the "prototype" models of the Doric 10000 & 10002 were less "plain Doric" than the others, including the use of red inverse scales and even ornamented with a red star on both the slide and the cursor. Perhaps the type of Xylonite being used didn't work very well with the red ink? We've seen many rules from a variety of makers have inks that leaked or bled into the rule. In such a case, "Doric" would be a good way to hide that these white Xylonite rules could only be manufactured with black ink.

As for the "Doric" label, only the 9068/4168 rule would keep that identity permanently, even up to the end of the slide rule era in 1975. Perhaps this is to pay homage to that legacy? It was, after all, the original Doric, with 4 historic slide rules based on the 9068 platform.

As such, many collectors think of the Doric as synonymous with the lone 9068/4168 style pocket duplex rule, the only one left standing in the 70s, despite the fact that 9 such rules were originally "Doric" in heritage.

However we think of the Doric rules today, it was these slide rules that either directly spawned or inspired all "Modern Polyphase" and "Modern Duplex" rules that would follow in the 1960s and 1970s. And as such, when we discuss the newer "modern" rules, we fall back on the Doric Family of rules as their legacy.

From the 1952 catalog. K&E would begin to rethink their "family" naming scheme for their slide rules. This is likely because if a consumer wanted a pocket rule, it would be easier to show all of their offerings on a single page. In 1972, the same catalog page would grow up to 6 distinct pocket rules, regardless of their construction.

The Modern Polyphase Family

The history of the Mannheim platform evolved and endured over almost the company's entire history, from their basic outsourced rule in the beginning, to high-quality rules built in-house, to the advanced functionality of the Polyphase rules, and through the modern era of the company. Regardless of the rule, by the 1950s, the Polyphase Mannheim had become the desired scale set for non-duplex rules.

With the introduction of better plastics, most all slide rule manufacturers dabbled with all-plastic construction at some point. For a company like Pickett, plastic rules were mated to their entry-level, entry-priced types of slide rules. While a company like Sun-Hemmi would eventually use all-plastic construction for even some of their most sophisticated rules.

K&E would be more like Hemmi in this regard. When the technology became available to make high quality rules out of plastic, the company had no problem applying it to their high end rules. We see this especially in the Modern Duplex Family of rules to be discussed later, whereas K&E's most revered slide rules of all-time are all-plastic in construction - the Jet-Log, Deci-Lon, and Analon rules if you were curious. While K&E did produce some budget rules from plastic, they were always well-constructed, with some heft and substance. Thus, plastic doesn't have to be the equivalent of "cheap."

But even the venerable, single-sided Mannheim/Polyphase types of rules could benefit from an all-plastic refit. So K&E evolved these into all-plastic and semi-plastic models over time. This is easily noted in the early 50s, when K&E discontinued six of their Mannheim and Polyphase Mannheim models within a 5 year span - those with "Favorite" and "Ever-There" legacies - and began focusing on a variety of new "Ivorite" plastic rules. In fact, through the 1950s, the only wooden rules of the non-duplex types were the N4058W "Beginner's" rule and the flagship Mannheim-type rule, the Model 4053-3, and even this latter rule became more increasingly plastic over the 50s.

We could very well be describing these rules as part of an "All-Plastic Family" of K&E slide rules; but I resist doing so for two reasons. First, there would remain a separation in functionality between rules of the single-sided (Mannheim) type of construction and those of the duplex style. As such, we need some differentiation when discussing rules of both types. And second, K&E would still maintain a price structure with their rules already established historically in their product lines. While K&E could normalize the production cost of all their slide rules by virtue of all-plastic construction through common manufacturing methods and tooling, it's not like we would expect the company to significantly lower the price of their flagship duplex rules, even if they COULD have been priced closer to the more basic single-sided rules.

Therefore, when writing about K&E slide rules and attempting to describe a family of rules with the "Modern" moniker, then we are mostly certainly talking about their common all-plastic construction. In fact, with the later (or modern) era K&E slide rules, we start to see some blurring of the lines between their single-sided and double-sided rules. We already mentioned this in our discussion of the "K12-Prep" (68-1892) rule's replacement of the Model 4058 beginner's models, but this was true of another model, the "GP12" (68-1565), as well. Even the famous 68-1400 Analon rule, while very much of duplex construction, was in all practicality and functionally, a one-sided rule.

Note: Such a single-sided rule in a duplex format is typically known as a "simplex" rule, which Pickett had made popular. While the GP-12 could be talked about here as a Modern "Single-Sided" rule, it most certainly does not resemble a Polyphase in any form. Because of its obvious relationship to both the Deci-Lon and Analon rules, I will discuss them collectively in the later section called Modern Duplex Rules.

Lastly, approaching the modern era, it became clear that K&E wanted to recategorize how their slide rules were presented. While the words Polyphase, Duplex, Mannheim, Log Log, Deci-Trig, et al, would always carry on in their titles for individual rules, they would no longer be listed in "family" designations within their catalogs. Instead, as we would see in the 1972 catalog, they would group them as "Pocket Slide rules" or "Limited Purpose" or "Special Purpose" or the "Deci-Trig Group." This shifting of labels is logical since most slide rules at this point would have common plastic construction, or a blurring of the traditional Mannheim (single-sided) versus Duplex (doubled-sided) point of view. So rather than listing them as model lines within these broad categories, K&E wanted users to think in terms of how they would be used practically. We can see this in how all K&E "Pocket Slide Rules" were listed in the 1952 catalog (see image above). Previous catalogs would have never grouped single-sided and duplex rules on the same page like this.

The Model 4161 Polyphase Mannheims

This model was first introduced in 1948 as the 9061-1, which was part of the Doric series of rules (see Sidebar: The Doric Family of Rules). However, by 1951, the 9061-1 was renamed the 4161-1 (again, see image) as a 5" pocket rule made of an ABS plastic known as "Ivorite," dropping the Doric label. This is the same plastic that would be used in all of their plastic rules toward the end of the slide rule era.

Evidently this became a popular slide rule for K&E, as a 10" 4161-3 version of the rule was added to the product line in 1954. It's like an all-plastic version of the classic 4053-3 model, except with the additional folded scales. It's also like the 4088 Polyphase Duplex and 4097C Ever-There rules, except adding L and ST scales. As such, it would be the most powerful implementation of the Polyphase Mannheim scale set to date.

So the scale set for both rules...

Front side: K, DF [ CF, L, CI, C] D, A
Back of slide: T, ST, S, C

The 10" model has additional utility over the pocket version by including inch and centimeter rulers to the top and bottom edges.

Of special note is that the trig scales on these rules are in decimal trig, though it would not carry a "Deci-trig" label. Three US Naval Academy professors (Kells, Kearn, and Bland) did write the manual for these rules, names that will be familiar to us throughout our reading here. Likely, like in many other rules discussed later, they had more to do with the rule than just writing the manual.

Both 4161 model rules rules would prove popular. Solid, clean, and durable, with an "unbreakable" metal frame cursor, the model name would become 68-1586 for the 5" and 68-1576 for the 10" in 1962, marketed as the "Modern Polyphase." Five years later in 1966, both rules would be reclassified as the "Jet-Math Modern Polyphase" rules, even though they had been mostly unchanged for 15 years.

In 1967, K&E would terminate the model 68-1586 (4161-1) in favor of the same rule - since 1956 they had been producing the SAME 5" rule in a leather case that came with metal clip. This rule, known as the Model 4161-1C (for clip), would be called the Model 68-1581 in the 1962 reorganization of model numbers. Therefore, at the discontinuation of the 4161-1 in 1956, buyers no longer had a choice to buy the rule without the case clip. Priced at $11 in 1962, the rule was very well priced, 50% to 80% less when compared to two other rules from different families built from the same form factor (which will be highlighted later).

As for the 10" Model 68-1571 (4161-3), it would continue to be sold until 1975.

Chapter 3: Rules of the Double-Sided, Duplex-Type

When K&E licensed the rights for the "duplex" design from William Cox in the early 1890s, they knew that it was full of potential, and thus it would be a technology that other slide rules makers would desire to have for themselves. By locking up the exclusive rights to the design, K&E set themselves up for early success, especially once they started selling to a hugely growing, revolutionary, industrial market in the U.S. And it was a name to be trusted - K&E surveying equipment and drawing supplies had already built half of Manhattan, including the Brooklyn Bridge, so rapidly expanding the product lines of slide rules couldn't have failed.

K&E would offer a variety of rules of many different types or "families" since the early 1890s. And unlike in Europe, which didn't really appreciate the duplex design of rules until closer to the modern era. Americans seemed to like and appreciate a good duplex slide rule. Pickett, a company which started post World War II, would never make a rule that WASN'T a duplex design. And, of course, this is largely due to the success of K&E's efforts in that product market.

Aside: German-maker Dennert & Pape actually manufactured the first K&E duplex rules during this early period, yet would not produce duplex rules of their own until 1948, the same year the company would be renamed to match their famous plastic rules, "Aristo." So it's not that they didn't know about duplex rules or were incapable of making them - they just didn't see reason to introduce them to the European market.

At anytime throughout K&E's history, the "flagship" slide rule for Keuffel and Esser would always be of a duplex design. As a general principle, duplex rules would always be more costly than their single-sided, Mannheim-type relatives, likely due in part to more surface area being covered by the celluloid laminations, as well as more scales required to be finely etched. In the modern era, most duplex rules would shift to all-plastic construction, but they would always produce their flagship, classic rules of mahogany. And these rules would always be more expensive than their plastic counterparts.

From p. 276 of the 1954 K&E Product Catalog, this detailed drawing of the Model 4070-3 Polyphase Duplex slide rule shows the detail that went into duplex slide rule construction. This rule, and many others, will be discussed within this chapter.

The duplex rule would evolve over the 70+ years of its manufacture, which I will describe more fully within the historical context of each model itself, but other than cursor materials and design, font usage, and edge laminations, the celluloid-covered mahogany rules would remain remarkably similar throughout these build histories, a testimony to the long-lasting quality of these rules.

Collector's Note: For the most part, the dimensions of the duplex rules remained the same as well. Where there are differences in a rule's length within a model line, the thickness of the stock did change as a function of its length. For example, the 5" version of these rules will be right at 1/5", the standard 10" rules are always 1/4" thick, and the 20" versions of the rules are approximately 1/3". The profile of the cursor block was made thicker to compensate so that the same frames and glass could be used. This is important for today's collector looking for replacement cursors for the longer slide rules, since cursors from the company are not interchangeable between rules of differing lengths. As a new collector, I once thought that maybe a 10" cursor from a donor 10" rule might fit on the much more rare long-scale version, but this is not the case. However, such a cursor can be customized by shimming the cursor blocks with other material, like an old credit card.

K&E rules of this type are easy compare to those from other manufacturers, particularly the bamboo-constructed rules made by Hemmi in Japan and the metallic rules from Pickett. As collectors, we love them all, but in general the K&E rules are absolutely ubiquitous in the United States, so without too many exceptions these slide rules are easily acquired on this side of the world. As such, beyond those older rules of the Original Duplex Family, which are mostly all scarce, much of what we will talk about in this chapter is not necessarily rare, or with extraordinary value, except for one notable exception in the Modern Duplex era.

It, and other K&E duplex rules will now be fully discussed, broken down by historical "family" type.

The innovative Duplex design allows access to both sides of the middle slide simultaneously. If the indicator/cursor covers both sides of the slide rule, then a single setting of the slide can allow access to double the number of scales versus the single-sided, Mannheim design.

The Original Duplex Family

Certainly, slide rules based on the duplex design had the potential to be powerful rules, especially evidenced by what they would become by the end of the slide rule era; however, without question, K&E achieved remarkably little with the "Original Duplex Family" of rules from a technology perspective. Those advances wouldn't come until decades later with other duplex family types.

In fact, K&E was unable to use the extra real estate inherent in the Duplex design to good effect. This is due, somewhat, to a technology-limitation early on, mainly because of a lack of capability of their dividing-engines to etch scales across the entire surface area. As such, it would be long while before the K&E duplex rules grew beyond the 8 scales of the original series.

K&E may have thought that they had time to develop this rule conservatively, given the sole right to produce the duplex design. But other slide rule makers were quick to dig into their own bag of tricks, inventing designs that would take away the chief duplex advantage, namely the ability of the rule to do three multiplications (or divisions) with one setting of the slide. See SIDEBAR: Three-Number Multiplication at right for more detail.

The Wm. Cox patent of Oct. 1891 called for his invention to have three defining characteristics:

1) The now familiar duplex stator and slide construction.
2) The use of a "wrap-around" runner.
3) The implementation of the specific scale set with inverted scales on the slide.

It is that third quality that makes three-number multiplication possible with the Cox rule, becoming the technology's main selling point.

As such, the early 1744 series of rules known as the "Wm. Cox rules," were very specifically based only upon those patent qualities, including a very limited scale set by more modern standards. And this seems to be the only goal for the first series in our discussion here, which is essentially to function as a "double-sided, original Mannheim" rule with an additional inverted scale capable of performing that simple and efficient multiplication technique.

There would be room to improve the rules over time, offering more functionality. But instead of adding to the Cox design overnight, improvements happened by way of evolution, not that they didn't seem to try. This period of time would be the most furious experimentation with a product you'll ever see!

To start, K&E did recognize that the Cox patent version of the rule did not have trig functionality, so in a strange implementation, they would offer version of all the rules in this family equipped with an option for a "trigonometric slide," with S, L, and T scales replacing the BI and CI scales on one side of the slide. And if the consumer was especially well-to-do, he or she could opt for a model version with BOTH slides. And then there was the feverish pursuit in cursor-design, with no fewer than 5 different cursors appearing on models over the course of both the 1744 and 4070/71 model series.

Note: A discussion of K&E cursors deserves more space than a simple sidebar, so please see Appendix 3: A K&E Cursor Study.

Sidebar: Three-Number Multiplication

The two important inventions of the Cox patent were the ability to use scales simultaneously on both sides of the rule during a computation AND the ability to do what we refer to here as Three-Number Multiplication using one position of the slide.

This first of those inventions is logically the most important from the standpoint of history, being that for the next 80 years of the slide rule era, the most powerful slide rule in the world, regardless of maker, would be of the duplex style.

The Model 1744 series of rules as shown in the 1895 Product Catalog.

Yet, it is the second of those inventions that was the main selling point in their 1895 Product Catalog (see above), which essentially copied the information and diagrams from the 1891 patent. So while the scale set would be limited, it was the inverted scales that made the design capable of the multiple operations with a single setting of the slide.

This happens with any duplex rule where D and CI are on one side of the rule with another C/D scale on the other. For example, to compute any multiplication of three numbers, the first product can be figured by aligning each number on the D and CI scales on one side of the rule, and then merely flipping the rule while setting the third number on its C scale and reading that result off of that D scale. For a mixture of division and multiplication, you just decide whether or not to use the inverted scale in your computations.

Yet, K&E had to understand that single-sided rule designs could accomplish the same thing if it had both inverted AND folded scales, or perhaps some other innovation in the way scales are laid out. As such, many other slide rule makers knew this and claimed the same "advantage" as the Cox duplex design.

Collector Mike Syphers provides a terrific look at the nature of this three-digit multiplication in a vignette called the "Scale Wars."

He highlights here many rules from other U.S. makers produced at the time in response to the Cox Duplex rule. They include a rule from Thacher & Scoffield (distributed by Dietzgen beginning in 1901) called the "Engineer's Slide Rule." Another by Dietzgen in 1904, knowm as their Model 1762 "Multiplex." And yet another by Kolesch & Co. in 1907 coined the "Triplex." These rules were rather ingenious, involving new innovations to provide equally efficient multiplications as the Cox rule, all without breaking the Cox patent.

Additional rules from Europe also began to feature scales that would provide three-number multiplication capability. These included the Tavernier-Gravet "règle des écoles" produced in 1904, the Nestler 34 slide rule introduced in 1911, and the Halden's Calculex Pocket Watch Rule of 1906, which used an inverted scale on its back side.

But the most damming evidence I mentioned earlier in the text, being the first Rietz rule made by D&P in 1902. It, predating the strikingly similar K&E Polyphase Mannheim, offered strong efficiency with chained multiplication.

So whatever advantage that K&E had hoped to gain in that regard with the Cox patented rules was gone within 5 or 10 years of the introduction of the Model 1744 duplex rule.

And subsequently, in addition to the new Polyphase Mannheim Model 4053 produced in 1909, K&E would push out newer duplex models: the 4088 Polyphase Duplex in 1913 and the updated 4092 Log Log Duplex in 1922, each yielding the potential for FIVE multiplications in only TWO settings of the rule.

Do yourself a favor and read more about these slide rule on Sypher's "Following the Rule" website!

Consequently, and rather strangely, this Original Duplex family would boast the largest number of individual models of any slide rule family in the company's history. And all of the model numbers with all of the slide options, as well as cursor options, make it very difficult to follow, particularly since rules that remain today will seldom have BOTH sets of slides for a particular sample, nor would they be labeled with the model numbers on the actual rule in a way to know which version of the rule you have in hand.

This family of rules would prove to be K&E's shortest lived, introduced with the Model 1744 series in 1895 and ending around 1916 (or perhaps 1917). So by the end of the approximately 20 year run, there would be only ONE scale dissimilar to the Polyphase Mannheim Model 4053, which favored a K-scale instead of the Duplex rule's BI scale - and that scale is honestly unnecessary since the rule already offered the CI scale to accomplish triple-number multiplication.

As such, it's a telling point that K&E really didn't seem to know what to do with these Original Duplex rules. Truly, some of the choices for scales layout and models didn't make a lot of sense in hindsight.

So, in essence, the Original Duplex family, even after a couple of decades, did not provide for a wider range of functionality over the slide rules already in their stable.

Yet, this doesn't keep ALL rules in this family from being some of the most collectible and valuable K&E slide rules in company history (please see Collector's Outlook for this series at right).

The two main series in the Duplex family are described below...

The Cox patented 1744 model rule, as offered in the 1895 K&E Product Catalog. Note the railroad scales and typical Dennert and Pape construction.

Models 1744, 1744A, 1744B, and 1744 1/2

Also known as the "Wm. Cox rule," the 1744 was the first "duplex" rule ever produced by any manufacturer. Early samples of the rule clearly show a Dennert and Pape construction, much like the 1746 Mannheim rules offered by K&E at that time.

There has been some debate these early rules being constructed by K&E, but collector Bob Otnes has stated that he was informed of the likely happenings by Hans Dennert of the D&P/Aristo company lineage. Dennert confirmed that the rules celluloid-laminated, wooden components with completed scales were sent unassembled to K&E so as to avoid the US duty tax. K&E then finished the rules by stamping the labels and numbers to the scales, affixing the body of the rule together with end brackets, and then supplying the chisel metal cursor. He further stated that the rules have no D&P markings on the rule. According to the Oughtred Society, which relates what I just told you here, there are only two known samples of the rule in existence. One shown below and the rule shown in the link.

The construction is similar to the imported Mannheim rules made of celluloid-covered boxwood and with cursors and end brackets made either of brass or metal-framed glass. It was added to the 17XX line-up of rules (which included the Mannheims) in 1895, costing buyers $6.50.

The scales were not labeled on the rule, which was typical of almost all known rules back in the day, even if the patent showed scale labels on the left side of the rule. It was a basic scale implementation strict to the Cox patent:

Front Side: A [B, C] D
Rear Side:  A [BI CI] D

The wrap-around cursor, obviously readable on both sides of the rule, was of the "chisel" type, also made of brass.

Marketed as an "Arithmetic" rule with "Arithmetic Slide," this 10" model, as I said, would allow for three multiplication or division operations in one position of the slide. Simultaneously, K&E produced a 1744A trigonometry option for the same price, replacing the BI and CI scales on the back of the slide with S and T scales (sine and tangent), as well as a "scale of equal measures," or L scale (logarithms) squeezed in between. And for a $1.50 more, K&E sold their 1744B variant which shipped with both slides from the other two models, made to be interchangeable. William Cox describes this in the December 1893 issue of The Compass (Vol. 3, No. 5, p. 74).

As such, with the variant options, it is very clear that K&E saw the need to offer more capability with their first duplex rules, but they would not be capable of doing it with one single slide rule.

Interestingly, K&E offered a 1744 1/2 version of this rule, the 1/2 denoting "half-size." This would be their first "pocket" duplex rule. You might think you could save money getting the half-sized rule, but it was priced equally to the 10" 1744 at $6.50. That combination of price and size likely didn't sell very well, so I'm not planning on finding this rule "in the wild" anytime soon.

I have already voiced my criticism of the rule, which I also discuss in the related SIDEBAR, yet the most obvious problem to mention now is that providing the option for both types of slides in any of these rules greatly limits how the user can operate the rule and the types of computations that can be done. Operations must be constrained to either one type or the other, so mixed operations between algebraic and transcendental functions - trigonometric, exponential, and logarithmic - cannot be accomplished seamlessly without first swapping out the slide. And truly, it is uncertain today how many of these rules in either the Cox models or the following 4070/71 models where actually sold, as so few sample of any of these rules exist in the first place.

But for the collector, these Model 1744 "Cox" rules are rare, and thus very desirable. As with most of K&E's 19th century slide rules, none came with the model number on the rule, however samples of this rule include the Cox patent written on the rule by the "Cox" name, as well as having the traditional D&P railroad track style scales.

Rules in the 17XX series, including both their Duplex and Mannheim rules, were the only slide rules offered by K&E from 1887 to 1900. But a year later, everything would change.

The Model 4070 & 4071 series

When K&E ramped up production of their slide rules in 1901, their Duplex Family expanded immensely, becoming their most emphasized and produced slide rules. The 10" Model 4071 was the main rule in this series of 10 slide rules, all of which were variants of original 1744 series, and all available through the 1901 K&E catalog. These rules, now made of celluloid-covered mahogany with engine-divided scales, typically came in a choice of brass indicator OR clamshell glass cursor, a choice of arithmetic-only or both-slide (arithmetic and trig) models, and in 5", 10", and 20" varieties. And, importantly, these would be the first production duplex rules built in-house.

Among the 10" rules, the Model 4070 (all-brass indicator) and Model 4071 (clamshell glass cursor) directly descended from the arithmetic 1744 rule. As such, these had A [B C] D scales on the front side and A [BI CI] D scales on the back, meaning there would be no reason to ever invert the slide. At an introductory price of $8.00 for the glass-equipped 4071 and $6.50 for the brass-equipped 4070, these represented K&Es flagship line, and thus their highest priced slide rules.

The previous Model 1744B, shipping with dual arithmetic and trigonometric slides, became the new Model 4075 and Model 4076 rules in this lineup. Again, both 10" rules, the former with brass and the latter with glass, and both shipping with interchangeable arithmetic AND trig slides. The option with both slides costs an extra $1.50 over the single-slide 4070/71 models. Note that the 1744A trig-slide only variety was no longer represented, so if you wanted trig scales you would have been given little choice but to spend the $9.50 for the Model 4076 with the extra trig slide.

K&E also offered a full selection of pocket Duplex rules in this series, based on the original 1744 1/2. These 5" rules were the arithmetic-only Model 4060 (brass) and Model 4061 (glass) versions, as well as the Model 4065 (brass) and Model 4066 (glass) which shipped with both arithmetic and trig slides. Again, K&E offered no discount with these rules over their full-sized brethren.

Finally, the first 20" duplex rules were included in this series. These were the Model 4078 and Model 4079, again with brass and glass cursors respectively. These only offered the arithmetic slide option, but they still set back the buyer $16.50 for the 4078 and $18.00 for the 4079. The latter would cost around $630 in today's money, or about the price of your iPhone. At least they didn't have to recharge the slide rule.

This series of rules would morph slightly over the next 15 years, beginning with 20" options that came with interchangeable, double slides to add trig functionality. These models, introduced in the very next catalog in 1903, were the Model 4080 (brass) and Model 4081 (glass) rules priced at $20.00 and $21.50 respectively. These rules would be renamed when the line-up was completely revamped in 1906, and those model numbers would be reused by K&E nearly 30 years later for two of their most important slide rules in company history (see Log Log Duplex Rules in a later section).

Speaking of 1906, while recovering from a fire to one of their Hoboken, NJ, warehouses the year earlier, K&E was undeterred from making a plethora of changes to the 4071 series of slide rules, streamlining some of the models while also adding many others, including a variety of 8" and 16" rules. Additionally, K&E shifted away from shipping both slides with their rules, going back to single-slide, arithmetic and trigonometric options. Where they did this, the models were denoted with an "N" suffix. These retained the $1.50 higher price tag over the arithmetic-only rules despite no longer shipping with two slides. This is because K&E found a way to add the trig scales to the arithmetic slide, with A [B S C] D on the front side and A [BI T CI] D on the back, with the L-scale on the edge of the rule. The cursor was modified to provide an indicator for the rule's edge. This was true for both cursor options, which remained the same for the models, except "metal" was substituted for brass in the non-glass variants. And like the models before it, all slide rules would come with a sewed-leather case and directions.

The left building was K&E's new office building in Hoboken, NJ as built in 1906, next door to their largest factory on the right.

And this is what it looks like today, as shown on Google Earth.

Because of the number of models in this series, 20 in total in 1906, it is easier just to list them by length...

The 4060 ($6.00) and 4061 ($7.50) 5" inch rules did not change, but the 4065 and 4066 rules became the 4065N and 4066N, with arithmetic and trig slides respectively. Those rules are priced, once again, at $1.50 premium over the arithmetic-only, 4060 and 4061 slide rules.
A new series of 8" rules was added, given model names of 4067, 4068, 4069 and 4069 1/2. The 4067 ($6.00) and 4068 ($7.50) arithmetic rules came with metal and glass respectively. The trig-only rules, the 4069 (metal) and 4069 1/2 (glass) carried with them a $1.50 higher price tag. Why the "1/2" suffix? In this case, they ran out of numbers, as the 4070 starts off the 10" rules.
The 10" rules, which bears the family name, were the 4070 and 4071 models. These did not change from the previous model years, other than the conversion of the brass cursor to "metal." The 4075N and 4076N (note the added "N" suffix) were converted to the new all-purpose single-slide "with trig scales," as explained in the 1906 catalog. Like the 5" and 8" rules, these 10" slide rules retailed at the same prices.
Also new to the family were 16" rules, the 4082 (metal/arithmetic), 4083 (glass/arithmetic), 4084 (metal/trig), and 4085 (glass/trig). Prices were $14, $15, $17 and $18 respectively. (Note that the 4082 and 4083 model names would be repurposed in later years).
For 20" slide rules, the 4080 and 4081 "both-slide" models were changed to the 4088 and 4089, now becoming trig rules. Curious is the lack of the "N" suffix here, where apparently the change of number was good enough? The 4078 and 4079 arithmetic rules of the previous line-up would also get new model numbers, now called the 4086 and 4087. These rules actually got a price-cut, with the 4086 and 4087 priced at $16 and $17, while 4088 and 4089 were $19 and $20.

1909 would feature one last revamping of the 4071 series, whereas the entire lineup would dispense with the metal cursor option. As such, all model numbers originally with brass or metal cursors would disappear, reducing the total number of rules offered in the 1909 version by a half. K&E also made a change to the end brackets for the 1909 model year, as shown in that catalog's illustration of the rule for that year. The bracket for the older rules was in the shape of a "C," but in 1909, the brackets became and "inverted-C" shape. Two years, later, they would switch to the "L" shaped end brackets which are the standard for the all future K&E duplex rules.

Of these ten rules introduced in 1909, five of them would change model numbers and add a "T" suffix. This designation now indicated the rule was of the "trigonometric" variety. As such, there would only be two rules for each length, with a single number model, both with and without a "T."

The 5" rule would now exist as the 4061 and 4061T models, priced at $5.00 and $6.50 respectively.
The 8" rule became the 4065 and 4065T models (a number borrowed previously from the 5" trig rule). These models were priced the same at the 5" rules.
The Model 4071 and 4071T took permanent grasp of the both the series name and the 10" selection of rules, again priced identically the 5" and 8" versions.
The 16" variety, now known as the 4083 and 4083T rules, were priced at $12 and $15 respectively.
The 20" rules were condensed down to the 4087 and 4087T models, at $15 and $18 each. These prices represent a $2 discount to the previous price and 16% off the highest historical price of this family of 20" slide rules. I believe it's quite obvious that K&E was aware of how exorbitant some of these prices were and worked conscientiously to reduce cost to the consumer, which in the case of the longer rules, was chiefly commercial customers.

This series of rules would disappear entirely a decade later. Listed in the 1916 catalog, there would be a 5 year gap before K&E's next catalog in 1921. While there is no catalog to show exactly when the line-up disappeared, it is clear from collected samples today that there were no more production dates after 1917.

Dating most of these rules is difficult. These ended before K&E's use of serial numbers in their rules in 1922; and while there are production numbers imprinted on the ends of the rules, it is not known how those can be used to date the rule today. Instead, collectors are left to use evolutionary changes to the rules like cursor, end-bracket and scale changes; as well as catalog descriptions to approximate the date of these early duplex rules.

I mentioned earlier why I felt this model line-up could have been short lived, and certainly, it seemed K&E was never content with it, constantly trying to improve it, but never really addressing the real truth of the matter, namely that the Duplex Family of rules didn't really give additional power for the user to do calculations that couldn't be done with already existing K&E slide rules. This is something that could have been fixed - quite simply, there's no reason to have A and D scales on BOTH sides of a slide rule that has a capability of being read on EITHER side of the rule with the same setting of the cursor. I find it curious why it took K&E so many years to understand that? It is, after all, a major selling point for duplex rules.

I suspect that K&E just didn't see the urgency. If they are selling slide rules by the thousands, then what would they care if a slide rule wasn't as good as it could be? Or perhaps by 1909, they'd be trapped by their own product placement, not willing to compete with their flagship slide rule, the Model 4092 Log Log rule, introduced the same year (see the Log Log Duplex Family in a later section).

Even so, from the perspective of history, for all the potential that the duplex technology possessed, the 4071 series and the Duplex Family of rules on the whole were rather unremarkable.

This would change with the introduction of the Model 4088, the first rule in the improved Polyphase Duplex family, which we will talk about next.

The Polyphase Duplex Family

In 1913, four years after the introduction of the Model 4053 Polyphase Mannheim slide rule but also four years prior to the death of the 4071 series, K&E introduced what they called the Polyphase Duplex slide rule design. Before we discuss each of these models though, let's continue our exploration of what K&E offered to that point and why these rules represented the more complete fulfilment of the promises of the duplex design.

The 1913 catalog, which is copyright in 1912 incidentally, has a few addendum pages pasted within to reflect newer products in 1913 at the time the catalog went to press. The new Polyphase Duplex Model 4088 rule was described on one of those addendum pages. The first sentence reads, "The Polyphase Duplex Slide Rule is a combination of the Polyphase and the Duplex Rules, with the addition of several special scales."

I think this statement is disingenuous, at best.

If K&E were honest, they would admit their Duplex family of rules was not meeting the potential of the duplex design. Remember, that line-up was revamped in 1909, only then giving it a near-equivalence in scales to the 4053 Polyphase Mannheim. There was simply no room with existing Duplex rules to innovate with other scales. Likewise, we could see that other slide rule designs, such as their own Model 4053, as well as other single-sided rules from competitors, had made the functionality of the Original Duplex Family of rules less unique.

As such, the Polyphase Duplex family of rules was K&E's opportunity to make the most of what a duplex rule could do, to the point where K&E would kill production of the original Duplex family within 4 years of the introduction of the Polyphase Duplex.

That said, so what were the "special scales" K&E mentioned? These would be the folded scales of the C, CI, and D scales. This is the first appearance of the CF, CIF and DF in any K&E slide rule, which were valued because now users didn't have to worry about off-scale computations - a solution will always be on the rule with folded scales regardless of which index is used. And as mentioned earlier, that combination of scales permitted chaining of multiplication and division operations with great efficient, now capable of doing 5 such operations with only two settings of the slide.

And as a tertiary advantage, because the scales were folded over pi (3.14159...), then values on the D-folded scale were also the product of pi and the D scale setting. This made quick work of problems that dealt with circles.

Of historical note, this is rather late for the introduction of folded scales on a slide rule. In his comprehensive 1909 work,  A History of the Logarithmic Slide Rule and Allied Instruments by Florian Cajori, he states that as of the time of his writing that folded scales on slide rules had not taken "foothold" (p. 66) in the States and England, that France had seen much increased use of it, especially in their technical schools - using a Tavernier-Gravet slide rule with folded scales known as the " règle des écoles" - and Germany was just beginning to offer this rule for sale. The 1913 introduction of the folded scales on the K&E Model 4088 rule was certainly better late than never!

Specifics of the models in the Polyphase Duplex Family, starting with the Model 4088, are as follows...

The Model 4088 Series

The model series began with a single Model 4088 slide rule in 1913. This model number is not to be confused with the brass-cursor 4088 model (4071 series Duplex Family) that was discontinued in 1909. The new 4088 model was introduced in two lengths, an 8" version predictably designated the 4088-2, and the full-scale 10" version known, of course, as the 4088-3.

The 4088-3 (10") and 4088-2 (8"), as introduced in 1913 with the same "column cursor" pushed out to all K&E slide rules, contained 11 scales. This is the same number as the 4071T duplex rule that was currently being produced and which it would eventually replace. So what makes it better?

In this new rule, K&E does away with the redundant scales. Gone were the A and D scales on both sides of the stator rails. Instead, the front side of the rule would be configured as DF [CF CIF C] D and the rear of the rule would be setup as K A [S T CI] D L.  It's an improvement over the Polyphase Mannheim and Duplex families because of the historical addition of the folded scales discussed earlier, but also there is a K-scale which was missing from every Duplex rule K&E had produced.   Certainly, this is the best arrangement of scales on any K&E slide rule to this point, even better for general math computations than the Model 4092 Log Log Duplex rule introduced in 1909 (to be discussed in the next section). It would become even better when K&E added a B-scale on the back of the slide in 1922.

Perfectly timed with the ending of the Duplex Family line-up, K&E added the 20" Model 4088-5 in 1917 and then the 5" Model 4088-1 five years later. Very few other changes were made to this rule over time, except for the cursor modifications they made simultaneously with all other slide rules. Those changes included the switch to the frameless glass cursor with metal rails in 1915, followed by a switch to plastic rails in 1916.

(Note of general amusement: This version of the cursor was patented in August, 1915, as number 1,150,771 by "Willie" L. E. Keuffel.)

In 1936, K&E revised the 20" rule and added an "N" designation. They supplied this N4088-5 with the "new-improved" cursor like all other K&E rules - this new cursor with a metal rim around the glass kept users from breaking off the corners of the glass by overtightening - which was indeed a god-send. It is unknown why the new prefix was used for this rule since no other such "improved" cursor rules added the "N" to the model name.

EDIT: After some deeper digging, it appears that there are samples of the N4088-5 with serial numbers, cursor, and scale fonts that date back as early as 1926. This is beyond curious, as it isn't until 1936 when the N- prefix is first shown in a product catalog. Later we will see the Model 4092-3 adding an N-designation in the catalog, yet not on the rule. I see this as indication that K&E didn't not always have correct designations in their catalogs. More research is needed.

K&E would cease production of all versions of this rule in 1939, replaced by similar, but more powerful Polyphase Duplex rules that we'll talk about next.

The Model 4070 Trig and 4071 Deci-Trig

In 1939, K&E discontinued all models of the 4088 and replaced it with these two slide rules, both with 10" scale length. Not to be confused with the original 4070 and 4071 Duplex family of rules that were discontinued more than 20 years prior, these new rules increased trig capabilities to give more accuracy for angle measures that are less than 5.73 degrees. Because sine and tangent are mostly the same for such angles (actually approximating the radian equivalent for that angle), then a single scale of greater resolution can be used to evaluate both of those functions. This would be known as the "ST" scale on this rule, which ranges from 0 to 5.73 degrees (on the Model 4071 Deci-Trig) rather than the typical 0 to 90 degrees on a normal S or T scale.

The secondary benefit of this is extra precision on the S and T scales, meaning that those scales could now begin at 5.73 degrees instead of the normal 0 degrees. As such, by essentially continuing the S and T scales where the ST leaves off, you gain at least a half a rule's worth of precision for angles bigger that 5.7 degrees, since half of the typical S and T scales normally take up half the rule. So, in essence, the ST and S/T scales work like one continuous scale when used together.

Now this was somewhat of a new scale for K&E. It would mark the second occurrence of an ST scale on a K&E rule, following the 4080 Trig/4081 Deci-Trig Log Log models introduced two year earlier, as we will see in the next section.

This ST scale (also known as the SRT scale in later decimal trig rules) would become a fixture of all but the most basic of K&E slide rules.

The overall scale set is something we could call the "Improved Polyphase." It was described earlier with the Ever-There Model 4097D. It is as follows...

Front Side:  DF [CF, CIF, CI, C] D, L
Back Side:  K, A [B, T, ST, S] D, DI

So, why two new models? Taking a cue from the Log Log Duplex Family of rules (see next section), K&E gave buyers a choice with how they wanted their trig angles expressed. Those who preferred a traditional degrees, minutes, and seconds (DMS) format might have chosen the Model 4070 Polyphase Duplex "Trig" version of the rule. But the trend toward a decimal degrees rule was strong by this time, whereas the Model 4071 Polyphase Duplex "Deci-Trig" could be preferred. This distinction with angle input preference had proven successful when first introduced in the Log Log Duplex models in 1933, so it made sense to do the same with the Polyphase Duplex rules in 1939.

The 10" new models immediately superseded the 27 year old 4088 model in all lengths, except for the 8" 4088-2 that would hang on for one more year, I suspect because they might still have had stock available. But I find it intriguing that until the Doric rules were introduced in the late 40s, a good decade passed by where the 10" 4070 and 4071 were the only Polyphase Duplex models available. In fact, this was a decade where there were ZERO K&E duplex rules manufactured in sizes less than 10". And as I said earlier, I suspect that the Ever-There series had something to do with that. A pocket version of a 4070 or 4070 duplex model would have cost double that of the Model 4097D Ever-There that sported the same, yet Improved Polyphase scale set.

Note: These rules and the Doric rule that follows have manuals written once again by US Naval Academy professors, Kells, Kearn, and Bland. By 1945, their patent number is listed on the slide rule itself. Coming two years after the 4080 & 4081 models that they designed, it is clear that they had some part in the design of this rule as well.

Sidebar: The European Influence on K&E

An aspect that I believe becomes obvious upon digging deeper into K&Es product line and the historical timing by which they rolled out certain innovations is also something I haven't seen talked about very much on the Internet, and that's the level of influence that European companies and their markets undoubtedly had on K&E. As German immigrants, founders William J. D. Keuffel and Herman Esser leaned strongly on happenings in Europe during the mid-to-late 19th century and brought some of those practices to their own company, which they founded in 1867, to the United States.

While K&E was the leading seller of slide rules in the United States for the entire duration of the company's existence - challenged early by Dietzgen and Post/Hemmi and later by Pickett for market share - K&E was known as the leading producer in the States, especially in their main business of surveying equipment. But they also would hold a variety of important patents regarding slide rule technology, especially once William Keuffel hired his second cousin, W.L.E. Keuffel, in 1884. Also named William (or sometimes "Willie), he would eventually become the Vice President of Manufacturing. He should likely receive more credit for K&E slide rules than any other single individual, as he was the guiding force for pushing the company in the direction of in-house production of all rules, including the writing of seven patents from 1898 to 1916, as well as the development of tooling to make slide rule production happen. His own son, A.W. Keuffel, would eventually write 11 slide rule patents himself.

As a company, K&E was always evolving; pushing diversity; able to make money on a variety of products. But for them, slide rules were a highly visible aspect of their business. They did not need slide rules to be tremendously profitable in order to reap the benefit of the exposure they gave the company. They would not compromise on quality for this reason. But this did not mean profit wasn't important. In some cases, they ruthlessly protected their bottom line, especially in their dealings where licensing agreements are concerned, an example of which can be read about here.

But in the early days of slide rules, we know they leaned heavily on what they could learn from European slide rule makers. Companies like Dennert & Pape (Aristo), Nestler, Marc/UNIS, and Tavernier-Gravet predated K&E by several years, particularly in slide rule production. After all, Great Britain had been developing and using slide rules since the 1600s and, by consequence, became popular in use throughout Europe much earlier than in the U.S., beating the US in popular use by a good 20 years. The original Mannheim was in use by French military in 1859, whereas it wasn't until 1881 that Americans began to be exposed to the Thacher Cylindrical rule. K&E was always aware of these histories, as well as the development by D&P with celluloid-lamination of wood in 1886, their making of the Rietz design in 1902, and the nature European markets, especially in London, Paris, and Berlin.

K&E wasn't alone in this. Dietzgen, AW Faber, Frederick Post, and Hemmi each setup shop prior to the turn to the 20th century, all of which were finding their way to the slide rule market as contemporaries to Keuffel & Esser. They too followed the lead of the earlier European companies. As such, the market for slide rules accelerated rapidly and there were many makers for Keuffel & Esser to monitor. In fact, it is said in these early years that the founders made yearly trips to Europe to learn and grow their business as much as possible, something that becomes obvious with the company contracts and products they would soon offer.

Likewise, the reason for European partnerships is simple. It was the best way to fast-track products to their rapidly expanding product markets. K&E's factory, which produced surveying equipment as K&E's main business, was not tooled for the additional production of slide rules. They needed established partners to fit the bill. Although consumer interest for slide rules in Europe remained in the single-sided or "simplex" designs, based on the traditional Mannheim, Rietz, and Darmstadt scale sets, in the United States there was no extensive history to dictate consumer preference. So being quick to market, rather than taking years to build up their own production capabilities, was the first mandate for Keuffel and Esser. They understood the importance of giving consumers many slide rule options, and they sold in large numbers, particularly their duplex rules.

And, importantly, this wasn't only a focus of their slide rule business. From their inception, K&E would rapidly produce an extensive catalog of a wide variety of goods. While their main lines of products were made in-house, the vast majority were out-sourced to other suppliers. And by the turn of the 20th century, K&E had thousands of products available in their catalogs, with store-front/distribution centers in four major cities across the U.S., including New York City, Chicago, Saint Louis, and San Francisco.

Of course, this business model was not unique. Perhaps no other company was more influential to K&E's business model on the whole as the W.F. Stanley and Co. in London. The Stanley Company was enormous in England by the turn of the 20th century, having long broken ground in the arena of drafting and survey equipment themselves.  Not only would K&E sell many Stanley-made products over time, K&E likely purchased most of their own tooling from Stanley as well, particularly their dividing engines that Stanley invented in 1861. Stanley laid the blue-print for such a business some 15 years prior to Keuffel & Esser, having more than 3000 products in their own catalog by 1881. London had to be a frequent, annual stop for Messrs. Keuffel and Esser.

This idea of borrowing ideas, designs, tooling, and supply from other companies shouldn't be a surprise, nor does it make K&E any less innovative. It's the way of any good business. We see this with the Frederick Post Company, who would never manufacture a slide rule of their own, instead farming out their manufacturing to Sun Hemmi in Japan. Likewise, Chicago-based Dietzgen was in no way particular about from whom they would acquire licensing rights. They too formed partnerships with Dennert & Pape and Nesler to accomplish their own goals. .

Instead of spending money on the research and development (R&D) of new products, it's much cheaper to let somebody else do that, put your own name on it, and rapidly build a customer-base. With any young company getting one's feet wet in a new market, it's just smart business to produce reliable, known products with a history of consumer demand and commercial success. Where they came from didn't really matter.

Of course, concerning slide rule production, we know that the earliest K&E models were imported from European companies like Tavernier-Gravet and Dennert & Pape. And for them, tapping into a growing market across the Atlantic made financial sense. So, it is reasonable to believe that these companies desired to flood the US and European markets simultaneously with slide rules.

Keuffel & Esser were quick to understand the advantages of celluloid on slide rules. The technology was obviously superior to anything that preceded it, making slide rules more accurate and much easier to read once engine-divided and painted in relief. Quite simply, Keuffel & Esser did not hesitate to bring this to the States. Interesting that by 1901 with the introduction of the new product lines, K&E simply referred to the celluloid laminations as "white facings" - they had become that ubiquitous.

Evolving from boxwood models provided by the Parisian company, Tavernier-Gravet, and moving to mahogany rules from German maker Dennert & Pape, this represented a rapid development from an invention patented in 1886 and not fully implemented by D&P until 1888, where in the same year there is evidence (Gieseler, p. 149) of K&E offering these rules to the American public. In an age without phones or internet, with an enormous barrier of water in between, I find the speed of this to be rather astonishing. We see this with the following models and their associated product catalogs:

10" Model 1746 Mannheim and 20" Model 1748 Mannheim in 1890
Model 1744 Wm. Cox Duplex in 1895
20" Model 1749 Stadia rule in 1895
10" Model 1745 Gunter in 1895 (adding celluloid facings to the original rule)

These rules, of course, are the precursors to the K&E-made, complete line of 4041 series of mahogany rules, the Model 4070/71 Duplex family, and a variety of specialty rules (stadia, sewer, et al), all of which would come out in 1901.

The Model 9071-3 Polyphase Duplex Doric

After 1947, the 4071 Polyphase Duplex would get a plastic, Doric equivalent rule known as the 9071-3 Doric Polyphase Duplex. Strangely, this rule was never described in a K&E catalog; only in a 1949 parts list. The rule was likely in production for a year or two around that time. I would suggest as early as 1947, which matches the copyright on a similar Doric plastic rule, the Model N9081-3, which was the all-plastic brother of the Model 4081-3 Log Log Duplex rule. It would appear that these rules were intended to test the market for the concurrent sale of both plastic and mahogany slide rules that provide the same functionality, something I will discuss more with the N9081-3 in the next section.

Made entirely of plastic, but no plastic like the Ever-There rules, the Model 9071-3 demonstrated the transitional and experimental nature of K&E's use of what they were still calling Xylonite plastic, something discussed fully in the "sidebars." Being fully "Doric," it lacks any ornamentation as one might expect, with black-only numbers and fonts.

The 9071-3 Doric is quite rare from a collector's perspective as it doesn't pop up often at auction. It is certainly attainable, however, and reasonably so, as it won't be confused by many as a high-dollar collector's rule, with an average price of maybe $25 dollars on eBay when it does come up. However, very few have come up for sale over the last 5 years and may be valued higher for that reason.

EDIT: I acquired this rule in 2023. More description coming soon.

The Model 9068 (4168) Polyphase Duplex Pocket Doric

So when I discussed the Modern Polyphase Family of rules, I made mention of the Doric series of rules which served as a transition line of rules in terms of construction. One rule, the 9071-3 Doric we just discussed. The other, the more impactful of the two rules, requires a more detailed look here.

The 5" Model 9068 Doric was a beautiful, well-proportioned pocket duplex rule with a simple Polyphase-type scale set...

Front Side: DF [CF, CI, C] D, L
Back Side: K A [B, ST, S] D, T

I mentioned earlier that the Doric can be thought of as un-ornamented, but this doesn't have to mean "basic" just because it uses no red ink. The scale set is logical and powerful for as simple as it is, originating, naturally, from the 4088. The ST scale (in DMS measures) is very convenient, giving 4 significant figures of precision despite being a pocket-sized rule. While typically called a 5" rule based on length of the actual scales, this rule is actually 12.5 cm in scale length, not 5". This will come into play when I discuss this format of rule as it appeared to be cloned by other slide rule makers (see SIDEBAR: The 9068 and the Clones below).

When K&E shifted direction away from providing a long-term Doric family of rules, the 9068, which likely originated from a different "prototype" rule known as the Model 10000 Doric as early as 1946 (see more here regarding the rarity of that rule), was reassigned the model number of 4168 around 1950, or most certainly prior to the 1952 catalog. In 1956, the same rule would be offered in a sheath with a leather covered clip, with a model number of 4168C. In 1962, it would become either the 68-1555 or 68-1550, the latter number if the customer bought the version with the case clip. Finally, in 1968, the non-clip version was discontinued, only offering the version with the clip until the end of the K&E era in 1975. It would be the only rule to retain the Doric label over time, despite no other Doric references to slide rules since 1949.

This same model would also be made for one year only in 1968 as the Model 68-1555 Celanese Celcon, a beautiful and highly desirable slide rule (see also Sidebar: 9068 and the Clones for more). This rule, carrying the Doric label as well, is somewhat rare and will cost the collector at least $75 to $100 when they appear yearly, perhaps, on eBay. The normal 9068 or 4168 version of the rule will be in the $20 to $30 range, more with pristine accessories.

The 9068 Doric model was offered at $8.50 in 1949, suspiciously priced at $10.35 in the December 1951 price list at the 4168, only to be returned to $8.50 two months later in the February 1952 price list. In 1962, the clipped case model 68-1550 would be $9.50, while the clipless version was $1 less; I suppose the former is money well spent if it keeps the rule from falling out of one's pocket!

Log Log Duplex Family

We now come to the "flagship" family of K&E duplex rules, which preceded the Polyphase Duplex rules by four years. This line of rules would prove to be K&E's most successful line, introduced in 1909 with the Model 4092 and continuing in some fashion until the end of the slide rule era. In fact, when you consider a "standard" duplex rule to compare with rules from other makers during the high-point of slide rule production in the 1950's and 1960's, it would be hard to argue that the later version, the Model 4081, doesn't deserve top placement on any tier list you might create (see Professor Herning's video, at right, where he discusses these leading slide rules of the era in 1955).

"Log Log" slide rules were introduced relatively early in the history of the device, invented by physician Peter Mark Roget (of Roget's Thesaurus fame) in 1814, which allowed for the computation of arbitrary exponentials and roots.  This was almost a full century before the first K&E Log Log rule. Why so long? Historians would suggest that there were not practical reasons to have a Log Log rule until the engineering boom of the early 20th century, though I feel the real reason is one of production capabilities.

Unlike previous Log Log rules that used a single scale, it is a certainty that more makers wouldn't have thought production of a Log Log rule to be useful unless multiple continuous scales could be utilized, providing worthwhile resolution. Such a design already existed known as the Yokota layout, which required three continuous log log scales on a stator rail.   Enter European slide rule maker (and industry leader) Dennert & Pape, who developed their first "Yokota" designed Log Log rule in 1908, one year before K&E produced their first Log Log Duplex rule.

Note:  As previously discussed, dividing machines up until the mid-1900s could only function at the physical edge of a stator rail, so only 2 scales on the rail were possible. This seems odd to me, as companies could put 3 scales on a slide for many years previous, but nevertheless there was a technological limitation to that point in time.

K&E likely retooled with these new capabilities simultaneously with Dennert & Pape, but it wouldn't be until a year after the Dennert & Pape effort that they could roll-out a product to market. But it was obvious that K&E's first order of business was to make such a slide rule, likely in what I feel was a partnership effort (see Sidebar: The European Influence on K&E" for more).

Note:  Since the C-scale represents the log of a number "x", then a "log of a log" scale for some base "b" allows the computation of b^x power. Because of the need for precision, these Log Log scales are very fine, typically three continuous scales called LL1, LL2, and LL3, folded to produce a single long scale for computing exponentials from base 1.01 up to 22,000. (LL1 ranging from 1.01 to 1.11, LL2 ranging from 1.11 to 2.718 or "e", and LL3 ranging from 2.718 to 22,000.) Other scales can be added for negative powers as well, typically known as LL01 (LL/1), LL02 (LL/2), or LL03 (LL/3) scales. Later rules from many makers will have up to 8 total log log scales for your exponential solving pleasure.

Such Log Log rules require three or four scales to be useful, which means that unless a company produced a rule ONLY for exponential computations, then there would not be enough space on the typical, multi-purpose rule, such as the Polyphase Mannheim layout championed by K&E the very same year. With the duplex-design and newly-found ability to have three scales on a rail, then a Log Log Duplex rule could be produced without sacrificing the utility of their own updated Mannheim layout (only the K scale of the Model 4053 is missing from the new Model 4092 Log Log rule), making for a very powerful slide rule.

One should note that the original K&E-made Duplex rule, the Model 4071 series, had been closing in on a decade of production at this point. As mentioned when discussing that rule, I felt that K&E had greatly underutilized that design because they couldn't add more scales and were unwilling to remove redundant scales (A and D scales on both sides, for example). While splitting that series into arithmetic and trig models that same year in 1909, it's no surprise that the days were numbered for this original duplex design. Eight years later, which was also 5 years after the superior Polyphase Duplex 4088 Model, the old Duplex family of slide rules would be gone.

By 1913, the pricing of these rules seems to support this: $6.50 for the 10" Model 4071T (Duplex Trig), $7.00 for the superior 10" Model 4088-3 (Polyphase Duplex), and $8.00 for the new 10" Model 4092 (Log Log Duplex). There would have been a place for the old Duplex at around $4.50 in this pricing structure, but not at a cost nearer the 4088, which was an improvement in every way.

As mentioned, there is a long history of models in this family of K&E rules, so let's dig deep into a model-by-model summary.

The Model 4092 Log Log Duplex

Introduced in 1909, and priced at $8.00, the new Log Log model became K&E's most expensive, flagship 10" slide rule. Front side scales included A [B S C] D and the back side included LL1 LL2 LL3 [C T CI] D L. I find it interesting that a Model 4092 1/2 version of this rule is mentioned in the 1914 instruction manual that shipped with the rule. The length of that slide rule is not given, but 5" is assumed. The manual mentions that the shorter rule does not have the D scale on the back side of the rule, but otherwise is identical. This rule was never listed in a catalog, nor are samples known to exist. No shorter rule based on this scale set would be offered by K&E until the 4181-1 "Jet-Log" introduced in 1953.

Aside: Note that there is mention of a N9081-1 Doric rule in 1948, but this rule is never known to have been produced. The fact that such a rule took 40 years to happen should not be a surprised. A shorter wooden rule wouldn't have been cost effective, so ultimately waiting for the newer plastic technology makes perfect sense.

This Model 4092 would remain entirely unchanged (except for the company-wide cursor changes) until 1922, when a 20" model, the 4092-5, was created. Of course, this necessitated a name change of the 10" rule to the Model 4092-3.

With this new rule came a drastic revision to the scales. Not surprising based on the success of the 4088 at that point and the increased potential for more scales, the 1922 version of both the 5" and 10" 4092 model incorporated folded scales as well. This design was filed for U.S. patent by A.W. Keuffel in that same year, assigned as patent number 1,488,686 two years later.

Moving all trig functions to the back side of the rule, and reworking the Log Log scales, the new layout was entirely revamped:

Front scales: DF [CF CIF CI C] D L
Back scales: LL0 A [B S T C] LL3 LL2 LL1

The addition of the LL0 scale is lovely here, adding the possibility of computing exponentials for bases between 0 and 1. Granted this scale is somewhat imprecise compared to future Log Log rules that would split these values across more scales (usually called LL01, LL02, and LL03 in future rules); however, this one scale did provide two significant figures of precision.

A K-scale for cubes is all that remained from the typical Polyphase scales, but it could be argued that a K scale is redundant compared to performing the same computations on the new 4092 rule with the LL0 scale.

For example, $8^3$ can be computed rather imprecisely to two significant figures using the K scale of a typical 10" slide rule - cubes with large bases are mostly impractical because precision is poor. While worse could be said using the LL3 scale of the 4092, by using the LL0 scale instead (which, like the trig scales, was keyed to the A & B scale of the rule), users can rewrite the problem as $(.8 * 10)^3$ or $.8^3 *10^3$. As such, setting the base at 0.8 on the LL0 and raising to the 3 power yields .51 to a similar level of precision as having a K-scale. Thus, then multiplying by $10^3$ is the same as moving the decimal 3 places to the right, for an answer of approximately 510.

Of course slide rules are much less effective with larger numbers, so the real power of the Log Log scales occurs when you have exponential bases closer to 1. For example, something like $1.2^3$ can be computed to 4 digits of precision using the LL2 scale on the Model 4092 but only 3 digits with the K-scale. Plus, with Log Log scales, the user can just as quickly get all the powers of 1.2, even decimal ones, just by moving the cursor.

No matter, this would be fixed in 1924 when K&E added a K scale to these rules, with a change in designation to both 10" and 20" rules, adding the "N" prefix. Interestingly, these were catalog-only designations...the 20" 4092-5 rule is the only one that added the "N" onto the rule itself; a curiosity indeed!

The 4092 rules would continue in production until 1939, even hanging on 6 years after the introduction of the next two Log Log Duplex rules, as its scale set was very convenient, powerful, and versatile, especially from the perspective of history. Later similar rules, from K&E and others, would seem to compete for the title of "most scales" on a rule, raising the complexity of their use. The 4092 keeps to the essentials. Likewise, this rule is nicely dimensioned. I haven't talked much about the form factor of K&E rules, especially the duplex rules, but to help with the look of additional scales, the width of the rule is increased compared to other K&E duplex rules. For example, the 4092 rules are consistently 4 cm wide compared to the 3 cm of the 4088 Polyphase Duplex. This 4 cm width would become standard for all of the more powerful duplex rules produced by K&E.

As such, I find the 4092 a joy to use, especially in the later variants that added the K-scale. The inclusion of the DF [CF CIF CI C] D with the "N" model, as with the Model 4088, facilitates terrifically efficient chaining of multiplication and division operations with the least amount of slide motions.

It is beautifully constructed. Its extra-width and uncluttered arrangement of scales seems to strike the right aesthetic, especially after 1922 when the rules shifted to laminated edges and after 1927 when the scales were printed in a non-serif font. I own two original 4092 samples made around 1920, as well as two 4092-3 samples made in 1933 & 1934. The later pair are among my favorite slide rules.

Sidebar: The 9068 and the Clones

What you see is not an illusion. The image above shows four rules that look almost identical to one another, and two of them are not K&E slide rules. The third rule is the K&E Doric 4168 Polyphase Duplex, likely introduced around 1950. As mentioned in the Doric Sidebar earlier, this is the direct descendant from the original 9068 Doric model. It would continue in production for 11 years until 1962, get renamed the 68-1555 for 6 years, and then be sold for 8 more years as the 68-1550 with a leather clipped case. Once K&E dropped the Doric "family" concept, no longer mentioning the word in the 1952 catalog or after, this is the only original Doric rule to retain a Doric label.

Below this rule in the image is a one-time variant known as the Celanese Celcon (68-1555), made of a green "acetal co-polymer" thermoplastic known as Celcon, produced by K&E for the Celanese company for one year in 1968. According to the ISRM, in an interview with the research director, it is the first use of the Celcon resin in an product. Celcon, and the Celanese Company, still exists today - they are a Fortune 500 company* and the world's leading producer of acetic acid and polyvinyl acetate. This digression aside, other than the plastic used in both the body and the cursor rails, these pocket rules are identical.

And now, to the top two rules. We likely shouldn't be surprised that Dietzgen is represented here, as they were known to outsource their production to outside makers, but you'd be mistaken to think K&E built the Dietzgen No. 1776 Redirule as pictured. Despite the shape, the dimensions are subtly different throughout, including the length of the rule, size of the rail grooves, size of the adjustment screw in the end brackets, and of course the cursor. The scale length is different too. The Dietzgen has a true 5" scale length while the K&E is actually 12.5 cm. The Dietzgen also adds two more scales, those of the "new" Polyphase scale set.

The top rule is the Bruning No. 2398, and yes, in construction, it is identical to the Dietzgen in every way, with the possible exception of the plastic being used. While age can account for color variations in plastic, the Dietzgen is most certainly colored different from the factory. However its surface feels like the Dietzgen while the K&E rules feel different. Interestingly, the Bruning has the same scale set of the K&Es, not the Dietzgen.

Part of the problem with reconstructing slide rule history is the lack of documentation of agreements, contracts, and production timelines for any of these companies. The best we have is trademarks, product catalogs, and patents in order to understand, in this case, who's cloning who?

The Charles Bruning Company deserves special mention. Bruning produced their own rules, but a greater number of them existed as rules licensed to other companies. As I said, Dietzgen was known for out-sourcing many of their rules, especially the plastic ones, but not to K&E in this case. Bruning is the licensed contractor here, as well as for Dietzgen's No. 1771 rule, among others.

The hard question to ask is how Bruning might be related to K&E? And is their any chance that Bruning produced these 9068 rules?

The latter question can be answered first - not likely. Most certainly K&E was held back by World War II in the sense that R&D efforts would take a back seat to customized production with the military as their top customer. So another company, like Bruning, could get a jump on them.

For example, Bruning would produce "Ivorite" types of slide rules even during the war. It would not be until ~1950 before K&E shifted to Ivorite construction in their plastic rules. But the early 9068 Doric, and the "prototype" Ecco 10000 version of it, is reputed to be something else - improved Xylonite (?) - or most certainly different than something like the 10" Bruning 2420 "Polyphase" Mannheim rule made around 1943 (from which the Post 1452P rule was also produced).

A second aspect, one of supplying to the military, means that production of typical products would have been redirected into other areas more specific to what was needed in the war effort. We discussed that with the 8858 Special War Time Issue slide rule mentioned earlier, but we also see a glimpse of this in the K&E 1944 Product Catalog...

While we suspect that production numbers of slide rules did not wane overall, as the serial number reference table (see much earlier in this article) shows rather normal estimates of 60,000 rules in 1943 and 77,000 rules in 1944. But if we were to look at K&E rules in the wild during this period, we wouldn't see much of their normal production rules.

What K&E did instead during this time is hard to figure completely, but we do know they produced their M4 and M16 Graphical Firing Tables during this time (looking suspiciously like William and Cox Load Adjuster "slipsticks" of the same era) , and likely their Model 4108 "US Military" rule. Their rules, with some speculation of their origins, will be covered in the section on Specialty Rules later. But importantly, the serial numbers of these rules seem to match the production table used to date our K&E rules. Regardless, these rules alone are not enough to make K&E busy during the war.

Back to the relationship with Bruning - they produced exactly the same rule as the Model 4108, doing business as the American Blueprint Co. Again, both K&E and Bruning making the same rules during this time? Certainly, this is something worth further understanding!

The idea of "cloned" rules is not anything unusual. While we are inclined to believe that K&E likely produced rules licensed to Bruning, I would leave open the possibility that it could have happened the other way around. And if that's the case, then it opens up inklings that Bruning could have produced the Doric series as well. I don't think so, but who really knows how business was run during the post WWII era? Yet, in the very least, Bruning most certainly influenced K&E with their slide rule construction during the 1950s, as Bruning was not limited in R&D efforts as was Keuffel & Esser.

*Note: The Fortune 500 company, Celanese, is based in Irving, Texas, within 10 miles of my residence in the DFW metroplex. I haven't worked up the courage yet, but I wonder if I walked through the front door with my own sample of the Celanese Celcon slide rule if they would even know what it was?

The Model 4090-3 Log Log Trig and 4091-3 Log Log Deci-Trig Duplex

The idea of a slide rule with decimal trig scales had been around for a good while, and when K&E produced one in 1929, a new "vector" rule no-less (see the next section), its decimal trig scales got the attention of other consumers. Thinking at first that only electrical engineering students could benefit from decimal trig scales, K&E was quite quick to offer them with their other duplex lines as well. As a math educator myself, doing trigonometry with decimal angles is just much easier for all concerned, not just electrical engineers. While I did not live back during those times and cannot state with authority that education was trending away from DMS (degrees/minutes/seconds) angles, I think it's obvious to see that K&E was not going to deny consumers the opportunity to choose either option where their duplex rules were concerned.

And thus was born the concept of the Trig and Deci-Trig versions of these rules.

I mentioned earlier that, first, the 4088 would give way to new Polyphase Duplex 4070 Trig and 4071 Deci-Trig rules in 1939, but here, in 1933, K&E did it first with the Log Log Duplex rules, producing the Model 4090-3 Log Log Duplex Trig and Model 4091-3 Log Log Duplex Deci-Trig slide rules. Again, like the 4070 and 4071 that would come later, these rules were identical, only differing in the way the trig scales are divided.

Unlike the new Polyphase rules, these two new Log Log rules would not replace their original rule in the series. In this case, K&E kept the excellent Model 4092-3 and produced a new, revamped Log Log scale set. It was this new scale set that would be implemented into the Trig and Deci-Trig options. So in 1933, and for six years after, K&E would carry three Log Log Duplex rules in their product line.

The new scale set for the 4090-3 and 4091-3 rules:

Front: L LL1 DF [CF CIF C] D LL3 LL2
Back: LL0 A [B K CI] T S1 S2

And scales of the earlier 4092-3 Log Log Duplex:

Front: K DF [CF CIF CI C] D L
Back: LL0 A [B S T C] LL3 LL2 LL1

The first thing to observe with the new scale set is the emphasis on trigonometry, as you might expect. The original 4092 gave up its additional C scale and the S scale for the S1 and S2.

These split sine into S1 (0 to 5.73 degrees) and S2 (5.73 to 90 degrees). Since the sine and tangent for angles in the small angle domain are largely the same, then the S1 scale could be used for tangent evaluation of those angles as well.

This is the only implementation of the extended sine scales S1 and S2 on a K&E slide rule, though their Log Log Vector Duplex rule introduced 4 years prior had inverted S1, S2, and T scales. As we will see in our discussion of that rule, there is an interesting backstory there. But suffice it to say, there might be another reason that K&E would re-label these scales for the next version of this slide rule 4 years later.

Two rules from my collection, the 4080-5 Trig and 4081-5 Deci-Trig, top and bottom respectively. As per all Trig/Deci-Trig models, the only difference is the divisions for the trig scales. The minute marks on the angles of the ST scale on the 4080-5 is the obvious indicator, but note also that all three trig scales (T ST S) are divided differently, degrees etched in 6ths on the top rule and degrees etched in 10ths on the bottom rule.

I do find it curious why the 4092 hung around for six years simultaneously with the new 4090 and 4091 rules, and wasn't discontinued like the Model 4088 Polyphase Duplex when usurped by the trig models? I would suspect this had something to do with the fact that the original 4092-3 also came in a 20" version, the 4092-5, and because such a long-scale version was not offered with the 4090 and 4091 series, then the 4092 in both 10" and 20" were rather safe, at least for a time.

But mostly, I would suspect that the newer models just weren't as good as what they attempted to replace. To me, they seem to sacrifice too much for their new capabilities. First, it is natural and usual to have all the trig scales on the slide, which the majority of K&Es have always had. While you gain additional precision with small angle trig on the new rules, you do so at the penalty of having the trig scales on the stator rail. Because these are referenced off of the C & D scales, the user would either need to flip the rule to read from D or flip the slide and perfectly align it to assure an accurate reading off C.

Second, as a Log Log rule, having all the LL scales on the same side of the rule makes sense when you only have 4 such scales. This is an aspect that makes the Model 4092 one of my favorite K&E slide rules. Noting that the new rules move the LL0 scale to the back, it seems like a case of, "If it isn't broke, then don't fix it." Future rules would also put the LL0 scales on the opposite side of the rule, but in those cases it's forgivable since they will give will give you more of them, impossible to fit them all on a single side of the rule. While this is an admittedly minor reason to dislike the newer 4090 and 4091 models, it is a change that I believe didn't resonate well with customers who would have seen this as an unnecessary complication.

Adolph Keuffel, company Vice-President and head of the slide rule division for K&E by this time, expressed surprise at the public acceptance of this rule in a testimony given during the civil lawsuit of K&E against Pickett in December, 1948. Concerning the 4090 and 4091 rules, he states:

"[The 4091] was considered an improvement over the previous rule, because of the - I would like to use the term 'amplified' trigonometric scales, or more useful trigonometric scales. After this rule was on the market we had some good reactions, but we were rather surprised that certain reactions from our users, who were more acquainted with the 4092, the previous rule, and there seemed to be some confusion, and we were rather disturbed that this rule was not taking hold as we thought it should. We didn't seem to grasp the reason for it. I mean, we did not grasp the reason for it." - Transscript of Record No. 9939, US Court of Appeals for the 7th Circuit. Keuffel and Esser Company vs. Pickett and Eckel, Inc., p.299.

Indeed, I would also consider this layout of the new trig rules poor for what it attempts to do. Too many trade-offs needed to gain a small amount of accuracy with the extended trig scales. And even then, if you wanted more accurate trig evaluation of smaller angles, then sticking with the superior 4092 layout and dividing any small angle by 57.3 will give you the same levels of precision (this is essentially what the ST/SRT scale would later do). In fact, we even see earlier K&E rules, like the Log Log Vector rule (see the next section), add a "radian" mark on the C/D scale for exactly this reason.

Thus, it should not be a surprise if K&E wanted to replace these rules as soon as possible, which leads us to...

The Model 4080 Log Log Duplex Trig & Model 4081 Log Log Duplex Deci-Trig

The run of the 4090/4091 rules would indeed be short lived. In only 4 years, they would be replaced with these 4080 Trig and 4081 Deci-Trig models in 1937. However, the 4092-3 and 4092-5 would hang on for two additional years. Introductory pricing in 1937 for both rules was $11.75 with synthetic leather case, with a real-leather upgrade for one dollar more. The Model 4092-3, still available, would save you a dollar.

The writers for the manual that would ship with the new series of rules were Lyman Kells, Willis Kearn, and James Bland, all from the U.S. Naval Academy. This is significant for several reasons.

First, the manual was not outsourced, rather it was written by the designers of the slide rule itself. As such, all we need to do is look at the patents written on the 4080-3 and 4081-3 models, particularly U.S. patent number 2,170,144, to see that Kells, Kearn, and Bland entered into a patent agreement with K&E for these slide rules. So, essentially, this is K&E either buying out USN Academy professors for the rights to the design or paying royalties to them for their production and sale.

Of equal importance, this rule is the first indication we have that K&E worked closely with the US Naval Academy, beginning around 1935. This would lead to the production of what appears to be as many as eight other slide rules, including three of the radio rules we will discuss in Chapter 4 and the Model 4110 Power Trig rule discussed in Chapter 6. And as we've already seen, the trio had some impact on the 4161 Polyphase Mannheim (Chapter 2) and the 4070/4071 Polyphase Duplex models described earlier in this chapter.

If I hadn't mentioned how puzzling the previous 4090/4091 rules were, one might be inclined to think that the 4080/4081 were simply a re-designation of those model numbers, as K&E was sometimes bent to do. However, not only do these rules fix the usability issue of the 4090/4091, the new models add three additional scales from the previous 17-scale models; bonus capabilities as well.

The change for the rules - from 4090/91 to 4080/81 - was well-received. Adolph Keuffel voiced this once again in court during the K&E vs. Pickett lawsuit in 1949, stating that they viewed the new invention as a "solution" to a "problem" concerning the 4090/4091 rule. Keuffel met with the professors and Admiral Hewitt in 1935, head of the U.S. Naval Academy, to discuss the rule, as was introduced to Keuffel by a letter the Admiral had sent. Hewitt was desirous of having the new model available to his new cadet class of 1936. Keuffel states in his testimony that he took charge to make that happen, completely converting a production line over to it. When asked by the court if that was a usual practice for them when they learn of a new potential product, Keuffel admits that it is "very unusual." (Transcript, K&E vs. Pickett, p. 313)

It is said that this new series of Log Log Trig and Deci-Trig rules became not only K&E's top seller but was, historically, the top selling slide rule of all time, regardless of maker - considered by many, including Professor Herning above, the "standard bearer" of slide rules. It leaves us to hope that K&E compensated Kells, Kearn, and Bland really well for the design!

And I can easily see why this slide rule was successful.

The front side scales reverted almost identically to the excellent 4092 model, placing the trig scales back on the slide while also squeezing in a CI scale for good measure. This alone would have been good enough. But the back side of the rule would also show improvements. The full scale set is as follows...

Front side: L, LL1, DF [CF, CIFF, CI, C] D, LL3, LL2
Back side. LL0, LL00, A [B, T, ST, S] D, DI, K

First, the "ST" scale arrives to the first K&E slide rule. As mentioned earlier, instead of an {S1, S2, T} set of trig scales, having {S, ST, T} just makes more sense from the perspective of what the ST can do, namely to give functionality to sine and tangent equally. And it also puts these scales back on the slide, read off of the C and D scales directly below. This was unlike the Model 4092 Log Log Duplex which referenced the trig scales off of the A and B scales, a difference actually noted within the newer patent by the professors; and as such, was the reason for the patent.

These K&E duplex rules provide some nice added functionality. The LL00 extends the LL0 scale, splitting exponential bases between 0 and 1 into two scales to give another significant figure worth of precision. In this implementation, LL0 is extremely accurate now, covering only bases from .999 down to .905, while LL00 handles .905 to 0. The added DI scale inverts the D scale across the rule, nice for a variety of general math uses.
Moreover, the organization of the log-log scales are nice, with scales for bases greater than one on the front side of the rule, and scales for bases between 0 and 1 on the back side. It's really a good design and, as mentioned when I introduced the Log Log Family of rules, the Model 4081-3, in particular, would become a "standard" rule by which others could be compared.

At this point, there would be nothing new in terms of construction of these rules, as most all duplex rules of this era remain 4cm wide, with celluloid covered mahogany, engine-divided, and sporting the same "New Improved Cursor" as most 1936 and greater rules would have. Not all is great, however, from the standpoint of the collector, as cursors of this era suffer from the KERCs-infestation. Thus, while great rules, many of these models can be hard to find with a good cursor (I bought two 4081-3 models before finding a good one).

Because this rule in either Trig of Deci-Trig versions was so successful at the time, both the 10" and 20" 4092 models would be discontinued two years later. In dropping those rules, 20" versions would arrive in 1939. These rules, the Model 4080-5 Log Log Duplex Trig and Model 4081-5 Log Log Duplex Deci-Trig, were introduced at the price of $25.30 each, or $27 for the chamois-lined leather case. As such, these 4080/4081 series rules, in all lengths, became the true functional successor of the Model 4092 series, something that the 4090/4091 abominations failed to do.

Nothing would change with the 4080/4081 until 1947 - at a 30% to 50% price increase for almost all of K&E's rules - when all versions of this rule in both -3 and -5 varieties would take on the N- prefix with majorly revised scales. The objective of the change would be to alter the red LL scales (those with bases less than 1 so that they too could be referenced off of the C and D scales. This had two desired effects in that now 1) all scales on the entire rule could be consistently referenced off of C/D and 2) the single decade aspect of C/D allowed the previous LL00 scale (running from 0 to .91) to be split into two higher resolution scales named LL02 and LL03 on this rule. As such, the new rule grows from five log log scales to six. To accommodate the change, the DI scale was removed from the rule.

As such, the new N4080 and N4081 rules of 1947 would have scales as follows:

Front Side: LL02, LL03, DF [CF, CIF, CI, C] D, LL3, LL2
Back Side: LL01, K, A [B, T, ST, S] D, L, LL1

I find it interesting that this change necessitated a new patent, assigned in 1947 to only James Bland. This U.S. patent number 2,422,649 would be known as the "Bland Invention," referenced as such in the later product manuals written by the trio. Yet, like the patent for the original 4080/4081 design, this too would be assigned to K&E under patent agreement.

So, as summary, the original 2,170,144 patent by Kells, Kearn, and Bland assigned the trig scales to the C/D scales and this second 2,422,649 patent, by Bland alone, followed suit with the log log scales.

It's with this configuration of scales that the 4080/4081 model line would dominate the "slide rule era" of the 50s and 60s, except for one minor change in 1954 where K&E would relocate the L scale to the back-top rail above the K scale and add the DI scale to the back-bottom rail, as in the original version of the rule. K&E did drop the N- prefix from the rule with that change, but I feel it was a response to match "N-less" model numbers with the new all-plastic version of this rule, the 4181-3, that was likely introduced a year or two prior. If you are curious, K&E did change the wood models in 1956, relabeling the ST scale to the "SRT" scale, matching the scale set of that plastic rule.

Their rules would continue being produced through the modern era, advertised as their "highest grade" slide rules in order to differentiate them from the new all-plastic rules. But it's important to note that when those rules DID arrive with the same functionality in the 9081/4181 for a substantial price savings, there's no question that 4080/4081 would lose some share of the market in this category.

Sidebar: Laminated vs. Visible
Edge Construction

We have explored the majority of duplex rules to this point in our text. And while construction techniques with the rules have been very consistent, other than the evolutionary changes to the cursors/cursor rails and the addition of plastic in the duplex family, one of the more distinctive elements you will notice about duplex rules is that sometimes K&E banded the edges of the rule with celluloid - other times they did not.

K&E duplex rules were constructed with only the faces of the rule covered with celluloid, with two exceptions...the original Duplex Family laminated the edge of the rule beginning in 1909 as did the Model 4088 Polyphase Duplex upon its introduction in 1913. The Model 4070/71 versions of the Duplex, those with the "T" suffix, would place the L scale on the celluloid-laminated top edge of the rule with a cursor that could indicate also on the edge. While that feature only lasted maybe 4 years in that configuration, when the L scale shifted back to the rule's face, K&E didn't bother to remove the edge lamination. It would stay in this style until 1917 when the entire Duplex Family lineup was discontinued. The Model 4088 would always enjoy laminated edges through its run.

The edge of everything else showed the mahogany construction of the rule. This would remain true until, in 1922, when the company started using serial numbers; they also began to laminate the edges of the rules with celluloid plastic. And this is without exception.

For 30 years thereafter, all K&E duplex rules had laminated edges, where the only place the mahogany could be seen is where the slide meets the grooves of the rails and the ends of the rule where the wooden end-grain is visible. K&E would put the new serial numbers toward the end of the bottom edge.

And then, in 1952, the edge banding disappeared, or rather was altered to a strip of inlayed celluloid on the edges, where the mahogany shown on both sides of the inlay.

For the Log Log Duplex and Log Log Vector Families of rule, K&E would clean up the rule faces of all maker-marks (except a new K&E logo at the end of the slide's face) and relocate them to the inlayed celluloid strip on the edge. Here we typically see the model number, patent numbers, and "Made in U.S.A." black printed text (see image below the Sidebar).

At this time, I cannot give a solid reason as to why K&E would go to edge laminations outside of the notion that, perhaps, they felt customers would want it that way. Or perhaps they just felt it was more aesthetically pleasing? Or perhaps, with competitor rules like Dietzgen and Post/Hemmi offering complete celluloid-covered wooden rules, they felt some external pressure as well?

Certainly it was not because they felt it necessary to move the serial number to the rule's edge. While this is something they did with the edged-lamination, they could have continued that practice when they shifted to the inlays, where instead they used the last three digits of the serial number on the faces of both rails near the ends, as well as on the slide. Labeling both in this manner assures that the rails and slides are well-mated during production.

There is some notion that covering around the entire slide rule would prevent seasonal movements of the underlying mahogany, but mahogany is more stable in this regard than most hardwoods (a reason it's used in guitar necks) and edge grain of the wood is less prone to expansion or contraction as the face were it not laminated. However, I do have some duplex rules in my collection that do show a slight lack of uniformity in rail/slide alignment over eons of time - or even a slight cupping in the edges of a stator/rail. So it does exist, but the amount at which that becomes objectionable is debatable since any expansion would be latitudinal and not in the direction of the scales themselves.

This could have been a concern of K&E's at which point the company could have taken a "better be safe than sorry" approach. This notion would support the incorporation of the edge inlay in 1952 as perhaps they did not feel it was necessary any longer? But more than likely it was because the new inlay design would prevent delamination at the face/edge join of the rule, which is something that many rules of the time did indeed experience (see image below for examples).

Verdict? K&E likely felt edge-lamination was desirable and practical for all the reasons I mentioned. But these were also aspects of their duplex rules that could be sacrificed if unsightly delamination of the celluloid became a bigger issue.

Delamination at the edges, as shown with the 4088-1 and 4090-3 rules in my collection. If this was happening shortly after they were produced, then the 1952 shift away from lamination of the entire edge of the rule would make sense. It may not affect the rule functionally, but it isn't good for the overall aesthetic.

This is especially true when the 68-1xxx designations came in 1962, the same year the Deci-Lon was introduced. The 10" 4080-3 would be rebranded as the 68-1318, the 10" 4081-3 became the 68-1210, the 20" 4080-5 relabeled the 68-1308, and the 20" 4081-5 shifted to the 68-1200. These would be the designations on the rule, though the catalog numbers would show more 68-1XXX numbers depending on the case accessory. [sigh]

To illustrate the headache of the 68-XXXX numbers, there were three separate catalog numbers for the 10" Deci-Trig model. 68-1220 designated the rule with the synthetic leather case, 68-1215 for the real leather, chamois-lined case option. And 68-1210 for a version package with both the nicer case AND a hard-back user manual.

The 20" 4080-5 regular trig model would be discontinued first after 1966, which should tell you something about which rule was more in demand between it and the 4081-5 Deci-Trig model, which would endure until 1972. Same story with the 10" models, as the 4080-3 version would end after 1967, outlasted by the 4081-3, also lasting through 1972.

Afterword (8/17/2024): With the discovery of the 1968 pricing, a few observations can be added. The remaining two Deci-Trig rules came at a price of $33.75 for the 10" version and $70.00 for the 20", these with the basic case. This 20" rule, at $76.50 with the real leather case, would be the most expensive rule offered by K&E, even more pricey than the 4083-5 (68-1424) Log Log Vector rule ($66.50 for synthetic case option) which had always been the highest cost K&E slide rule. But no longer. Interestingly, and to my amusement, both rules would be more costly than the 6.5 ft. long Deci-Lon demonstration rule (68-1929), which remained $60. How much is $76.50 today in 1968 money? How does $692 sound?

From the collection...neophytes to K&E rules need time to learn the differences in models. Here we see the two "log-log" models, both in degree/minute trig and in decimal trig form, as well as the "vector" rule, bottom, to be discussed in the next section. Note the differences in scales here. Quick tip...sometimes it's difficult to detect what model is being sold on eBay if the seller doesn't post a model number, but the faster you learn the scales for particular models, the better you can identify the rules in that setting. I've gotten the jump on some rather nice, inexpensive rules by recognizing what they are when the pictures seem lacking in detail.

The Doric N9081-3 Model

This Doric model, introduced in 1948 and appearing in the 1949 catalog as one of the three members of the Doric family, was an exact conversion of the Model 4081-3 scales into a plastic rule. Interesting to me, there was no "N9080-3" version of this same Doric rule. We could make a judgment about the Trig vs. Deci-Trig choices being made during this era of slide rules. K&E's own choice of the "decimal degree" version of the rule instead of the DMS version likely demonstrates which version was the best seller. As such, the Model N4081-3 Deci-Trig was deserving to be rewarded a doppelganger rule.

The Doric N9081-3 was excellent in most every regard, albeit a little spartan and less impressive than the flagship wooden rule that it copied, as were all of these "Doric" rules. It is understandable why K&E wouldn't want to replace the 4080 and 4081 models as they had become part of the fabric of the company itself. But as we will talk about with the Modern Duplex rules, the opportunity to offer a similar rule as a lower price point would prove to be the reason why this Doric rule appeared in the first place. In this case, the 1948 catalog price for the N9081-3 rule was $12.00, which saved $8.50 over the mahogany version. Remember, this is for a complete, functionally equivalent rule to the company's most successful slide rule to date. If a customer couldn't afford more than $20 for the celluloid-covered mahogany rule (equivalent to more than $250 in 2023 dollars), then they would have thought the all-plastic version ($150 in 2023 dollars) was a godsend. These savings represented by all of the Doric rules signaled the transition to all-plastic construction; a harbinger of the future.

This N9081-3 rule (along with the Doric 9071-3 Log Log Duplex) served somewhat as a proof-of-concept, albeit K&E was certainly comfortable enough with the product to offer it for public sale rather than to classify it as a prototype slide rule. Why exactly that is the case is curious? But I think it is rather obvious that this Doric rule offered an opportunity to test the market with two functionally identical slide rules of two vastly different forms. This rule, and the 9071-3, were unique in that none of the other Doric rules had a direct wooden analogue, and thus there was something to be learned by offering both rules for public sale simultaneously. This data would give them valuable knowledge of how to proceed once the inevitable did happen - the shift to mostly all-plastic slide rules in the coming decades. Could K&E continue to sell wooden rules after the all-plastic rules entered the picture? Would customers view the company as compromising their own legacy of making well-made, elegant, top-of-the-line American slide rules?

Of course the 9071-3 Doric quickly disappeared with no modern plastic version, so it was this N9081-3 rule that demonstrated that the best selling and highly-regarded Model 4080 or 4081 made of mahogany could be replicated as a functioning replacement; and even be sold at the same time. While the Doric rule would give way to a better version 3 years later, these rules would be sold alongside the wooden version until the end of the slide rule era.

Notwithstanding, history would show that the N9081-3 rule was very much transitional; but an important slide rule historically. Its replacement, the Model 4181-3, would become the first "Modern Duplex" rule for the company. It will be discussed fully in that section.

Collector's Outlook:
Double-sided, Duplex-Type
K&E rules

Note: Suggested prices are for slide with case on eBay. I don't include shipping in these estimates. Any extras, especially those considered complete, with box and documentation, can double these prices.

Our writing here spends an enormous amount of effort describing the Duplex Family of slide rules. It would be nice to collect them all if only to hold-on to some of the most delightful slide rules ever made. For me, that is what defines something as collectible. But I suspect that most people only care if something has monetary value.

And while all rules are interesting, most of them can be found for $15 to $30. This includes all of the Polyphase Duplex, Log Log Duplex, and Log Log Vector families of rules. So any of the 4088, 4092, 4080/81, and 4083 models (and their derivatives) are plentiful on eBay. It should be noted once again that finding a good sample with an intact cursor can be challenging. A nice, clean selection, especially with box and documentation, should be in the $40 to $60 range.

Among the Modern Duplex family, two rules come to mind as being of value. The first is the K&E Analon slide rule, which was produced only during one production year in limited number. It is somewhat of a 'holy grail" item for collectors and will cost in excess of $300 to $400 on an open market. The price goes higher if NIB (new in box) with hard-bound instructional guide. The second rule of value is the 5" Deci-Lon 5, Model 68-1130. It will cost upwards to $130 to $150 or more. It should, as it's likely the best pocket rule ever made. The bigger 10" version, Model 68-1100 Deci-Lon 10, has some variance here. I possess seven samples of this rule and never paid more than $30 for any of them, but I've seen some go on eBay for as high as $250 in a bidding war - for no known reason other than the completeness of the NIB package.

The real valuable rules are from the original Duplex Family, especially the earlier William Cox rules of the Model 1744 varieties made prior to 1901. These rules will run well over $500, usually well over, during their quinquennial appearance on eBay. The best opportunity is to purchase one is as part of a bundle that isn't specifically named in the description, where only a small picture from an eagle-eyed lurker sees the chisel-type cursor. But even then, expect a bidding war.

The follow-up series, the 4071-type models (not the newer Polyphase Trig and Deci-Trig remakes) can be tremendously valuable. Recall that these items were produced between 1901 and 1916 across no fewer than 20 different models, as K&E struggled to figure out what rules to actually offer. For this reason, some of these rules have never been seen, while others are no more frequent than "yearly."

Compounding the issue is those rules that came with dual slides or in many different configurations, since it remains unclear how available they actually were. For example, while the 1906 to 1909 versions of the 16" 4083 and 20" 4087 are rare enough - they are found occasionally for hundreds of dollars - each of those rules came in three other configurations with distinct catalog numbers. So if those other variations exist, finding them becomes complicated.

The same can be said about the 5", 8", and 10" versions too, especially when slide and cursor options were considered. Speaking of which, those rules that shipped with two-slides, it is rare to find a sample for purchase that still has both slides. And, likewise, we must keep in mind that many of these models were still marketed with the option for metal chisel-style cursors, even after 1906. But as we know, these types of indicators were being phased-out of the K&E lineup at that time; people simply weren't buying rules with chisel cursors anymore.

So, it is unlikely that we will ever cross paths with many of these selections, but if we did they'd likely be priced well out of reach of most collectors. The 4061, 4065, 4070, and 4071 models, in both the normal and "T" (trig) versions, will be the most common among this family of rules, but still very rare. I would expect prices in the $100 to $200 range for the yearly appearance on eBay. The longer rules, the 4083 and 4087 models, will be in excess of $200, and on a much more rare basis.

Also, because of the infrequency of these rules coming up for sale, the reader should keep in mind that this seems to be my general observation only, as too few of these rules have resurfaced to provide a good statistical sampling in regards to price. So expect variance here.

Ultimately, as with many highly desired rules, stumbling upon one at an estate sale might be the best way acquire any of the pre-1916 K&E duplex slide rules.

Log Log Vector Duplex Family

This was very much a special purpose slide rule and from that standpoint could be talked about among the Specialty Rules, but staying true to ISRM's K&E Model Map, we will discuss it here with the other duplex rules.

This history of the origin of the Log Log Vector Duplex is most unique among all other K&E slide rules. Being a scale set licensed from an individual, there would be decades of negotiations, disagreements, and even a lawsuit over royalties and breach of contract. (Credit to William K. Robinson for his study linked above. Much of our understanding comes from this research.)

But what must be said is that the introduction of a Log Log Vector Duplex rule in 1929 was an important development for K&E, not so much because it was necessarily a great seller, but rather what it could do was quite remarkable. The ability to read hyperbolic functions on a slide rule was nothing new, but the scale set, developed and patented in 1924 by A.F. Puchstein, could compute hyperbolic functions on the complex number plane, a task at which electrical engineers and students would find extremely useful. Secondarily, the user could easily convert these functions between rectangular and polar coordinates, a useful feature for a variety of vector operations. Doing all of this required both hyperbolic trig and standard trig scales on the same rule, since the computations always require a combination of both of scales.

Professor M.P. Weinbach of the University of Missouri, working with Puchstein, promoted the slide rule to K&E beginning as early as 1925, but a contract would not be reached for several years. In this arrangement, Weinbach would pay Puchstein 1/5 of the royalties received from the rule. The partners would hold exclusive royalty rights from the rule's first production in 1929 until 1947. This meant that for 18 years, K&E would not have competition in this market from other makers. Afterwards, Hemmi's Model 255 arrived after WWII and could solve hyperbolic trig functions directly, but it was obviously limited in the States. By 1948, the Pickett Model 4 and Dietzgen Model 1735, both of which had hyperbolic scales, finally challenged K&E for market share.

In all, two vector models would be produced by K&E spanning the 43 years in which the Log Log Vector rule was offered.

The Model 4093 Log Log Vector Duplex

Introduced in October 1929 after being on the drawing board for nearly two years, K&E introduced the Model 4093-3 Log Log Vector Duplex. The rule was constructed much like all other duplex rules of the era, with celluloid-laminated mahogany construction and with frameless glass indicator. The rule featured the circular K&E logo on the upper right stator and patent notifications on the lower right stator. Upon introduction, the scale set was as follows:

Front side: L LL0 DF [CF B CI C] D LL3 LL2
Rear side: Sh1 Sh2 Th [SI1 SI2 TI] D S T

For the uninitiated, the front scale set will look as familiar as any Log Log slide rule, bearing some similarity to the Model 4092. However, the rear of the rule would be like nothing seen before. An engineer would have recognized the utility immediately, where the only scale on the rear of the rule that is NOT a trig scale is the D scale. The top stator, with Sh1, Sh2, and Th scales are the hyperbolic sine and hyperbolic tangent scales. The slide had extended sine scales for small angle trig, like the 4090/4091 that would come 4 years later, but in this case, these scales are inverted. And a normal sine and tangent scale are below the D scale on the lower stator.

An important characteristic, being very much a rule designed by electrical engineers FOR electrical engineers, is that the trigonometry on this rule is all in decimal degrees. As such, this is the first K&E slide rule to use decimal degrees instead of DMS measures, even prior to the first "Deci-Trig" rule, the 4091-3, Log Log Duplex, which would arrive three years after. The instruction manual, written by Weinbach himself, supplemented the rule.

In short, after much debate with K&E about the design of this slide rule, Weinbach got exactly the rule he wanted to produce almost six years after his scale set was first patented. Upon introduction in 1929, the 4093-3 was $16, or $16.85 for the nicer leather case version. This was $6 more than the Model N4092 Log Log Duplex, just as reference. K&E would drop the price to $12 by 1932. According to Robinson, Weinbach's royalty was 5% per slide rule.

A longscale 20" version, the Model 4093-5 Log Log Vector Duplex, was added in 1931. Pricing of this rule made it the most expensive offering among all K&E slide rules, priced at $32 ($33.50 for the good leather). Of course this is a serious premium for the superior resolution rule. And it would stay that way, rising to $35 by the 1938 price list. This made the 4093-5 K&E's most expensive slide rule by at least 25%, even outpricing the expensive specialty rules, such as the N4096 Desk rule and the 4102 Surveyor's Duplex that we will discuss in the next chapter. Needless to say, K&E must have felt they would sell at that hefty price.

I mentioned some degree of contention between Weinbach and K&E that would eventually lead to a lawsuit in 1944. The initial conflict actually started with the Model 4091 Log Log Duplex Deci-Trig first produced in 1932. Weinbach did not like idea that K&E was producing another decimal trig rule, which at that point was a feature exclusive to his Vector rule, as was the extended sine scales. His concern, somewhat unfounded, was that engineers and students would forgo the more expensive Vector rule for the new 4091 Deci-Trig. Of course, K&E felt the concerns were unfounded as well. Their argument was that what made Weinbach's rule unique was the ability to do complex hyperbolic trig problems, and not necessarily have the monopoly on standard trig problems in decimal degrees. However, this could be the impetus for K&E to change to an ST trig scale when the 4090/4091 rules were upgraded to the 4080/4081 models 4 year later.

This did not make Weinbach waver in his position though. Although K&E would dispense with the extended sine scales for the 4080/4081 series upgrades, Weinbach also felt that he was owed a royalty for 4 years of the 4091 Deci-Trig for using mostly identical scales to his rule. In Weinbach's own notes, he mentioned that since K&E was using decimally-divided trig scales in the Deci-Trig rule (half of all his copyrighted trig scales of the Vector rule), that K&E should pay him half-royalties akin to the Vector rule contract, or 2.5% per rule (Robinson, pp. 21 & 22). According to Robinson, K&E would not concede on the Deci-Trig, but would offer to make the minimum annual royalty payments for this own rules $650 beginning in 1934, which did not seem to appease Weinbach despite only making around $250 on average the previous 4 years. In the middle of the Great Depression, and being guaranteed the price of an average car each year, I'm inclined to think Weinbach might have been standing too much on principle.

However, he would be worn down by the end of the year and conceded to the agreement in December, 1934. K&E, not completely philanthropic as they tried to appear, did want Weinbach to agree in writing to the loosely held termination date of their royalty agreement in 1947.

The parties, according to Robinson, would be mostly silent for 4 years. But in May 1938, Weinbach decided to contact K&E about a new idea for a slide rule. He was disappointed to be turned down, but there was a "bright side" to the letter. He was informed that K&E wanted to update the 4093 with a major scale revision and asked for Weinbach to give input. He was sent a prototype rule to evaluate and was asked if he would update the instruction manual in light of the new rules, which would be called the Model 4083-3 Log Log Duplex Vector (note the switch of "Vector" and "Duplex" within the 4093 name).

Weinbach loved the new design, submitted a couple of minor suggestions, and updated the manual extensively to provide more examples to demonstrate the increased capabilities of the new rule...

Two of the Log Log Duplex Vector rules in my collection. The lower 10" rule was likely made just before the move to inlayed celluloid edge in 1952. The upper 20" version shows a serial number placing it right around 1962. This slide rule came in a 68-1424 box with a blue/green case, which is the synthetic leather version.

The Model 4083 Log Log Duplex Vector

The quality that Weinbach loved most about the new version of his rule was that it would improve the scale set to make it more capable in a general math sense. He saw this rule as one that non-engineers might actually buy, and any opportunity to cut into the sales of the Deci-Trig was favorable to him.

The Model 4083-3 Log Log Duplex Vector went into production mid-1939 at a cost of $13, or $14 with the case upgrade. Interestingly, a 20" Model 4083-5 would be made ready with the roll-out as well, not something that K&E was always ready to do with their non-standard sized slide rules.

Of note, K&E asked Weinbach for approval of a price decrease for the larger rule. According to K&E, the rule could be discounted $5 because the new scale set required much less machine setup as compared to the old rule. Weinbach approved. The Model 4083-5 would be priced at $30, or $31.70 for the plush leather case.

As for the new scale set...

Front side: L, LL1, DF [CF, CIF, CI, C] D, LL3, LL2
Back side: LL00, LL0, A [ B, T, ST, S] D, TH, Sh2, Sh1

The obvious change is the replacement of the SI1, SI2, and TI scales with non-inverted T, ST, and S scales. Being functionally the same, the new standard trig scales are borrowed from the 4080/4081 rules and would become commonplace for all future K&E rules. The hyperbolic trig scales were moved to the bottom rail to make room for an A scale, which the earlier rule lacked. So easy squares/roots could be calculated, as well as basic multiplication and division on the back side of the rule without need to flip the rule over for hyperbolic trig functions. This general math enhancement, as well as the improvements to the log log scales, are what excited Weinbach about the new rule.

With the relationship seemingly repaired, Weinbach easily made money beyond the $625 guaranteed minimum royalty, something that happened the first time in 1936 with the 4093 version of the rule as the economy was leaving the Great Depression. By 1940, the new 4083 would earn double those royalties for Weinbach, where Robinson estimates in his writing that maybe 3000 rules were being sold on average between 1939 and 1942.

It's at this point, in 1943, when the relationship goes bad between Weinbach and K&E, under very strange circumstances. I spare you the narrative of everything here and point you to the Robinson paper, but a condensed version of the story is enlightening...

Weinbach received a visit by a couple of gentleman in 1943, presumed to be from K&E (according to Weinbach's later statements). The gentlemen inquired about the copyright of the rule, what the contract between he and K&E looked like, and when the agreement would end. Weinbach offered the information freely, without too much worry about it. Promising to reveal their associations later (something that would never be stated explicitly), the two gentlemen were certainly from a competitor, likely looking for information as to when their own company could freely produce a Vector rule of their own. Shortly thereafter, Weinbach revealed to K&E, in casual conversation, the happenings within that meeting, whereas K&E immediately stopped royalty payments on their contract. While not obvious to Weinbach - this shows naivety that I find surprising - it was certainly quite obvious to K&E that Weinbach had erred, having broken his non-disclosure agreement with their competition.

Sidebar: The New "Unbreakable" Indicator

Yet another new feature was being applied to K&E slide rules of a variety of types was something they called the "unbreakable" indicator, appearing first described in the 1953 Educational Product Catalog with their pocket rules. This new cursor design would use a clear plastic window replacing the glass in their normal "improved" indicator.

From that point forward, in addition to their pocket slide rules, it would be applied to many others, including the new 4181-3 Log Log Deci-Trig, their classic 4053 Polyphase Mannheim, the Merchant's rules, and the 4058 Beginner's rules. It was not considered as luxuriously appointed as their improved glass cursors and so it was not applied to their top-end duplex rules, at least not early on.

Introduced in 1962, despite not being their "highest quality rule," the Deci-Lon 10 (68-1100) would go down as their best, most powerful rule. Of interest here is the clear "unbreakable" cursor, a feature introduced in 1954 on all K&E rules except for their "highest quality" wooden duplex rules. By 1962, K&E would use it on all their slide rules.

This was not the first attempt at an all-plastic cursor by K&E. Their first Ever-There rules were equipped with an all-plastic cursor based on the same transparent Xylonite used in their drafting triangles and protractors. Prior to that, in the early decades of the century, the same type of indicator would be used on their Model 4058 Student's rules. While good transparency is less important with such tools, it's obviously important for slide rules, and therefore use of the clear Xylonite was short lived. The clear material was would become increasingly yellow over time with exposure to UV light. K&E would supply these rules with the improved glass cursors, first around the 1920s with the Student's series and then in 1936 with the introduction of the new Ever-There lineup.

Of course by the mid-50s, suitable materials for highly functional transparent cursors were plentiful, so swapping out the glass windows began to occur across all slide rule models, culminating with the "unbreakable indicator" described above for all slide rules by the end of the slide rule era.

So what clear plastic was in use here? My guess would be acrylic, which polishes very clear and naturally resists UV light, thus remaining clear over time.

Here we see the scale set change and the removal of the N-prefix to the vector rule. Sh1, Sh2, and Th are the scales that reveal the vector nature of the rule. These scales get swapped to the upper stator rail in 1954, with the SRT scale arriving in 1955. The lower rule dates to 1956, showing both changes.

As a consequence, and through a chance run-in with an old friend who happened to be a patent attorney, Weinbach decided to file suit against K&E, not only for breach of contract for withholding the royalties, but also to sue for back-royalty payments for all the Deci-trig sales over the previous decade. Weinbach never let go the idea that K&E was making money off of his technology without compensation. Robinson estimates the total profit loss to Weinbach was around $190,000, which was a very large amount of money at that time. Of course that figure comes from the fact that the Model 4081 Deci-Trig, which Weinbach claimed also used his decimal-based trig scales, had become their best seller and would remain so for decades later. This estimate also falls in line with the damages claimed in court documents.

The story does not end well for Weinbach. Although his agreement was to end in 1947, the number of sales of the Vector rule increased massively with the return of American soldiers after the war - Weinbach stated himself that enrollment in his university program tripled in 1945 - and therefore he would miss out on his large royalty payments, which would have likely matched his salary from his job. That same year, Weinbach lost his wife, as well as suffered from health problems that would put him in the hospital for lengths of time.

K&E, surprised by the lawsuit, responded with every delay tactic they could, in a way which would greatly increase court costs for Weinbach, but also seek to win the battle by attrition. We know the lawsuit never went to trial. Robinson suggests that a settlement was reached, and through interviews he ascertained that Weinbach's portion of the settlement was likely around $10,000 (with a share going to Puchstein), which is a figure that likely replaces the lost royalties of his own slide rule. Weinbach would die himself mid-1946, with the original royalty agreement ending in 1947. Not coincidentally, both Pickett and Dietzgen would produce American rules that incorporated hyperbolic trig scales in 1948.

Author's Aside: I make no serious judgments about the conflict based on the limited sources we have, though the letters brought to light by Robinson do give me some general impressions about it. First, K&E was a big business, something easy to see here, and their only concern is the financial ledger. When I think of all the outsourced products in their entire catalog, the hundreds of contracts and dealings that K&E had made over 70+ years at that point, the significance of one man, one contract, meant very little to them. Second, I feel that Weinbach acted too often like a man with more resources than he actually had. Often naïve; often unappreciative; even sometimes greedy. Robinson estimates nearly 24,000 of his slide rules were sold between 1929 and 1943, with total royalties paid out to Weinbach at approximately $14,000; this, for a very niche product. Considering the economic era in which these events happened, Weinbach would have had a very comfortable and meaningful life as a college professor. As a professional educator myself, I would consider myself having the perfect career if I were making another 25% to 50% extra income and the satisfaction of having my intellectual property out there as a service to others.

This, of course, was not the end of the Model 4083 Log Log Duplex Vector. 1948 would also signal a redesign of the 4083. Adding the N-prefix to both 10" and 20" models, the rule would shift around the log log scales and add another. It would also move all notations on the rule's front onto the top edge of the slide rule (see image at right).

Note: We have no price lists between 1944 and 1947. The 1947 catalog shows a substantial price increase to most K&E slide rules, especially with their duplex families of rules. The price of the 4083-3 (basic case) slide rule jumped from $13.50 in 1944 to $20 in 1947 and again to $22.50 in 1948 for the new N4083-3 model. The 20" version would jump from $31.50 to $48.50 over that same time frame. Even the nice leather case (an option that puts as "S" suffix on the model number) would cost an extra $3 and $5 for the 10" and 20" upgrades respectively. While I'm not an expert economist, nor an expert on the post-World-War-2 era, I would find further study of this inflation intriguing.

Like the rest of the K&E duplexes, the N4053 models would also shift to the inlaid celluloid edge and stylized K&E red logo in 1952, accompanied by another $2 price hike as well.

Another major change to the scale set would occur in 1954, significant enough to drop the N-prefix from the models. As you might recall, all of the Log Log rules (both Duplex and Vector families) dropped the N- prefix that year, which I speculated earlier might have been because of the new 4181-3 introduction of the Ivorite rule that lacked the "N," where K&E revamped the Log Log Duplex rules to match its scale set. In the case of the Vector rule (shown above), the scales were shifted to place the Sh1, Sh2, and Th scales to the top of the back stator rail. The L scale was removed and a DI scale added below the D scale. New green cover manuals would also come that year (for all their rules), with an additional 8-page supplement to support the new DI scale.

The following year, in 1955, the ST scale would be relabeled the SRT scale, with no other changes to the rule. It would be packaged with a 6-page supplement to the manual describing SRT functionality.

The typical 1962 renaming of the model numbers occurred, with 68-1439 assigned to the 4083-3 and 68-1434 assigned to the 4083-3S (upgraded leather). New numbers for the 4083-5 and 4083-5S are 68-1429 and 68-1424 respectively. No changes to the rule otherwise. Prices are $26.50/$29.50 for the 10" rules and $59.50/$65.50 for the 20" rules in the 1962 price list.

The 20" Vector rule would disappear in the 1967 catalog, with the 10" disappearing from the 1972 catalog. A long run for a very good, versatile slide rule with the most interesting of histories!

Modern Duplex Family

By the 1950s, leading high-end slide rule makers of the age, especially Faber-Castell and Aristo (Dennert & Pape) with their beautiful plastic rules and Pickett with their metallic rules, had transitioned successfully to more advanced materials and construction. Keuffel & Esser would do the same. But unlike the other makers (except maybe Hemmi in Japan), K&E would keep the wooden rules around out of tradition, legacy, customer expectations, and cachet. It is not, after all, like the wooden rules were some how made lesser by the plastic revolution. They are as they always were, long-lasting, powerful, and beautiful tools. Like Hemmi, it would be difficult to let such rules die even though their all-plastic rules were undoubtedly more cost effective.

But by this point, wooden rules were not high profit items. And for K&E, it is debatable if they ever were. Wooden rules, from the first day, were costly to build, especially in smaller sizes where the labor hours were mostly the same regardless of the size of the rule. This meant lower profit margins for wooden pocket rules and fewer customers that saw value in them. Many that K&E did produce over time, particularly the pocket versions (e.g. Models 4031, 4053-2, and 4088-1), would likely have been sold nearly at cost, just so they could fill market expectations.

Because the new 68-1XXX model number convention of 1962 lacked all manner of intuition, K&E helped customers out by publishing this conversion list in the 1962 catalog. Thanks a lot.

Author's Aside: This fact has led some to believe that slide rules were never a very profitable venture for K&E. I would disagree, instead choosing to say that some slide rules made them money while other ones did not. However, it is mentioned by Jack Burton, former VP of Marketing for K&E, that toward the end of the era, profit margins suffered due to the rejection rate of production rules and the company's life-time warranty. K&E, a victim of their own perfectionism, would reject most of the rules they made, as well as to freely exchange rules with customers who had the slightest complaint, even when the rule was damaged, worn by typical use, or the customer couldn't adjust it themselves. Burton mentions that instead of arguing with customers, K&E instructed their distributors and stores not to argue and just replace the rule. It is estimated that as much of 50% of K&Es wooden slide rules would have warranty claims during Burton's run during the 60s and 70s (Journal of the Oughtred Society, Vol. 8, No. 2, Fall, 1999, p. 28).

I will be talking about all of the following slide rules in rather glowing terms. These ABS plastic rules, made of "Ivorite," do not surrender capabilities and functionality when compared to their wooden brethren. In no way was the quality of the rules compromised despite being lower in cost to produce, as K&E made certain that the rules that can still be found today remain functional; a testimony not only to the K&E's quality control but also the endurance of the materials and method of construction. How profitable they were depends on rejection rates and warranty replacements; but that said, the performance/value ratio of the Modern Duplex family of rules are off the chart.

K&E likely could have sold these rules at prices near (or above) their "highest quality" wooden duplex rules. Instead, as we saw with the Doric 9081-3 in the previous section, these prices will greatly undercut those wooden rules.

But before we discuss the several rules in this family, it's important to note that K&E was preparing for another rule to go into production in the 1970s known as the "Ke-Lon." Only a mockup prototype of this rule exist, but the Ke-Lon, prototyped on a Deci-Lon body, was essentially a Deci-Lon with three added scales for hyperbolic trig. All "Lon" scales would have moved to the front of the rule. The idea of this slide rule would have been to produce the "ultimate" slide rule in the sense of unifying all previous Duplex families into one flagship model. However, in my opinion, it would have been just for cachet only, impractical in the sense that few people would find the Vector functionality useful. But K&E did drop the Log Log Vector family completely in previous years, so perhaps it made sense to them to revisit them on this new proposed slide rule? No matter, K&E would be in worse financial trouble by the early to mid-70s, so this rule never really had a chance (see Sidebar: The End of an Era).

But at least the slide rule market was still good for K&E in the 60s. So let's talk about these Modern era rules that DID find success, starting with the replacement to that aforementioned Doric N9081-3 slide rule that would signal the beginning of the new, all-plastic era...

The Model 4181-3 Log Log Duplex Deci-Trig (Jet-Log)

In the 1952 catalog, K&E writes, "Nothing has been found to surpass the combination of selected, seasoned mahogany and xylonite, of which the famous highest quality K&E slide rules are made."

But if the real intent for the new plastic rules was not known, I think with a little more reading we see K&E's full plan...

Product positioning is important. As I've tried to express through the article, any line of consumer goods needs to cover the gamut of possible consumer demand. Since the inception of their rules, K&E had struggled in trying to lessen the cost of their slide rules in order to meet a demand of the every-man. By the 1950s, we see the need for slide rules within the markets of the common person, chiefly to students and to workers at normal jobs, and to those who might find it convenient to keep another slide rule in their pockets. While they would still sell rules to engineers and builders, willing to spend the most for the best, K&E knew that if they could provide plastic rules for ~70% of the price of their "highest quality" rules, and if those rules were functionally equivalent, then they would have positioned all their products to maximize market share.

As such, the Model 4181-3 Log Log Duplex Deci-Trig, first introduced as a repackaged Doric N9081-3 in 1952, would prove to be one of K&E's most important rules. Priced initially at $15 in the 1952 catalog, compared to the $22.50 price of the N4080-3 and N-4081-3 models, we can see the savings to be had with all-plastic production.

Shown in the September "Slide Rules" only revision of the 1955 catalog, the rule would be transformed from the familiar Doric N9081 characteristics to that which we would become more familiar, dropping the N- prefix, dispensing with the Doric label, and adding red ink for the labels and the inverse scales. The catalog description for this rule (and the plastic pocket rules) states that "Ivorite" construction is used. We see this description also in an edited version of the preface that we saw in the previous 1952 catalog (see below). "Ivorite" is trademarked here, which might simply mean that K&E, while their rules were already transitioned to Ivorite with the Doric rules, could now be safely called what they wanted all along. They seem proud...

Judging between the N9081-3 and newer 4181-3 versions of this Log Log Duplex slide rule, they are slightly different in feel. The former seems heavier, stiffer, smoother, more glossy, and more square, and the latter would appear to be lighter, thinner, and more beveled. The color of my samples of these rules are slightly different, with the N9081-3 Doric retaining more of a pure white tone, though I would not make judgments whether or not this is true across all samples of these rules. Regardless, they appear to use a slightly different plastic, both of which are leagues removed from the white Xylonite advertised by K&E as used in their Ever-There rules beginning in the early 1930s.

Of course, as talked about with the Doric rules and as we witness here, it becomes hard to find consensus in what the actual plastic composition is. My thoughts on this, and a little more about plastics in general, can be found in the Sidebar: A Little About Plastics at right.

Scales for this rule were identical to the wooden 4081-3 rule, and would continue to adopt the same scale evolution as it also changed. The N4181-3 (9081 Doric) version of the rule was based on the 1947 Model 4081 scale set, with the newer 4181-3, described in the 1955 supplementary catalog, taking on the same revised, "non-N" 4081 scale set that moved the L scale to the top rail and put a DI scale on the bottom.

Sidebar: A Little About Plastics

In a way, we've been talking about plastic construction the entire time. Celluloid is a bioplastic, produced from a cellulose substance (like cotton fibers), nitrated (with nitric acid), and then mixed with a plasticizer (like camphor). This was originally produced in the 1860s. It wasn't until 1907 when Bakelite, the first all-synthetic plastic, was formed.

Since then, most plastics begin with a fossil-fuel and then polymerized with a compound of substances, synthetically comprising the plastic being manufactured. For example, ABS is an emulsion of acrylonitrile (produced from propylene), a polymerizing butadiene, and styrene. This, and many other modern plastics, are thermoplastics, meaning that they can be melted, molded, formed, and even recycled to become something else.

As we talk about the modern K&E rules and their evolution to all-plastic construction, it should be expressed that the actual composition of plastic being used is never explicitly revealed. This would be true of the plastic industry as a whole. As plastic became more widely used, their composition and methods would have been proprietary from company to company. So as collectors of slide rules trying to understand their evolution, it can be confusing for us.

Our chief indicator is simply what slide rule manufacturers advertise them to be. Over the 40s and 50s, we see a transference of title from Xylonite to "plastic" to Ivorite in the K&E catalog descriptions. We know that the "Ivorite" copolymer is a Acrylonitrile Butadiene Styrene (ABS) resin, which has become standardized across industries for a variety of molded components and products ever since. And we know that "Xylonite," a name first given to celluloid types of compounds (plasticized nitrated cellulose), was a catch-all term used for a variety of celluloid-type evolutions for nearly a century prior.

Joseph Soper, who began a career at the K&E Salisbury plant in 1966, eventually becoming Plant manager, says in his book, K&E Salisbury Products Division Slide Rules, that the molded Cycolac ABS blanks were produced by a company called American Insulator. He states that all such slide rules were made of the resin in 1966 and that earlier molded rules would have been made from any of a variety of nitrocellulose-based compounds, likely because they found it difficult to find a place with enough capacity to produce the rules. (Soper, p. 113)

As such, there would be variance between model lines up until the point at which they could agree on a compound, which probably had more to do with waiting for the plastic industry to catch up to the demand that K&E wanted to fill. As I speculated earlier in our discussion about the Doric rules, the very temporary aspect of that series could be explained in that K&E wasn't ready (or able) to settle on one consistent formula or supplier.

Soper confirms that, stating that the ABS resin was in limited use around the country in 1960. Interpretation of the word "limited" is obviously a matter of degree, so the question does arise, at what point prior to 1960 could K&E find the right supply? Regardless of timing, it does confirm for us that finally settling on a plastic would have been an evolutionary process. And with the Doric rules, they all lack a consistent look and feel, unlike the Modern Polyphase and Modern Duplex rules we discuss here.

For this author, lacking scientific methods for determining exact composition of the rules, the best I have to go by is the feel of the rule in my hands and the way that K&E decided to talk about them. From that standpoint, it would definitely seem that once K&E settled on the term "Ivorite" in their language use, then we are talking about slide rules of very similar ABS resin composition, in this case from the 4181-3 model up until the latest rules of the slide rule era (1975).

This new rule would be renamed the "Jet-Log" in 1960, referred to as such in a pocket slide rule brochure in that same year as the big brother of the Model 4181-1 "Jet-Log Jr.", a very important pocket rule that deserves its own discussion (next). Two years later it would receive the Model 68-1251 designation. It would hang around until the end of the era, becoming the sole Deci-Trig option once all the wooden slide rules disappeared in the 1972 catalog. Despite filing for bankruptcy and ending all slide rules production that same year, the importance of this rule as K&E's lone successor of the long Log Log Duplex line demonstrates how good this rule had become. As the end of that line, I could have presented it entirely under the Log Log Duplex family of rules. But because of its new construction, I have slotted it into the Modern Duplex category.

Model 4181-1 Pocket Log Log Deci-Trig (Jet-Log Jr.)

The pocket version of the 10" Model 4181-3, this 5" Model 4181-1, in many ways, became the more successful rule. This is largely because as a pocket rule, it is just really good!

Like the big brother, the Doric version of it was described early on in the 1949 catalog. But unlike the 9081-3 Doric, the 9081-1 was never actually produced, at least not to our knowledge. It would come into production in finished form, described in the 1953 Educational Products Catalog, looking very much like a smaller 4181-3 (non-Doric version). (Note: The 4181-3 in that same catalog is described as the Doric version.) And by finished form, I mean complete with dual-color scales, red K&E logo, and unbreakable indicator. There is some indication from internet sources that the 4181-1 initially appeared without a model number on the rule; however, I have not seen this to be the case myself. This would also conflict with the aforementioned 1953 catalog that is shown pictured with 4181-1 on the slide.

The scale set was the same as the 10" version, which made it the most powerful pocket slide rule upon its introduction, regardless of make. And its for this reason, from 1953 and a decade following, this rule would be K&E's chief competition against the powerful pocket rules being produced in the United States by Post (Pocket Versalog 1461 in 1957) and Pickett (Model 600 Magnesium introduced in the late 1940s). Priced at $9.25 for the synthetic leather sheath ($1 more for the ungraded leather case), as compared to $15 for the full-sized rule, I'm quite sure there were many consumers that saw the value in going straight to the pocket version of this rule.

Note: The Post Versalog 1461 would seem to be a response to K&E's excellent 4181-1 pocket rule, but it would come 4 or 5 years later. And as a celluloid-covered bamboo rule (by Hemmi), it was expensive to make, priced around $6 more than the K&E rule in 1962. However, there's an argument to made that the competitive threat was coming from Pickett. Their Model 600 Pocket Log Log Duplex rule had been in production since the late 40s, and while it's not as powerful as K&Es rule (two fewer scales), it's very likely that K&E felt some sense of urgency, and that the 4181-1 was a response to that Pickett model. The magnesium Picketts were troublesome, which was common knowledge, but K&E wouldn't have viewed any time to spare. Pickett would replace the magnesium models with aluminum by the late 50s, and their new N600-ES rule would have been very much a competitor against the K&E 4181-1 during the entire decade of the 1960s.

4181-1 Pocket Log Log Deci-Trig - Front

4181-1 Pocket Log Log Deci-Trig - Back

As mentioned earlier, 1960 would bring about a new marketing name for the 4181 rules, now referring to them collectively as the "Jet Log" series of rules. The 10" 4181-3 would be known as the Jet-Log and the 5" 4181-1 would be referred to as the Jet-Log Jr. Strangely, until 1962 when the rules would receive their 68-1XXX model designations, the version of the 4181-1 with the upgraded sheath (with leather-covered clip) would be listed as the E4181J. Even more strangely, only its successor, the 68-1251 with the upgraded leather case would be called the Jet-Log Jr, with two lesser case options, the 68-1282 and 68-1287, known collectively as the "Deci-Trig Pocket" rules. These model numbers would not be printed on the actual slide rule.

In 1968, only the 68-1251 Jet-Log Jr. would be offered, along with the 10" Jet-Log, both outlasting the wooden rules in the Log Log Duplex family of rules, as the classic 4080/4081 legacy wooden rules would be missing from the 1972 catalog.

I find it ironic that K&E would hang on as long as possible to their all-wooden rules, reminding consumers that those were their "highest quality" rules. In retrospect, it was an effort to convince customers that these expensive to produce slide rules were still worth the highest price in their product line. K&E seemed to soften from that in 1962, where the catalog became less descriptive and more matter of fact. By this time, unfortunately for the classic wooden rules, they would not be able to compete with Modern Duplex construction, high quality or otherwise. The Ivorite constructed rules would prove in the end to be cheaper to produce, and I think quite surprising to many, would still maintain a standard of quality deserving to become their "highest quality" of slide rule.

From my collection, here is the K&E GP-12; front side.

The Model GP-12 (68-1565)

"GP," meaning "General Purpose," it is clear what K&E intended with this slide rule: to supply a higher-quality duplex rule than the K-12 Prep rule, with more capability, but not equating to a large price tag. The 1966 K&E catalog, the year it was introduced, clearly intends this toward general math/business/science applications. Note: The rule is marked with a 1964 copyright, so a 1966 introduction might not be totally accurate, lacking a 1964 or 1965 catalog.

Without knowing this upfront, it would be one of the most curious slide rules of the modern era in my estimation. It might still be. Regardless, it is certainly a very functional and attractive rule.

The Model GP-12, also mostly unknown as the 68-1565, is of a duplex design, made of Ivorite, but it would be considered a simplex rule (a duplex rule using only one side for scales). This is only mostly true, however, as we will see. It has a lighter feel than other full-size plastic rules, like the 4181-3 Jet-Log and the Deci-Lons (described next). This is largely due to the use of plastic end brackets rather than metal.

The top and bottom edges are grooved to accept a single-sided, clipped-on cursor that rides in the grooves, but it could have easily been supplied with a full-duplex cursor if they wanted to put indexable scales on the reverse side. Instead, the back is populated with useful formulas and conversions...with a couple of surprises!

Front Side: Sq1, Sq2, DF [ CF, T, SRT, S, CI, C ] D, A, K
Back Side: single decade log scale [ centimeters inches ] L scale

For the front side, it seems like it was created by taking as many scales as you can off of the Deci-Lon rule to fit them into a one-sided rule. Perhaps it's no coincidence, but the arrangement seems to pattern the Aristo 901 Junior {DF [ CF, CIF, CI, C ] D, A, K} only with the double square root and trig scale thrown in.

But the back side of the rule is quite different. It is populated with "Conversation Scales and Factors," but it has a logarithmic scale on the top rail and an L linear scale on the bottom, with centimeter and inch rulers along the edges of the slide. These scales, however, are not calibrated to the front face, nor is a cursor needed. How they work can be seen below...

The back of the GP-12. Note the two uses. On the left, note the inch/cm indicators on both rails. Here the bottom indicator is aligned to 4 inches, whereas the top indicator reads ~10.15 centimeters. So, the GP-12 allows for direct inch-to-centimeter conversions. Or, since these scales are on the slide, it can be removed and used as a 10" (or 25 cm) ruler. On the right side, note the indicators are now on the slide, pointed toward the scales on the rails. As set here, the number 3 is indexed to the bottom log scale. In this case reading ~.477. As such, the log(3) = .477. Experienced users will also recognize that .477 is the mantissa for the log (3 $\times 10^X$). As such, log(30) = .477 + 1 = 1.477, log(300) = .477 + 2 = 2.477, or log (.3) = .477 - 1 = -.523.

As such, the rear of the rule is more than merely conversion tables. It provides what is essentially an L scale for computing base 10 logs and exponentials, and gives easy inch/centimeter conversions. And while it's not designed to do so, because the linear scale on the bottom rail is divided up in inches like the ruler above it, it can be used to do easy addition (and subtraction). For example, in the right image above, the rule is positioned to see that 446 + 925 = 1371, among other sums involving 446.

Price for the GP-12 is unknown since there are no published price lists for anything later than 1962. However, in 1962 prices, there is a large gap between the K-12 Prep rule at $2.25 and the 4181-3 Jet-Log at $21. Those being the two least expensive full-sized duplex rules, K&E likely felt this slide rule was needed to fill that gap. 1966, I would place it roughly equivalent to the Model 4161-3 (68-1576) Modern Polyphase in terms of materials and capabilities. This $11 classic Mannheim-designed slide rule (in 1962), also with all-plastic construction, had 12 scales (as did the GP-12, disregarding the back side), yet it was very much a different rule. So I feel like K&E would have naturally priced the GP-12 around $11 to $13 in 1966 without too much concern that it would steal sales from the Model 4161-3, which would become called the "Jet Math" in 1966.

Afterword (8/17/2024): With the discovery of the 1968 K&E Catalog Price List, the speculation of the previous paragraph is over. And was I ever wrong! The 1968 price for the GP-12 rule was $6.50. This is perhaps half the price anticipated and well under the $11.50 tag placed on the Model 4161-3 (68-1576) Modern Polyphase rule for that same year. Comparing the rules and their prices, it's uncertain how people would ever choose the Jet-Math over the GP-12 at those price points?

The more that the GP-12 is used and held, the more that this user can appreciate the overall utility it provides. I find the rear of the slide rule to be more useful than I first imagined, perhaps because of its simple functionality. Additionally, the rule is really attractive, feels great in the hand, and it gives the impression that it's more akin to a top-end rule than it's price would have certainly been. And speaking of top-end rules...

On the short list of the best pocket slide rules ever made - the Deci-Lon 5 as introduced in 1962.

The Deci-Lon Series

Shifting to a new model numbering scheme wasn't the only thing that K&E accomplished in 1962. They also introduced, arguably, their best slide rule in the company history. It came in both pocket and full-sized versions; the Deci-Lon 5 68-1300 and the Deci-Lon 10 68-1100.

The Deci-Lons were the fulfillment of everything K&E had learned about making slide rules. From the modern Ivorite plastic construction, unbreakable cursor, and a very powerful, 26-scale set, these functional rules could have very well taken over the entire industry. But what made this slide rule really wonderful is its form. This was K&E's exit from modernity, coming to age into what a future slide rule should look like.

This rule throws out tradition entirely. Gone are the right angles and the conservative approach to design.

Now angular, deep-bodied, muscular, and even angry, as if it has something to say. Colored vertical lines on the ends of the slide declare, "Hold me here." We've seen K&E produce innovative rules before, even giving them bold names like Deci-Trig and Jet-Log, but this shouts its name on the rule itself.

Deci-Lon!

Exploring the scale set, we have 13 scales on the front and back each:

Front side: Sq1, Sq2, DF [CF, CIF, L, CI, C] D, Ln0, Ln1, Ln2, Ln3
Back side: Ln-3, Ln-2, Ln-1, Ln-0, A [B, T, SRT, S, C] D, DI, K

There is nothing new here. It's all been seen before...just not this much of it!

Basic arithmetic operations are handled with ease with all the Polyphase Mannheim scales. Efficiency of these operations are improved by a full suite of folded and inverse scales. Squaring, cubing, and rooting functions can be found with high precision with the A, K, and a pair square root scales. Trig functions are handled by the full {S, SRT, T} set, in Deci-Trig divisions (hence the "Deci-" in Deci-Lon). Exponentials and logarithms powerfully executed with eight log log scales based on "e" known as Lons (hence the "-Lon" in Deci-Lon).

It's the perfect, complete general math rule with a cool name in the prettiest package imaginable.

But such a slide rule would not be useful for many if it were priced out of reach. Whereas one might think that K&E's most powerful slide rule would command the highest price, this is not the case with the Deci-Lon. Being a modern rule, based on modern, finely molded plastics and inexpensive production techniques, the rule could be offered at a much lower price point than their traditionally "highest quality" mahogany-based slide rules. The 10" Deci-Lon entered the 1962 catalog with a $25 price tag. This is $4 greater than the Ivorite 4181-3 (68-1251) Jet Log and $3.50 less than the wooden 4081-3 (68-1210) Log Log Duplex Deci-Trig. So from the standpoint of usefulness, I'm not sure how the venerable wooden Deci-Trig - or even something like the famous Post Versalog - could compete? The Deci-Lon was most certainly a hit at that price point.

As for the pocket rule, the Deci-Lon 5, with a 68-1130 model number, was priced exactly half the price of the full-sized rule at $12.50. This should have been a very popular option, as by the 1960s pocket rules were very much in vogue. The 5" Deci-Lon gave buyers even more reason...it's functionally every bit as powerful as the larger rule. Where some precision is obviously lost, many would believe that the trade-off in portability and the huge discount compared to the 10" rule would have made this an easy choice. At least this is my thinking. The reality is such that the Deci-Lon 5 is actually a quite rare slide rule today, very collectible, in fact. As such, I wish we knew how in demand the 5" version really was?

Keeping in mind the popularity of the 4181-1 Jet-Log Jr., K&E would have a complete array of six pocket rules, all reasonably priced, once the Deci-Lon 5 arrived in 1962. So we do know that K&E had consumers covered in pocket rule options!

Significantly, this is a long way from the days when K&E offered a wooden pocket rule (Model 4031) at the same price of its full-sized version (Model 4041). Because production for the modern plastic rules is mostly the same regardless of size, they can be priced more in-line with the cost of actual materials used. This was not the case with the wooden rules, which required longer production times across the board, which meant the pocket rules also had to be priced to cover the labor involved. When you recall that very few pocket rules were made by K&E until the 50s, and that one of their longest running models, the 4053 Polyphase Mannheim, didn't receive a pocket version until the Ivorite rules in the late 50s, then it should be clear the advantages of plastic when six pocket rules appear in the 1962 catalog.

The Deci-Lons would stay in production for a decade, largely unchanged, until the early 1970s when K&E removed the vertical stripes (grips) from the ends of the slide on both Deci-Lons and ceased putting the actual model number on the body of the rule. The latter point is not unheard of, as there were many instances in the company's history when a slide rule did not show a model number. But the disappearance of the vertical stripes, which in my opinion are one of the interesting and distinctive features of the Deci-Lons, obviously requires some reasoning. The popular story there is that the mold for the Deci-Lon was damaged on one of the slide's ends, so instead of fixing the mold, they dispensed with the vertical stripes altogether. Honestly, I am not sure I concur with this explanation as I write this, since a new mold would be needed to omit the stripes anyway. But I have a right to change my mind upon further research.

Yet another curiosity with the Deci-Lon is something I mentioned when talking about the Model 4098A pocket rule in the previous chapter. These rules are not intended to be used with a reversible slide, though it could be physically removed and flipped over, just as with the majority of duplex rules regardless of manufacturer. However, on a Deci-Lon, doing so could have caused an issue with the way the scales lined up front-to-back, as indexes often do not line up with the slide in the reverse position (something I also noted with the Model 4181-3 Log Log Duplex). The amount of error is small, but noticeable. It is a curiosity, but can be explained by the cast nature of the plastic rule. Such rules were no longer "engine-divided," but rather are imprinted in the casting process. Certainly there can be front-to-back error in the mold that wouldn't have been there by using a traditional dividing machine. But this error doesn't affect the rule functionally unless the slide is used inverted. And I suspect K&E knew this, likely hoping that the angular ends of the slide would mean that users couldn't have been confused about the slide's orientation.

However, somewhere in the middle of the production life of this rule, K&E altered the tongue & groove joint on the slide to assure the slide can only be slotted one way. This shift in construction does not seem to align with the aforementioned delineation of the two variants, as two samples of my Deci-Lon 5 slide rules, both with stripes and model number on the rule, are different in that one (SN 011482) allows the slide to be inverted and the other (SN 021968) does not. Same story with my many Deci-Lon 10 samples, though it seems the change was made closer to the same time of the change in the striping. The reason for the change, as I speculated with the 4098A, might have been to counteract a proliferation of warranty claims that might have come up in later years from customers who felt that the scales should be perfectly aligned no matter how the slide was oriented. Keep in mind that the traditional duplex rule didn't really care how the slide was oriented. It would have been usable and accurate either way. I can see how users, new to the resin-cast rules, might have been surprised the first time they tried using the rule with an inverted slide!

I believe that K&E learned that if the user cannot physically invert the slide, then it's unlikely the user would ever know of any misalignment when the slide is incorrectly positioned, nor would it generate any public concern that the newer plastic rules breached a century's worth of K&E's high standards. Ignorance, in such cases, would be blissful for K&E, who we know was losing a lot of profit to warranty returns because of the overly generous return policy, as I noted in the "Author's aside" at the beginning of this section.

For the moderately serious collector today, the Deci-Lon is one of the first rules that can be found in their collection. It's unique among all slide rules from the standpoint of design and represents the pinnacle of what K&E learned about slide rules over their century of existence. The 10" Deci-Lon 68-1100 in either variation should cost between $30 and $100 on eBay depending on condition and completeness. They are by no means rare - there are 24 samples available on eBay as I type this - but they do command a healthy price for the most part. On the other hand, the 5" Deci-Lon 68-1300 pocket version is much more rare, appearing perhaps monthly on eBay. Prices will be set between $100 and $150 for the slide rule with leather sheath, and often much higher. This seems about right for something that just might be regarded by the majority of collectors as the best pocket slide rule ever made.

Sidebar: The End of the Era

If you were an engineering student at a university in 1972, you would have entered your first year with a slide rule in hand. By the time you graduated, your slide rule would have been stowed away in your desk, giving way to the new Hewlett-Packard or Texas Instruments hand-held calculator that had become affordable enough to own.

The end of slide rules happened that quickly - and without remorse. There was no asking whether or not switching to electronic calculators was a good idea? Or whether something would be lost by the dismissal of the slide rule? All that mattered was that computations formerly done on the slide rule could now be done with 8 digits or more of precision, all without having to track the decimals yourself.

Speed? People of the era would say that they could do computations on the slide rule just as quickly, but being a generation removed from the era myself, and only now learning my way toward slide rule proficiency, the calculator is most certainly faster and more convenient.

Precision? How precise do you really need to be? We round our answers routinely to the hundredths or thousandths anyway.

As a high school math educator for 28 years, I now see where we have missed the slide rule pedagogically. Numeracy among students is far worse than it ever has been. My students give little regard to magnitude and significance to the output of calculator computations, not knowing if they got the question right until they see the amount of red-ink on their graded exam.

Much of this could have been prevented during the analog to digital transition. Traditional pedagogical methods could have held, withholding calculators from young students until they understood the principle of "garbage in, garbage out." Teachers could have continued to insist on deriving estimates to solutions a priori. Our school system could have hammered the rules of numbers and their operations solidly prior to letting them be lazy with a calculator. Or better yet, educators could have kept teaching children the slide rule to give them a better sense of numbers, operations, proportions, fractions, arithmetic rules, and reasonableness of solution within math problems.

I recall a time early in my educational career when the importance of "manipulatives" were stressed, to supplement understanding of material with hands-on, visual reinforcement. And while today's "apps" on our smartphones can somewhat do this, it's in no way standardized in an educational system still trying to push TI graphing calculators on the kids, all because of a huge lobby at the state level to assure that they do not go away. Where were the slide rule lobbyists in 1973?

All rants aside, there are so many advantages to old slide rules in modern education that I wrote an interesting and detailed article about it linked here, if you are so inclined.

Despite going public on the NASDAQ in 1965 and advertising their biggest sales year in 1966, Keuffel and Esser rapidly declined shortly afterward.  Slide rules were in no way their big money maker, but these were still going strong into the 1960s. It was their technologies in other fields that were being supplanted by improvements. All of their traditionally analog products were quickly becoming obsolete. Their surveying instruments, transits, and theodolites were giving way to digital equivalents, which would be 5 to 10 years ahead of pocket calculators. So while we tend to think the digital world began with pocket calculators, that is only true for the general consumer like us. Within industry, digitalization had already begun it's transition.

K&E would continue selling slide rules until 1976. But for the most part, their production of slide rules had abated beginning in the late 1960s. By that point, they were selling from massive amounts of inventory that they has accumulated, which is likely why, after 1962, there are so few K&E product catalogs and price lists to let today's collectors know what was actually going on during the late slide rule era.

Likewise, when we say that a model was still being offered as late as the mid-70s, this is only because they still had inventory that they hadn't been able to sell. So when we see that K&E still offered a particular rule in 1976, then that means either the rule was less desirable than others that sold out long before, or they were top selling rules in prior years that K&E had produced in over-abundance.  But all we need to do is count the amount of slide rules offered in the 1967 Catalog (29) and compare that with the 1972 Catalog (13) to know how much life was left in the venerable slide rule.

K&E did not die over night. They would stave off Chapter 11 bankruptcy until 1982. They hung around for 5 more years before selling their existing intellectual properties and products to the Azon Corporation in 1987. After other transactions through the years, K&E's optical tooling technology eventually found its way into the hands of Brunson Instruments Company, a competitor to K&E in that field since 1927. Today, Brunson still sells a line of products based on the original K&E designs.

Many of K&Es products have become collectibles, not only their slide rules. But while K&E would always reap the most profit from the rest of their consumer goods, they would be best acknowledged by the general American public for the long history of excellent slide rules.

The Analon (68-1400)

One of the more unusual slide rules ever produced, and certainly a pinnacle rule for slide rule collectors, the Analon was introduced in 1967 as a slide rule for doing dimensional/numerical calculations within Engineering-Science applications. Simplex in design (duplex with single-side use only), the Analon was very similar to the GP-12 in build, with gold-colored plastic brackets instead of the GP-12's black.

With A, B, C, and D scales in their traditional places, the rule was populated with 7 other scales: three U scales on the slide and 2 V scales on each of the rails. These U and V scales had any of 30 different dimensional and physics variables (with a legend for these on the back side of the rule). These could be used to confirm the formulas that comprise those variables. In other words, it helps keep dimensional units straight when you do lengthy computations. At a more basic level, a formula like F (force) = M (mass) * A (acceleration) can be confirmed on the Analon by placing the C-index at "M" on the bottom rail's V scale, moving the cursor to "A" on the slide's U scale, and reading "F" off of the same V scale, thus confirming the formula.

The image below of the 68-1400 Analon shows this well, courtesy of Miguel Ramirez at his highly recommended "My Rules" website.

Without getting too much into the design history and theory, to make this happen with 30 of the typical variables in science & engineering fields, while keeping all the scales spaced out and readable (increasing accuracy), was quite the feat. As such, the scales are no longer logarithmic, but rather affixed linearly within a confined 10" space. In other words, it's easy to imagine starting with scales from scratch and then positioning the F, M, and A variables on the rule so that they perform the same example above. But how do you place 30 other variables for all the other formulas you might need to confirm while keeping them equally spaced on the scale and, importantly, keeping them on the 10" rule? For more on this, I encourage you to read Cliff Frohlich's article for the Fall 2014 Journal of the Oughtred Society.

Design considerations aside, as remarkable as they are, it's the collectability of this rule that has made it popular. In truth, it is a "holy grail" for collectors. I still do not own one as of November 2022 (edit: nor yet in August 2024), though I've passed over a few that have come up on eBay, as I'm not ready to spend $300 or more on one. But why so expensive?

The Analon was only produced for one year, 1967, in very limited supply; perhaps 600 to 1000 samples. It is said that double that amount were manufactured, but half of them did not pass quality inspection due to errors with the painting of the rule. When you couple so few samples of the Analon with its very unique design, its something very desirable for collectors. For myself, I will remain hopeful to stumble onto a bargain somewhere.

In a very large sense, this is a specialty rule, and had K&E continued in its manufacture after 1967, the question becomes if it would have caught on? In Frohlich's article, after listing six specific flaws that might have ended the Analon's production, he summarizes it for us thusly:

"Nevertheless, the design of the Analon was highly innovative. If calculators had not replaced slide rules, other dimensional slide rules probably would have appeared, possibly targeting specialized audiences, such as chemists and even earthquake seismologists. Undoubtedly slide rule manufacturers would have experimented with other layouts and designs. Because there is a certain ‘geek market’ attracted to slide rules with numerous scales, dimensional scales-either like or unlike the Analon’s U and V scales might have been added to some of the more complex slide rules. However, these events did not happen, and the Analon is unique—there is nothing else like it."
- Cliff Frohlich, Journal of the Oughtred Society, Fall 2014, p. 27.

Had the Analon not been produced so close to the end of the slide rule era, I could imagine that other slide rules would be produced, the K&E product line would have shifted, and we might be thinking today of a different way of categorizing all of their slide rules historically. But despite being far from a general-purpose rule, it was a still a Modern Duplex family member very much akin to the Deci-Lon and GP-12. It made sense to talk about it here, instead of treating it like the specialty rule that it is, which becomes the subject of our next chapter.

Afterword (8/17/2024): I had not tried to speculate on a price for the Analon rule because its 1967 introduction was well after the last known 1962 price list. However, now that we know 1968 pricing, we can deliberate a bit more in that regard. The original price point was set by K&E at $18.35.

Comparing this to other K&E rules of modern construction, the very similar GP-12 was $6.50 and the 10" Deci-Lon was $28.50. This shows great potential profit margins to the former rule, yet is much more attractive in price compared to the latter rule...or in fact, to the venerable Model 4081 Log Log Duplex (68-1200) mahogany rule which would command a price more than $6.25 higher.

Compared with the other wooden K&E specialty rules of the era, the Analon was priced MUCH less than the Model 4083-3 Log Log Vector ($33.25) and the Model 4139 Cooke's Radio rule ($29.75).

Perhaps the best price entry comparison is with K&E's Model 4143 Kissam's Stadia rule priced at $20.50 in 1968. Both all-plastic and both specialty-purpose rules, K&E seemed to regard them as similar both in terms of market size and profit margin.

Introducing my recent find of the 1968 price list to subscribers over on the "Slide Rule Fan Club" Facebook group, collector/owner of an Analon, Bob Cherry, recalls the purchase of his sample in 1978 at the price of $35. He states that his came in a box complete with rule, case, hardback book, and quick-use guide (on paper). This would make sense, as K&E was doing the same with the Deci-Lon, Jet Logs, and Jet Maths by the early 70s.

I appreciate Bob's candor on this, as it becomes easy to see how a boxed-set of those items, costing collectively around $25 in 1968, would have increased by $10 by the end of the 70s. Bob confesses that he "wasn't searching for an analytical rule" and that he bought it "as kind of a novelty." Apparently, what Analons remained by the end of the 70s were already being considered collector's items shortly after the death knell to slide rules sounded.

It also brings about another thought as it concerns the collector's outlook some half-century later. With their venerability and demand among slide rule enthusiasts today, I am often given to think that a slide rule as fabled as the Analon MUST have sold with minutes of coming off of the product line. But apparently, if full boxes of such a limited edition rule are still hanging around in shops more than a decade later, then it yields surprising illumination for me. In such a case, I love that my own bias in that regard is proven in error.

Chapter 4: The Specialty Rules

The majority of slide rule products sold by Keuffel and Esser in the 19th century were actually highly specialized tools already developed by others. This was due to K&E's growth focus on becoming a sales distributor of products they felt aligned with their major markets of engineering, construction, and design. And keep in mind that it wouldn't be until the 20th century until K&E felt there was a market toward the every-man consumer. They built their foundation on providing quality products and tools to industry, those that would be building and rapidly expanding the U.S. after the Civil War.

To this point in our survey of K&E slide rule products, we have covered only those slide rule products that resonate with those who practice general mathematics. While there were certainly engineers in need of a Log Log Duplex slide rule, there are a wide variety of specialty products and slide rules that must be addressed in our effort to be complete. Beginning in 1901 with the introduction of the 4XXX naming convention, K&E started using 41XX series numbers to denote many of these specialized types of slide rules, particularly the linear rules. The notable exceptions to this naming scheme are many of the cylindrical or circular types of rules, as well as the Merchant's Family of rules, all of which carry 40XX model designations.

To differentiate this chapter from the one that follows, Chapter 4 focuses on Specialty rules within specific families, as K&E historically cataloged them. As such, these families of rules represented a longer history of specialty types, including a variety of rules intended for merchants, surveyors, electricians, engineers, and industrial workers. This chapter will also represent those items that fit well into a category, such as K&E demonstration rules, sectors, and rules related to chemistry or medicine. This is unlike Chapter 5, which highlights unique, one-off rules and other designs that wouldn't be cataloged within a "family" of slide rules.

As we discuss the Merchant's Family of rules first, which comprises the 4094, 4095, and 4096 models, it will be easier to discuss them in a narrative form rather than by a rule-by-rule breakdown, mostly because there are so few models to describe. However, other families will breakout each slide rule by individual model type.

Not likely the original manual for the Merchant's Family of slide rules, but can we be so sure?

The Merchant's Family

In 1915, K&E introduced a new duplex rule for those that might be confused by so many scales. Seriously, this is what they state in the catalog. They suggest that businessmen, accountants, merchants, mechanics, foremens, and the like, need a rule that doesn't detract from what they need the slide rule to do, namely "multiplication, division, and proportions." The Merchant's duplex rule, the Model 4095, was designed to do this. 10" long and built with the same form factor of the Model 4088 Polyphase Duplex introduced just two years before, the 4095 sports a very simplified scale set of DF [CF, C] D on the front side and only two scales, the CI and D on the back. This scale set is essentially patterned after the "règle des écoles" rule that Tavernier-Gravet introduced nearer the turn of the century.

Of course, this scale arrangement focuses only on C and D scales, which are the multiplication and division scales. These are enhanced by folded scales on the front side, which allows any such computation to have an index that is always at the center of the slide rule, which assures that a product or quotient is not "off the rule," a common happening on rules without CF and DF scales. And naturally the C and D scales can do conversions and proportions by virtue of being one decade logarithmic scales. This four scale arrangement on the front of the rule allows for chains of multiplication and division operations to be done more efficiently and quickly, seemingly something that merchants might want to do. This includes the three-number multiplication technique mentioned earlier in my writing.

Thus, technology for this rule was nothing new, as they'd been developing upon the same industry standard of efficiency for the previous 20 years, beginning with the Cox-patent Original Duplex rule. K&E understood the virtue of using those scales to promote greater simplicity in operation on this Merchant's Family of rules.

The back side, with only a CI scale on the bottom of the slide and another D scale on the bottom rail, allows users to invert their multiplications to work more like a division problem, without any distractions from other scales.

Strangely, with only two scales on the back there is a conspicuous amount of empty space on the rule. K&E intended for users who do commonly performed conversions or computations to use this void on the rule to mark their own "gauge-points" to align with the index marks. Seems like a great idea, though it does make for a sparse presentation on the back side of the rule (shown below).

The Model 4095 came with a frameless glass indicator and a "Morocco" case (box) at a cost of $4.50, which was also the cost of the Model 4041 Mannheim. For further perspective, this is the least expensive duplex rule when it was introduced in 1915, with the Model 4088 priced at $7.00 and the Model 4092 at a dollar more. The older Duplex Family of rules, two years before being discontinued, were also available in 1915 at 50 cents more for the base version and $2.00 more for the "T" or trig version. The 4095 also beat the Model 4053-3 Polyphase Mannheim in cost, coming in at 50 cents less.

In 1922, the year of the introduction of serial numbers and the celluloid-wrapped edges, the 4095 was renamed the Model 4095-3, with 5" and 20" rules being added, the Model 4095-1 and Model 4095-5 respectively. The 4095-3 was $5.50 that year, with the case-upgraded "S" version at $6.35. The 4095-5 and 4095-5S were $13.00 and $14.50. The 4095-1 only came in as an S version, priced the same as the 10" model (with basic case). This 20" Model 4095-5 rule would only last 4 years and is a quite rare rule, with only two of them popping up on eBay since 1999 at a cost of ~$50, though because one hasn't been listed in a dozen years, it might sell for as much as a Chevy. The 5" Model 4095-1 would endure alongside the 10" version until 1943; however, it is almost as rare as the 20" version, popping up on eBay every couple of years, though with an average cost of around $75.

A year after the 4095-5 was discontinued, it would return in a different guise in 1927 as the Model 4096 Desk rule for $18 (see below). Enclosed in a nice wooden case, and able to be worked IN the case, the Mannheim-style rule was much larger in cross-section than the typical Mannheim, with attached metal feet that lifted the rule off of the table at an angle, yet solid enough to be worked with one hand (freeing a hand to write down computations). A knob on the slide allowed for slide movement. The scales were a repeat of the Model 4095's front side, with DF [CF, C] D. There were no scales on the back of the slide. In 1930, the desk rule would become the Model N4096 when it added a CI to the middle of the slide. This made it a more complete Merchant's model, though it wouldn't not be classified as such in catalogs.

Note: This rule is not to be confused with a very early All-Metal Mannheim Model 4096, a rule we will discuss toward the end of our text. It is an absolute unicorn and has nothing to do with the Merchant rules.

If we go by the product catalogs, then the Merchant's Model 4094 was born in 1930. However, there are samples with serif-font that date as early as 1927, albeit not in the 1927 catalog or 1928 price list. This was a standard Mannheim-formatted build out of celluloid-covered mahogany, like the current Model N4041-3, only with the Merchant's scale set. In fact, it was essentially the same rule as the N4041-3 in all regards except the scale set, DF [CF, CI, C] D. Inch and centimeter rules were on the top and bottom celluloid-covered edges of the rule. There were no scales on the back of the slide. Price of the rule was $5.00, which was 50 cents less than the N4041-3 for that year.

The Model 4094 and 4095-3 Merchant's rules with simplified scale set, both from my collection.

For 1939, K&E would add the Model 4096M, which was identical to the 20" desk model without the metal stands and metal knob on the slide. Thus, it was hand-held. Priced at $15 in a Morocco case, which was $5 less than the desk model for that year. I actually have this rule with a 1938 serial number in my collection. So while it wasn't in the May 1938 price list, it's a reminder that their first catalog entries may, or may not, be when the first samples first arrive!

1943 would signal the end of all remaining Merchant's 4095 models as well as the end of the N4096 desk version. The 4096M actually was rebranded as the "desk" model, but without the case or leg stands. It, and the Model 4094, would remain until they were discontinued in 1947. Strangely, these slide rules appeared in the 1947 catalog, but indicated that their were "temp. disc." The Model 4094 would indeed return in 1948 and the Desk Model 4096M returned a year later, along with the old N4096 version on the metal stands and with the desk presentation case. Albeit, once the case version was reintroduced, the 4096M version disappeared the following year.

There had not been a pocket Merchant's duplex model since 1943 and never a pocket model on a Mannheim build (unless you count the Ever-There 4097B which was functionally the same). Of course, in 1949, this is where the Merchant's Doric rule is first introduced. The 5" Model 9050-1 Doric filled that gap nicely, though as I mentioned in the discussion about the Dorics, this version would not hang around long, being reclassified in 1950 as the Model 4150-1. This was the same rule as the Doric, keeping the Doric label on the rule for a couple of years prior to dropping the label altogether. At this point, the 4150-1, 4094, and N4096 would remain in some form until 1972 when all but one of the Merchant rules disappeared.

In 1954, the 4094 became the N4094, taking on the same transformation in construction as the Model 4053-3 Polyphase Mannheim, gaining a plastic body with printed on conversion charts and the unbreakable cursor. The "N" designation was only in the catalog, as the rule remained labelled the "4094."

In 1962, these rules would receive their 68-1XXX series numbers. They would also be marketed as "Business" rules, with the N4094 being the only "Merchants" trademarked rule. The 4150-1 and N4096 would have two case options each. Prices at this point were $5.25 for the 4150-1 (68-1791) and $6.25 for the 4150-1C (68-1786) with clipped case. The N4094 (68-1775) cost $16.50 with only a synthetic leather option. The Desk Model 4096 cost $29.50 for the 68-1754 with synthetic leather case and $39.50 for the 68-1749 with wooden presentation case. The 4150-1 would be the only rule to make it past 1972, however. It would be sold until the end of the slide rule era in 1976. As I mentioned in the "Sidebar: The End of the Era," it's not that the rule was that much more desirable at the time; instead, it's likely that K&E was trying to off-load their old stock of these rules as long as they were still in business.

On the whole, the Merchant's Family models are quite collectible. It's difficult to find any of the models today except for the post-1922 Model 4095-3 rule and the 4150-1 plastic pocket rule (either might be between $15 to $30 today). The pre-1922 non-N version of the 4095 will more than double the price of the later rule because of its scarcity. And as I mentioned previously, the 4095 in other lengths are especially scarce, so the next one sold might cost more than most collectors would be willing to pay. The Model 4094 in the Mannheim format is not as rare, but has been difficult to find in recent years. It typically sold for around $20 or $30 in the past, but as I write this, three such rule sit on eBay with "Buy-it-Now" prices of over $70 and another, the all-plastic N-version, is priced at $57. Finally, the Model 4096 in both hand-held and desk version will run somewhere in the neighborhood of $100 and $200 respectively. They are most certainly desirable slide rules, though they are more common than you might think. The high price is more because of the size (and novelty) of those slide rules.

In summary, the Merchant's Family of rules helped professionals and trades-people who needed to do only a limited type of computations on a daily-basis. It's like a 4-function calculator in a world of scientific calculators. From that standpoint, the much cheaper Student's rules of the Model 4058 line are actually more powerful given their full Mannheim scale sets. But given the build quality of the Student's rules, it's easy to see that people would want something even more basic, yet still luxurious, nice, and smooth. Such was the appeal of the Merchant's Family of slide rules.

The Stadia Family

Next to the original Mannheim family of slide rules, the earliest offered by K&E are rules from their Stadia Family. Recall that early demand for K&E products was from an industry that was actively building and expanding the country, which meant that land surveying was big business. A method known as the "stadia method" for surveying was the most advanced method used up until the technological age of laser range-finders and GPS, requiring optical tools for which K&E, of course, would provide. With these tools, a survey crew could measure the height and distance of any object given any "line of sight" to which the optical system is capable. The stadia method is still in use today, as the method is simple and accurate enough for a variety of range-finding and topographical applications, without having to spend thousands of dollars on automated surveying tools.

A lengthy understanding of stadia surveying is beyond the scope of this article; however, perhaps we can gain quick understanding of the process in order to know how to use most any Stadia slide rule. The important variables and factors are shown below, courtesy of Mike Syphers' amazing and comprehensive website.

In order to find both the height and distance of an object, four inputs are needed, with one derived from the first three observations:

1.) The angle theta $\Theta $, given by the tripod of the sight device.
2.) The angle alpha $\alpha $, given by the fixed measure of the optics, typically .010 radians. This is the fixed angle between two stadia marks in the optical field of view.
3.) The delta-y ($\Delta y $), given by the linear measure of a stadia rod/board with gradations measured in inches, judged through the sight device between the upper and lower stadia marks.
4.) The measure of "R" or "r" computed by dividing the $\Delta y $ with the angle theta. This is a simple inverse-tangent function.

On a slide rule, certainly on any of the K&E rules, an R scale for the sight distance will typically be on the top rail, with an A scale on the bottom rail. Both these scales are the same two decades of logarithmic scale, though A is marked 1 to 100 and R is marked 10 to 1000. Given such a rule, situations between 10 and 1000 feet are required. On the slide are a V scale on the bottom and a combination H (right side) and V scale (left side) at the top. These signify "horizontal" and "vertical." The V part of the scale on the upper slide is for measuring large angles of theta up to 45 degrees, with an entire bottom V scale allowing for small angles, yielding the ability to measure possible vertical angles in degrees and minutes. The H scale runs backwards from 0 to 45 degrees, with limited resolution.

The scales are derived from the formulas for horizontal and vertical displacement (defined later). The scales increase in angle from left to right, marked in black, but the H scale also has angles marked in red running from right to left. The black and red angles meet at 45 degrees around 1.5" inches from the right of the scale.

To use the stadia rule, you would first compute the R input from the inputs given you by the survey measurement, which is the amount of feet measured on the stadia rod divided by the .010 radian standard of the sight device. An easy example is if $\Delta y $ is 2 feet at an angle $\Theta $ of 15%, then "r" would be $\frac {2} {.010} = 200 ft$. On the slide rule, the right index of H/V would be set to 200 on the R scale. The cursor can be set on the H/V scale for the angle $\Theta $ (15 degrees in our example). When set on the LEFT side of the 45 degree mark, the resulting reading (off the R scale) of 50 feet is the vertical height gained over the distance. When 15 degrees is set on the RIGHT side of the 45 degree mark, the resulting reading (off the R scale) of 187 feet is the horizontal distance to the site being measured.

For grins, computing the hypotenuse (r) using the displacement figures to compute a new "r" allows you to know the total distance reduction given by the change in sigma (tilt of the stadia rod) for the measurement. In our example, $\sqrt{187^2 + 50^2}$ = 193.6 ft., where we would expect closer to 200 ft. given smaller angles of theta. Certainly, there are also slight errors given by reading the slide rule, as the horizontal displacement when plugged directly into  ($r * cos \Theta sin \Theta$) yields 186.6 feet, or ~5 inches off of the 187 ft. measurement we read. Not too bad!

For smaller angle of theta, the V scale would be used, as it is finely etched for angles of 3 minutes of arc to around 5.5 degrees. When this is used, the height is measured off the A scale, at better precision than using the H/V scale. For smaller angles of theta, the horizontal distance would approximate whatever R was, so that would not be computed on the slide rule.

More on the math: When given the hypotenuse (r) of the larger right triangle (in the above diagram), finding distance would typically be $r * cos \Theta$ and height typically $r * sin \Theta$, but the measure of $\Delta y $ assumes it is orthogonal to the sight device. Unless it is a level measurement (0 degrees of rise to the target), then there will be error caused by larger angles of "theta." As such, the computations for height and distance change according to some reworking of the trig, a process known as a "reduction." And this is where the stadia rule is designed to help, as it will compute the actual vertical ($ r * cos ^2 \Theta $) and horizontal ($r * cos \Theta sin \Theta$) displacement, or height and distance. On most of the K&E rules, this will be labeled clearly by $cos ^2 \Theta$ on the left side of the rule, to remind users that measuring to the left of 45 degrees is the vertical reduction. Likewise, it will be labeled $\frac {1}{2} sin 2 \Theta$ on the right side (the equivalent of ($sin \Theta cos \Theta$), indicating that values to the right of 45 degrees measures the horizontal reduction.

As such, a crew of two men, one operating the sight device and the other holding the stadia board can map out the topology of any area from any single spot within a sight radius of 1000 feet. Math is a wonderful thing...and a stadia rule makes it better!

Armed with this discovery, let's look at members of the Stadia Family over the nearly 90 years slide rules were produced by K&E.

The Model 1749 Mannheim Stadia

K&E's entry into stadia rules occurred in 1895 with this 20" Mannheim-style rule, Model 1749. Several items were designated the 1749 at the end of the 19th century, including another stadia rule, their beginner's rule, two sewer rules, and their sector rules, all with dash indicators. Being the first 1749, the Stadia rule was dash-less. This slide rule, of course, gave way to the 4XXX series of in-house rules in 1901.

Priced at $13.50, which was $3 less than the 20" Mannheim Model 1746 they sold at the time, the 1749 Mannheim Stadia was most certainly constructed on the same frame, with celluloid-faced mahogany (probably), imported from Dennert & Pape. At least this is the likelihood since there are no known lasting samples of this nearly 130 year old slide rule. Thus, if you find one, you might well have won the lotto.

The scale set is unknown, though based on drawings in the 1901 catalog of the successor Model 4100 Stadia rule (see next), as well as known samples of that rule, the 1749 was likely the same. Instructions, as on the 4100 model, were likely printed to paper and affixed to the back of the rule. The back of the slide would have been empty, although later models will add a B and C scale for doing general math computations and proportions. As such, this rule, and all stadia rules until 1913 (when the B and C scales are introduced), would have not included a cursor.

The 20" N4101 Stadia Surveyor's Rule. I stumbled upon this mint rule with case recently. Dating from 1943, produced during World War 2, the very lengthy single-sided Mannheim formatted rule carried an asset sticker on the back from the US Geological Survey. A wonderful piece of history!

The Model 4100/4101 Stadia Series

In 1901, the Model 1749 Mannheim Stadia was re-designated as the Model 4101 Stadia. This 20" model would endure until 1952. But it would be the 10" version, added that same year that would become K&E's longest lasting slide rule, being produced in some form until 1960.

The scale set on this new Model 4100 Stadia rule, like the forerunner Model 1749, was R [V/H, V] A. Nothing changed in the way the rule worked. Directions were spelled-out on the back, printed on paper stock, and glued to the rule. This rule was also blank on the back of the slide, and was thus without a cursor, as once the index of the slide was set to the stadia measure's R-value, then it was easy enough read the rule from the measured elevation angle.

The rule would change completely in the 1913 catalog, advertising a reversible slide with a new B scale on the back, located on the bottom of the slide. This, when used with the two decade A scale on the bottom rail gave the rule a general purpose application for multiplication, division, and proportions. For the first time with a stadia rule, this catalog pictures and describes a glass indicator. This cursor would have the metal frame.

Both rules were offered in a Morocco-covered case (box) for $4.50 and $12.50 for the 4100 and 4101 respectively. K&E did sell cases a la carte at this time, offering a sewed leather option for $.90 and $1.40, as well as a sewed case to accommodate a magnifying indicator. The cases were $1.40 and $2.10 for any 10" and 20" rules. The magnifying indicator for these rules was an extra $2 and $2.50. Other than adding the frameless glass indicator in 1914 (metal rails) and 1915 (plastic rails), both rules would remain unchanged over the next dozen years.

Both rules would be greatly improved for 1925. The Model N4100, priced at $6.50 and sporting a new N- prefix, added an HC scale in the middle of the front slide and a C scale to the back slide.

The C scale is obvious. As a single decade, it would now work with the A scale for squares and square roots. The HC scale, however, is not so obvious. This scale, very smartly I might add, allows for more accuracy with horizontal measures, akin to what the small angle V scale did for vertical accuracy. It too, like the bottom V scale, is keyed to the A scale. It is laid out with angles from ~1 degree, 48 minutes up to almost 18 degrees, 25 minutes. This works as an offset to the stadia measure (R value) used. As an example, if the reading is for 5 degrees elevation (theta) at 500 feet away (R-value), then setting right of 45 degrees to 5 degrees on the H scale, there's around a foot of error for the horizontal reduction. However, when setting the cursor to 5 degrees on the HC scale, the A scale reads ~3.77. As an offset, this number subtracts from the 500 feet R-value to yield a much more accurate horizontal reduction of 496.23 feet. Remarkable!

As a further bonus, the rule would also add celluloid to the edges, with an inch scale on top and a centimeter scale on the bottom. Because general math computations are read from the two decade A and B scales, resolution suffers compared to the normal Mannheim, but with the added utility of this N4100 Stadia, it would have been hard to be disappointed.

The 20" would not add the N- prefix until the 1928 catalog, and while the 1925 catalog states that the 20" rule is identical to the 10", there seems some discrepancy with known samples of the rule, as the 4101 appears to have waited until 1928, not only for the new prefix, but also for the new scales. I cannot concur however, since the price of the Model 4101 did not change from the 1925 price of $16 when the N-4101 was introduced in 1928. Curious indeed.

There were S options for both rules in 1928, with the N4100S costing an $.85 premium and a $1.50 extra for the N4101S.

Other than the typical cursor improvements in the mid 30s, both rules would remain unchanged for two decades. As mentioned, the N4101 would show as "temp. disc" in the 1949 catalog and subsequent price lists over the next three years. While some rules come back from that status, the 20" version of this rule never did, but apparently there was enough back stock to sell until 1952, as known samples age to that year.

The N4100 would continue, but it would see a conversion to semi-plastic in 1956, just as did the 4053 Polyphase and the N4094 Merchant's rules in 1954. Essentially, all three of these rules were identical by 1956, with plastic bases. The N4100 dropped the N- prefix to reflect the major change. It would also print the identical Conversion Factors onto the back of the rule, with stadia instructions now placed into the front channel beneath the slide. A strange look, indeed. Checking the price of all three of those rules in 1956...the Model 4100 was $14, which was 50 cents more than the 4053-3 and a $1.50 more than the N4094. Both the Stadia and Polyphase rules had "S" upgrade options to the chamois-lined leather case for $3 more.

Today, samples of the 10" Model 4100 can be found perhaps bi-monthly on eBay for a price of between $50 to $60, though 15 to 20 years ago the rule was in much higher demand, approaching $200. The same can be said for the 20" Model 4101, only more appreciably. It has sold for over $400 back in 2003, yet today will still cost maybe $150 if one actually comes for sale, which happens maybe once per year. But as with many rules that haven't sold in a while, a really nice, well documented version sold today just might surprise you how many Ben Franklins it can bring.

The Model 4143 Kissam Stadia

Apparently, the Model 4100 was discontinued around 1960, as it would disappear from the 1962 catalog. But introduced around that time was the Model 4143 Kissam Stadia. It was to the Model 4100 as the 4161-3 was to the Model 4053...an all-plastic spiritual successor. But the first mention of the rule wasn't until the 1962 catalog when it would be called the Model 68-1486 Kissam Stadia (4143). Because there are Kissam Stadia rules with 4143 printed on the slide rule, it's obvious that the Kissam rule would run for a year or two as the 4143 before the 68-1XXX switch.

The "Kissam" moniker comes from Phillip Kissam, a Princeton University professor who authored many texts on surveying during his long career beginning earlier in the decade. He shared a patent with Carl Keuffel for an "optical plummet," which would have been incorporated into the design of K&E's surveying transits. Presumably, Kissam had some input into either the design of the 4100 or re-design of the 4143. Equally likely is that slide rule was designed to work with Kissam's techniques described in his textbooks, which were quite ubiquitous and well-known. It is unclear if Kissam received a royalty for some intellectual property or if the moniker was merely a tribute to Kissam himself, but it's noted that the original 4100 model lacked such an association.

This Kissam rule, other than being all-plastic construction with the unbreakable cursor, was indeed quite different from the former 4100. The scales on the back of the slide were removed, lessening its functionality for general math computations. It kept the Conversion Factors on the back of the rule, but it removed the stadia instructions entirely, placing them on a separate plastic card (at right). The front side scales were also changed:

inches // R1 [V1/H/V3, HC, V2] R2 \\ centimeters

The Model 4143 Kissam Stadia in my collection. It was likely originally purchased in 1962, as this one came with the same plastic insert shown below, with the 68-1486 number. As with many K&E slide rules, they had no issue repackaging older inventory for a new catalog year.

With slide removed, showing all plastic construction and maker's marks in the slide well.

Unlike the Model 4100 that utilized the back of the slide for general math use, the Kissam left it void of scales.

The Kissam Stadia Model 4143 (68-1486) came with a plastic insert for instructions. This image courtesy of ISRM.

The R1 scale added another decade, now numbered 1 to 1000. The top scale on the slide was now a combined V1/H/V3 scale. The H scale, in red, would index from the end of the second decade, at 100 on the top rail. Horizontal measures would read right of the 45 marker. To the left, vertical measures for standard angles was the same. The right side V3 part of the scale allowed for computations with sub-minute sized angles, read off the R1 scale, but then divided by 10,000 and subtracted from the R-value to give the vertical reduction. For angles between a minute and a degree, the V2 provided vertical reduction for those angles with more precision, read off the R2 scale on the bottom rail. The HC functioned as normal, also read off the R2 scale as on offset to the R-value to yield horizontal reduction. To make that work, the R2 scale was entirely different from the previous A scale, a sacrifice made - no more general math ability - in order to give much more precision for stadia measures.

Priced at $19.50 in 1962 with a leather case and plastic instructional insert, this Kissam rule is substantially more than the 4163-3 ($11) and 4094-3 ($16.50) rules which shared the same form factor. This final version of the "4100" Mannheim Stadia rule would last through 1972, ending the long run of a popular slide rule for K&E.

Value for the collector is strong, though it seems to vary. As with the Model 4100, 15 years ago or more the Model 4143 would have been in excess of $200, with one sample on eBay achieving a bid-off ending at $338. Today, the rule pops up only occasionally for a likely price tag of maybe less than a $100.

The Model 1749-3 Colby's Stadia

Patented by Branch H. Colby of St. Louis, and one of two devices known as "Colby's Computers" licensed to K&E for sale, the Model 1749-3 Stadia is pictured in the 1897 catalog (shown at right). The rule is for an office desktop, at a massive 50 inches in length. It is comprised of an upright slide riding in a vertical, perpendicular groove. Advertised for stadia reductions, computing the difference of elevations between two points if the Stadia reading (delta-y) and vertical angle (theta) are known.

The July 30, 1895 patent for the device describes a "Log Scale" on the base of the rule to be three decades of logarithmic scale (an extended R scale) numbered 1 to 1000. The vertical slide is labeled "Arc Scale," keyed to the Log, based on angles of elevation from 0 to 18 degrees relative to the distances on the base scale according to the formula for vertical displacement. The device has indexes for meters, yards, and feet, covering any unit by which a survey could have been measured.

The idea is to have surveying crews recording distances and angles of elevation/depression data from a known benchmark, measuring points either in a site plan grid or radiused from the sight device using recorded azimuth measures. The data could then be brought back to the office where the all of the elevation differences are computed in higher precision than with a standard 10" or 20" stadia rule. This would give accurate topographical surveys of the fields being measured, whereas a map could be produced - all done from the comfort of the office. This was traditionally accomplished with reduction tables, which was accurate, but a very tedious process. Colby's slide rule attempted to fix that. The extent at which he succeeded is unknown?

The rule itself, according to pictures of a known sample, appears to be celluloid colored mahogany. It was sold in a long wooden case and priced at $20 in 1897. This rule was given the 4125 model number in 1901, but would be discontinued prior to 1904. Perhaps this short life-span answers my question as to success of the rule?

Only one of these devices to be sold on eBay happened in the year 2000 at a price of $400. This collector is holding out hope of finding another one!

The Model 4105 Webb's Stadia

A very unique rule, the Webb's Stadia rule was added to the K&E catalog in 1903. The price was $5, only 50 cents more than the Model 4100. It's unique in that the Model 4105 Webb's Stadia Rule is an elongated wooden cylinder with seven paper scales affixed around the surface. If you pictures a rolling pin in your mother's kitchen, then this slide rule wouldn't look all that dissimilar in appearance. Four scales are labeled "Differences in Elevation," which collectively serve as a single folded scale ranging from 1 minutes to 40 degrees. At 12.5" long, these scales are the equivalent of a single 50" scale. Three scales are labeled "Horizontal Correction," which serves a similar function as those for elevation, ranging from just under 2 degrees up to 40 degrees. Sliding through an outer metal tube with a slit through which the inner cylinder may be read, the slit is labeled with a single scale ranging from 100 to 1000 feet.

The idea is simple here: align the index of the cylinder to the stadia measure (R-value) by sliding it through the tube, and then sighting the vertical angle on the correct vertical and horizontal scales by rotating the cylinder to measure the reduction respectively. No cursor is needed. Quite nice! The device shows that such a rule is actually better formatted to a cylinder rather than to a linear rule, especially from the standpoint of increased resolution.

Designed by Walter Loring Webb, a Professor at the University of Pennsylvania, his namesake slide rule was licensed by Keuffel & Esser and manufactured in-house. K&E sold the device for exactly 20 years until it was discontinued in 1923. This 20 years being the length of his Webb's exclusive patent rights. It is unclear if other makers might have freely produced the slide rule after 1923. Cost for the rule was $7.50 in it's last year, but it will cost you more than $400 today for the rare sample that might come up on auction.

The Model 4102 Surveyor's Duplex

In 1915, despite already producing 3 other stadia slide rules, K&E introduced their first stadia rule based on a duplex form factor. However, this rule had a few special tricks up its sleeve.

The stadia functions of the rule cover the back side, giving the same functionality as a typical stadia rule.

My sample of the N4102 Surveyor's Duplex, as displayed on my classroom wall.

But it's one thing to use the stadia method of surveying any number of points, but they mean very little unless those points relate to a meridian line. So before a survey can be done, precise latitude and longitude as well as an exact north bearing measurement is required. At night, this is possible by sighting Polaris (the "North Star") and staking out when true north is on the ground. But this isn't feasible during the day. In that case, measuring the sun's azimuth and comparing it to it's known bearing provides that information. Known as "astrometrics," this is a practice still valued by modern surveyors today.

As for the 20" Model 4102 Surveyor's Duplex , this is where the front side, astrometric scales of the rule are useful. Once the altitude of the sun is ascertained by transit observation, the slide rule has scales to compute accurately the sun's azimuth (bearing from true north). This important measurement allows a survey crew to stake out a true directional grid, as well as conduct a radial survey of the area. Rear side of the rule has typical math scales, with both of the traditional stadia scales.

The astrometric scales are designed to compute the formula:

$\cos azimuth = \frac {\sin d} {\cos h \times \cos l} - \tan h \times \tan l$

In this formula, "l" is the latitude of the observation obtained by either map or direct observation, "d" is the declination of the sun as provided by ephemeris data, and "h" is the sun's altitude taken from transit observation and corrected for atmospheric refraction. According to the catalog description, it computes this with an accuracy of 1 arc minute, with the length of the rule giving more sub-divisions for increased precision. The manual did ship with the slide rule to provide instructions for use.

Scale layout is as follows:

Front Side: D, sin d [ cos l & h, C, tan ] tan I, Az
Back Side: Vert, A [B, CI, C] D, Vert/Horiz

This 20" duplex formatted rule first appeared in the 1915 product catalog with frameless indicator and Morocco boxed case for $18. It would adopt the N- prefix in 1927, with the only apparent alteration, other than a $24 price tag, being the change to a non-serif font (which happened across all K&E slide rules in 1927, without exception). It would upgrade to the improved-glass indicator (metal frame) in 1936. In the 1952 catalog, the N4102 was listed as "Temp. Disc." and would disappear entirely by 1954.

This rule is quite desirable and rare, coming up on eBay maybe once per year at an average price of over $400. It pops up on the occasional auction site as well where I found a sample for my own collection, fortunate to pay less than $100. My N4012 is a 1937 model with the new-improved cursor, yet as with many cursors of this type did come with cracked cursor rail (onset of KERCs). It does remain intact and fully-functional, however, reinforced with a little super glue (cyanoacrylic).

The Chemical Family

The idea of a chemical slide rule seems obvious. As easy as it is to mark pi on a slide rules with a gauge mark of the symbol $pi$, using gauge marks to indicate chemical elements and formula weights was inevitable.

Even before atomic theory had arrived, chemists knew the weights of elements and compounds empirically, through experimentation. By the 1930s, when the idea of the neutron (neutral particle) was finally understood, many such chemical gauging of slide rules had already been around for decades, as chemists would commonly mark their own general math rules with many of the most used chemical compounds and elements in their experiments.

For K&E, waiting for the discovery of the nuclear age was unnecessary, as they would produce the first actual chemical slide rule in 1913.

Model 4160 Chemist's Duplex Rule

The date is August 25, 2024 as I write this. I would have thought that I would have a sample of this slide rule by now, but it still eludes me. It is somewhat rare, popping up perhaps twice per year on eBay. Even so, good fortune has not brought one into my hands. However, hope does not elude me.

I share my emotions regarding the Model 4160 Chemist's Rule before describing it simply because it's one of the first K&E specialty rules that came across my radar, as I just think a slide rule for chemists is pure genius, even if real chemists might feel they aren't entirely useful (at least of among the chemists I know). My collection does include the Hemmi version of this rule, the "Model 257 Chemical Engineer," and I have felt that it would only be a matter of time before the K&E rule would join it.

The Model 4160 appeared in K&E product catalogs from 1913 to 1941.

Louis Gotlib, in his Fall 2012 article for the Journal of the Oughtred Society, expresses that the number of Model 4160 rules produced by K&E, as told to him by K&E Salisbury plant manager, Joe Soper, was quite small. While I would agree with that assessment - I've been a collector long enough to know that there is a direct correlation between the number of rules originally produced and the number that come up for sale today - Soper would have only known this indirectly, as the last catalog appearance of the Model 4160 Chemist's rule was in 1941, more than two decades prior to Soper joining Keuffel & Esser in 1966. It is thought, perhaps, that K&E might have continued to produce the rule despite being removed from their catalogs; however, this in mostly due to the fact that the chemical rules from all the other makers began and persisted into the 1950s and 1960s. But most would believe that K&E, with the Model 4160 being the first of such rules produced, had discontinued that rule well prior to other companies taking up the mantle.

4165 Urea Index Rule

The Sewer Family

4130 Colby's Sewer

4132 Crane's Sewer

4128 Nordell Sewer

The Steam Power Family

Model 4140/4141 Hudson's Horsepower Computing Scale

If I am K&E and I want to be known as THE source for slide rules in the USA, then I will not care so much if I made the rule or not. In the case of the Model 4140/4141 Hudson's Horsepower Computing Scale, K&E became licensed to sell the product from its maker, the W.F. Stanley & Co., Ltd. in England. We will talk more about Stanley later in the context of the Fuller Calculator, but there is no mistake that K&E followed this company closely in terms of their business model, especially working together with them in many areas. One of these is to license the U.S. sale of the Hudson's Horsepower Computing Scale.

Origins of this slide rule can be traced back as early as 1877 in the W.F. Stanley product line. These early versions were made of pure ivory, whereas later versions might incorporate celluloid covered boxwood, as does the version marketed by K&E known as the Model 4141, or simple cardboard construction, as with the Model 4140 version of the rule. Either version is only 4.5" long. Price of the two rules in the 1913 catalog is $3 for the cardboard Model 4140 and $6.50 for the wooden Model 4141 version.   K&E sold these rules between 1901 and 1916. Both versions came with a leather-covered sheath.

The rule has two slides which move independently between two fixed stator rails, bracketed on the ends. As such, it is fully duplex, as there are indeed scales on the back side. There is no cursor. While K&E did not start selling this rule until 1901, the fact that it existed as a duplex rule in its construction as early as 1877 most certainly predates the Wm. Cox patent for the duplex slide rule of 1891. It's important to note here that Cox might have used this rule as inspiration for his own patent, which called for logarithmic scales, inverted scales, and a "runner" cursor. The Hudson scales had none of these.

Scales for the Hudson rules are as follows...

Front Side: Indicated Horsepower [RPM] [Stroke Mean Pressure] Cylinder Diameter
Back Side: Mean Pressure Scale

The slide rule works on the principle that if you know any two of an engine's piston speed, its displacement size, or its power, then you can compute the third quantity. Displacement size would be a product of the stroke length and its cylinder diameter, giving 4 total inputs, which makes the dual slides a necessity in a cursor-less design. 700 RPMs is shown as the maximum measurement, as such would be for slow revving engines of the day, most usually steam engines, but not necessarily.

J.G. Hudson, designer of the rule, invented 3 other rules similar to this horsepower rule, each of which was produced by Stanley. These include a Pump Duty Computing Scale; the Shaft, Beam, and Girder Scale; and one known as the Photo Exposure Scale.

Why K&E chose only to sell the Horsepower rule is unknown, but all such rules are similar in style and function. The horsepower rule is the most common; however, as well as the longest selling, sold by Stanley (and AG Thornton) until approximately 1931. Even so, it's still quite rare, as only two Model 4141 rules have come up on eBay since 1999, averaging around $460.

Model 4135 Power Computer

A likely reason for K&E dropping the Hudson's Horsepower Scale in 1916 is that the company began selling their own version of very similar slide rule 3 years previously. The Model 4135 Power Computer rule, first appearing in the 1913 Product Catalog, allows for similar horsepower computations of steam, gasoline, and oil engines. Two versions of the rule would appear over the product's history. The first rule, made by K&E from 1913 to 1921, was based on the same design as the cursor-less, Hudson rule, with identical celluloid-covered boxwood and dual independent slides. It was longer, however, at 7 inches. The K&E version also makes better use of both sides of the duplex rule, with a general-purpose Mannheim scale-set on the front side of the rule (A, B, C, and D scales only) and the specialty scales on the back. Cost for this rule in 1913 was $7.00, only slightly more expensive than the Hudson version despite the much larger size and increased utility.

Like the Hudson rule, the Model 4135 is used to compute the formula:

$HP (horsepower) = \frac {P \times L \times A \times N} {33000} $,

where P is the pressure (mean effective) , L is length of stroke, A is area of one piston in square inches, and N is the number of power strokes per minute. In this case, 33000 ft.lbs/min. is James Watt's equivalent for 1 horsepower (hp).

As such, scales on the back side of the rule are like the Hudson rule:

Indicated Horsepower [RPM] [Stroke Mean Pressure] Cylinder Diameter

In 1922, K&E completely changed the format of this slide rule, adding the "N" prefix. The new model N4135 was converted to the 4088-1 model body, also introduced that same year. Thus, it was a normal 5" pocket duplex style using typical celluloid-covered mahogany. This rule added a glass cursor Scales were as follows:

Front Side: A [B, CI, C] D
Back Side: P [R, W] D, where P = horsepower, R = Stroke, W = RPM, and D = Cylinder Diameter

Essentially and functionally, the new format worked the same as the old, except stroke length and RPM are switched on the slide relative to the earlier version, and the cursor replaces the need for dual slides. This rule would last appear in the 1937 catalog and the 1938 price list at the price of $10. Of note, 1938 catalog pricing for the 4088-1 polyphase duplex pocket rule, from which the N4135 was derived, was $9.50.

Description from my 1921 K&E Catalog. Click on the image for a close-up of the scales for this rule. Note front and back scales are denoted A, B [C, D, E] F, G, adjusted for steam pipe calculations on the front and water pipe calculations on the rear.

4142 Allan Friction Head Duplex

This slide rule might as well be a unicorn - I am unaware of a single known sample of this slide rule anywhere. But it is described in every product catalog from 1915 to 1927 at a cost of $18.

Keeping to the theme of "steam" power, the Model 4142 Allan Friction Head Slide Rule is a 20" duplex formatted slide rule designed to do steam and water pipe computations. According to the catalog, the rule is setup to compute one of 5 variable inputs as follows: volume, friction, diameter, velocity, and pressure. Those relating to steam computations are handled on the front side of the rule, with scales for water calculations on the back.

The detailed illustrations in the 1921 catalog (at left) show a very clearly labeled and well thought-out slide rule. I would certainly love to see a scan or image of this rule, or better yet, the real deal in person. Alas, there is simply no internet record of an actual sample anywhere. More searching (and research) is necessary.

The Radio and Electrical Family

As a slide rule collector, those specialty rules designed to help with electrical applications are among my favorites. Most slide rule makers have a variety of slide rules dedicated to this purpose, especially since specific electronic computations are easily derived from special logarithmic scales and because of the need to do these computations in the field.

Radio communications is merely an extension of electronics, and traditionally a radio engineer needed the capabilities that an electronics rule would provide. Typically, such radio specific rules will add a scale (or three) specific to an aspect of radio engineering, or to those of the electronics and the general-math abilities they provide.

K&E would provide five different radio rules over the years, but prior to that, we should look at the company's only electrical rule.

Model 4133 Roylance Electrical

Introduced in a supplement to the 1913 product catalog, this rule, in the 8" single-sided, Mannheim format, had an interesting history.

The first question that arises is the origin of the name, "Roylance." Many places on the Internet declare the name to be "Roy Lance," but it should be noted that all catalog entries and instruction manuals, give the name as Roylance.

Secondly, this rule cannot be taken out of the context in which it was introduced, during the time when the world was first becoming "electrified." Creating the infrastructure for running electricity in every home took place in the early part of the century, and that required running an enormous amount of copper wire. Determining the amount and type of copper wire needed to run from AC transformer to AC transformer is the focus of this slide rule. As such, it allows for the computation of amperage limits for a particular gauge and type of copper wiring, as well as wire temperatures at particular power draws. The rule's exact construction is based on the 4035 Mannheim rule in its format, with scales as follows...

Front Scales: Inches || A [ B, C ] D || B&S Gauge
Back of Slide Scales:  [ S, L, T ]
Back Scales: Centimeters

Of note are extra markings, in red, on various scales for extra purposes:

Between 9.9 and 20 on the B scale, there is a scale for wire temperature in Celsius for doing resistance calculations.
On the right hand side of the A scale is a "W" gauge marking to represent the constant (.003027) for calculating weight in pounds per 1000 feet of bare copper wire.
The C scale is marked in red at 746 to assist with conversions of watts to HP (and vice versa).

The front edge of the rule has what's known as the B & S (Brown and Sharp) standard wire gauge. These numbers are the equivalent measure to the better known AWG (American Wire Gauge) commonly used today. On the rule, it's a three line continuous scale running from 0000 to 40, representing common wire gauges from large to small. Most electrical calculations on the slide rule begin with this setting.

Additionally, there is a scale beneath the slide on the main body with columns of numbers in 4 rows. Those numbers are used with the B & S Gauge scale to give the following information:

Top row: The amp carrying capacity for rubber-covered wire
2nd row: The amp carrying capacity for weather-proof wire
3rd row: The amp carrying capacity for the rubber-covered cable
4th row: The amp carrying capacity for the weather-proof cable

An example for the use of the Roylance rule is shown at right.

Two other features of the rule seem unique. First, the rule employs a three hairline cursor. It is unlike earlier two-line cursors used around the turn of the 19th century when the scales were a 0.5 cm from the end of the rule. In this case, if the diameter of a circle is set on the right hair-line, then its area can be read from the left hair-line. A very interesting feature! The center hairline is used for all other computations normal to the rule. Second, there is a centimeter ruler on the back of the bottom stator of the rule, and it is divided atop a celluloid lamination. This is the only instance of a single-sided, wooden Mannheim formatted rule running an extra celluloid strip on the back.

The Model 4133 Roylance sample in my collection. Its overall shape and size, use of celluloid on the rule's back, the "B&S Gauge" on the front edge, the utilization of the slide well, and the purpose of the rule itself all contribute to its uniqueness.

An example for use of the Model 4133

The Roylance was introduced with a leather case in 1913 with a $5 price tag, but quickly rose to $8.50 by 1922 when if would be sold as a N4133S version, indicating sewn leather case. As comparison, the 8" Model 4053-2 on which format the Roylance was built was $6.95 in 1922. The Roylance rule was somewhat enduring, lasting until the 1949 catalog, albeit there were times of unavailability during the war. Other than the typical cursor evolution, the only significant change to the rule occurred in 1925 with the addition of a CI scale to the middle of the slide. This added the "N" prefix to become the Model N4133.

The Roylance is very much a blue-collar, industrial slide rule, intended for doing large-scale electrical work, from power pole to power pole. The Roylance Model 4133 would have played a significant part of building the electrical infrastructure for the United States. Once electricity was provided, then it became possible to have devices that ran on electricity, giving rise to the electronic industry beginning in the 1920s. A remarkable amount of such electronics concerned the development of radio.

"The K&E Radio Rules"

For K&E, an electrified world meant designing more slide rules that could help with computations required in electronic/radio circuit design, of which K&E is an early pioneer - it was around this time when slide rule makers produced some of the first Elektro/Electric slide rules in which the collector would be familiar. Yet it was radio electronics where K&E became most interested.

Living in the 21st century, we have a difficult time understanding the need for slide rules designed for radio electronics. In an age of on-demand digital media, it almost seems that radio itself is as obsolete as slide rules! Yet, for the discussion of the following slide rules, it is helpful to understand the world as it appeared a century ago, when radio was a growth industry and its ability to change the world was at hand.

It is in this setting that K&E and other manufacturers would produce slide rules to help with the computations involved in radio transmission and receiver design. These following rules represent somewhat of an evolution of their radio rule, all of which were conceived, planned, and produced between 1935 and 1940. And as we will see, K&E allied themselves with the US Navy for this task.

Model 4091-3 Spec

The earliest attempt at an electronic engineering slide rule by K&E is also the one in which we have the least knowledge.

According to Clark McCoy (JOS, Fall 2015, p. 51), the U.S. Navy approached K&E to produce such a rule around 1935 or 1936. At the time, instructors at the Naval Academy were working with K&E on the development of the 4080 and 4081 Log Log Models that came out in 1937. Through this working relationship, the Academy inquired about producing this slide rule.

To my knowledge, McCoy owns the only three known samples of this slide rule, so "rare" is quite the understatement.

McCoy does explain that one of these samples has "RMS No. 72" inscribed on the rule's edge, yet he surprisingly has no explanation for that. RMS would indicate Radio Materiel School (RMS) at the Naval Research Laboratory (NRL) in Bellevue, District of Columbia. Its use in their six-month curriculum would seem to be obvious. This program pushed through approximately 200 US Navy sailors each year, beginning in 1924. These facts should indicate something about the growth of radio in this country during the era, especially its importance within military applications. It's this context in which K&E saw a burgeoning market. This becomes thematic of all the radio rules that K&E would produce, as described shortly.

While a general math rule could be used to compute solutions to the formulas required for electronics, the Navy wanted something more efficient, and they knew such a rule would have been simple to implement.

Hearing this, K&E took the Model 4091 Log Log Deci-Trig (decimal trig scales needed) and added this "special" scale, thus giving rise to the name of the rule as the Model 4091-3 Spec. The word "SPEC" is written above the normal "<4091-3>" model number found on the typical rule, highlighting the distinctive difference. The added scale is labelled "LC." "L" stands for Inductance - "I" is reserved for "current" in electronic-speak - and "C" means Capacitance. The LC scale would allow for the computation of resonant frequency when given a variety of inductances and capacitances. LC circuits themselves have both an inductor and a capacitor, storing energy at natural oscillation frequencies. They are an electronic building block within radio circuitry, and this rule was design to help engineers tune those circuits.

This scale was squeezed onto the top stator rail on the back of the rule, just above the LL0 scale. The only other difference between these versions of the 4091-3 rule was that K&E switched the front and back of the rule, putting the maker's marks and copyright notices atop the L scale - as there would not have been enough room above the LL0 scale for the new scale if kept as normal.

Not listed in any K&E Product Catalog and rolled out around 1936 (according to their standard serial numbers), this item is rare, perhaps serving only as a prototype or precursor to other radio rules, with very limited purpose to the US Navy only (see Sidebar: It's Classified at right).  It is difficult to know how many were made since only three such rules are known to exist. However, this rule was enough to give K&E a start with this type of slide rule, giving them something to improve upon, which is where the next rule comes into our conversation.

Note: This slide rule could have very well been discussed within Chapter 7; however, we discuss it here because of its contextual importance to the development of electronic engineering slide rules by K&E.

The ultra rare Model 4091-3 "Special," shown here courtesy of Clark McCoy.

Sidebar: It's Classified!

When looking through descriptions of K&E specialty slide rules posted over at the excellent International Slide Rule Museum (ISRM), I stumbled across a letter to the editor of "Radio-Craft" written by Nelson Cooke in 1943.

- courtesy of ISRM

Cooke would rise to become the Navy's top radio electrician, opening many of their electronics schools across the country and writing many of their curriculum manuals and text books. But for this context, we know that he designed the K&E rule which bears his name.

While the Cooke radio rule was the main contributor to the US Naval electronics programs, it seems apparent from Cooke's letter that the curriculum and slide rule used for the Navy's electronics instruction was indeed a "priority article," classified and restricted to only "preferred" military personnel.

This would indicate most of the intent surrounding the K&E slide rules developed in part by the US Navy, that being their intended use in these non-civilian programs. As such, we see why so many of them were never listed in a product catalog or, in the case of the Cooke rule, could have been in production long before being published to a catalog.

The point at which the Radio rule was declassified and made available for public distribution is unknown, but it would seem that the 1943 entry in the K&E catalog was premature, prior to the US Navy's consent to do so. Most certainly if it was never intended for public sale, then K&E would have refrained from posting the slide rule in every product catalog through 1967. So most certainly, the Cooke rule became an available product.

Thus, Lt. Cooke's letter to the magazine would appear to indicate that K&E jumped the gun in their publishing of the Cooke Radio rule within the 1943 product catalog. So, it appears like Cooke himself was trying to put out a few fires!

Nevertheless, an RMS inscription has been found on both the Cooke rule and the 4091-3 Spec. This would indicate their use within the "Radio Materiels School" of the US Navy.

It is unknown whether or not the 4082-3 Radio Special rule was connected in any way to the US Naval programs in a manner similar to the 4091-3 Spec and Cooke Radio rules, but it would appear highly likely indeed to be a slide rule designed and commissioned for the US Navy. To me, this seems very likely based on its development date, provenance and similar design to these other rules, not to mention that it too was not known about publicly.

As an endnote, the use of commissioned slide rules within electronics schools would be a common theme during the slide rule era, as we would later see with the Cleveland Institute of Electronics (CIE) as they commissioned their own slide rules from both Pickett and Aristo. In the case of a public program such as the CIE, the numbers of students who historically attended the program are numerous, making slide rules connected to that program quite plentiful for today's collector. Yet for many of the K&E radio rules, it would be easy to see why they are not quite so ubiquitous.

As such, both the rarity and the mystery surrounding many of the K&E radio rules is not a surprise.

Model 4138 Morrison Radio Engineering

One year after the 4091-3 Special," K&E came to market with a radio rule that customers could apparently buy. The Model 4138 Morrison Radio Engineering rule only appears in the 1939 "slide rule only" catalog for a cost of $20 (add a dollar for leather case) and likely disappeared as quickly as it appeared. Very few of these exist in the wild today, so it is apparent that not many of them sold. They come up at auction on rare occasion, with a handful popping up on eBay over the last 20+ years. Expect to pay in excess of $500 or $1000 for one!

Designed by J.F. Morrison of Bell Labs and produced by K&E in 1938, the rule accomplishes what the Model 4091-3 Spec does, yet adds specialized scales to compute the propagation of radio frequencies (RF). Because of this, the Model 4138 is one of the more unique of K&E's rules in terms of its look and scale set...

Front side: L, F, A [B, CI, C] D, T, ST, S
Back side: 9, 8 [7, 6, 5, 4] 3, 2, 1

Most of the general-purpose scales are placed on the front of the rule, while nine numbered scales are located from bottom to top on the back. The F scale in this implementation is a single inverted "D" scale folded at $\frac {1}{2\pi } $. It is self-documented on the right side of the scale with "FREQ." label. This allows for the fast computation of inductance or capacitance of a circuit for a given frequency when read off of the A scale (result would be given in ohms). This is the typical A scale, but it is self-documented on the right side with the "LC" label, which indicates that the rule is setup to work problems that use the resonant frequency formula, or $ LC = \frac {1} {(2pi*f)^2}$. Note that the A scale, being 2 decades, accounts for the squared quotient of that formula.

When used with the single-decade D scale, problems that work with the Inductive Reactance (equal to $2 \pi *f*L$) and Capacitive Reactance (equal to $\frac {1} {(2 \pi*f*C)}$ formulae can be solved. For example, to know the capacitor required (units in farads) for a given inductor (units in henries) at a given frequency (units in cycles), the user would set the frequency on the F scale, the inductance on the C scale, and then read the result from the D scale.

Decimal placement is more critical than in most slide rule usage cases, since many of the units are typically used with prefixes. As such, the capacitance could very well be in micro-farads, the inductance in micro-henries, operating on any number of kilo-cycles, yielding a reactance in kilo- or -mega-cycles (or hertz). Of course, for an electrical engineer, they would be accustomed to the domains in which they are working and would do quite well to get the correct unit magnitude. But even if they are not, the rule has decimal keeper scales, like the Pickett N16 Electronic, to keep things sorted out.

The F scale implementation here - and its usage relative to the A and D scales - appears again with the Model 4092-3 Radio Special rule that we will talk about next.

Morrison's design continues to the back side whereas, according to manual, the numbered scales allow for the computing factors involving radio wave propagation as shown at right.

These scales on the back side are self-documenting, with labels to what each scale involves, shown on the right of each numbered scale.

The Model 4082-3 "Radio Special" within my collection. Note the unique "F" scale, which replaces the "K" scale of the Model 4081 from which the 4082 is derived. This is a rare rule, with very few changing hands on eBay.

Model 4082-3 Radio Special

While the Morrison rule might find utility for general public use, it would have been overkill for most in the electronics field, which likely explains the rarity of that slide rule and its only appearance in the 1939 product catalog. More practical would be a rule that offered the feature set of that rule's front side only, or something more like the 4091-3 Spec that was custom-made for the US Navy and not offered for public sale. What was needed, specifically, was a general purpose slide rule that adds the "F" scale found on the Morrison rule.

One might be inclined to wonder why K&E did not just offer a version of the 4091-3 Spec rule that uses this "F' scale, replacing, or in addition to, the LC scale on that rule? First, recall that the US Naval Academy was writing the documentation for the upcoming 4080/4081 redesign of the 4090/4091 slide rules, yet their own rule, pre-dating the 4081 Deci-Trig model, could not take advantage of that actual redesign.

And, second, you should also remember my disdain when discussing the Model 4090 and 4091 earlier, as I felt using it is quite cumbersome. Therefore, it made sense to build a new "special" rule not on the 4091 design, but rather on the improved 4081 model that would come out in 1937. And thus, we have the Model 4082-3 Radio Special, a slide rule based on the excellent design of the Model 4081 Log Log Duplex Deci-Trig (because electronic engineering requires decimal trig) with the important addition of the "F" scale as discussed in the Morrison rule.

Front Scale:  L, LL1, DF [ CF, CIF, CI, C ] D, LL3, LL2
Back Scale: LL00, LL0, A [ B, T Cot, ST, S Cos ] D, DI, F

A typewritten manual was supplied with the original rule. Instructions for computing capacitive and inductive reactance of a circuit at any frequency are presented just as with the instructions given for the Morrison Radio rule. However, the Radio Special's manual also gives instructions for other electronically-related applications using the general-purpose scales. For example, it shows how when 10 on the LL3 scale is aligned to the index on the C scale, then by setting the cursor on any "power ratio" on the log-log scales will read the decibel conversion across the hairline on the D scale.

Curiously, this slide rule would not appear in any K&E catalog, so it is unknown if this rule was offered for public sale. We will discuss this more when we investigate the next slide rule, the Model 4139 Cooke Radio Rule, but in the meantime I am inclined to believe that this rule, like the 4091-3 Spec, was also designed and intended for US Naval Academy use only. Such would explain why it was not listed in a catalog at anytime between 1937 and 1943 when it is suspected that this rule was being produced. It would also address the rarity of a slide rule that I feel should have done better in overall sales has it been publicly offered. Moreover, other than the Morrison rule, all of the radio rules covered here seem impacted by K&E's association with US Naval Academy math professors, and the Model 4082-3 really fits into the design evolution of these rules thematically.

Regardless, It should be noted that K&E made two versions of this slide rule. The first rule in 1937 had the words "Radio Special" presented on the rule itself, or so it has been said by K&E collector Clark McCoy, who offered the scan of the manual linked above. I have seen no images of this earlier rule and they are not offered by McCoy.

This rule would be updated in 1939, also according to McCoy, but removes the "Radio Special" moniker from the rule itself. This second version also makes one alteration of the scales by switching LL00 and LL0 on the back side of the rule. This rule, as shown at right from my own collection, might have been offered at least through 1943, though my own sample has a serial number that corresponds to 1941.

The Model 4139 Cooke Radio Rule - Showing the front side of my more recent sample of two, this slide rule has a 1954 serial number. Significant in that it exhibits the <4139> model number and the rounded Cooke Radio emblem, which the earliest Cooke rules did not have, as well as the exposed mahogany edges.

Model 4139 Cooke Radio Rule

So if we take the two "special" electronics rules described above and merged them into one, then we might produce a slide rule desired by more people. Such is the Model 4139 Cooke Radio Rule, which takes the "LC" scale from the 4091-3 Spec and the "F" scale from the 4082-3 Radio Special and places them on a rule with a NON- log-log duplex scale set. This recipe culminates in K&E's longest enduring and highest selling slide rule designed for electronics.

This rule first appeared in the 1943 catalog (and 1942 price list) at the cost of $12.75 (plus a dollar for the S-case version). It would last appear in the 1967 catalog as the Model 68-1460. Designed also by a U.S. Navy man and academy instructor, Chief Radio Electrician Nelson Cooke, the Model 4139 certainly has a similar pedigree to the earlier effort (efforts?) by the Naval Academy, with notable differences.

The non-log-log duplex scale set we speak of is the Model 4071-3, which itself appeared first in 1939. It seems odd that K&E did not base this rule off of the log-log duplex rules as it did its predecessors, but they certainly felt that the smaller form factor - 40mm instead of 33 mm wide - and the lack of any log-log scales was acceptable for offering to the public. Are log-log scales unnecessary for electricians? Certainly they are a bonus in making a "do-everything" kind of slide rule. But is this really K&E's goal with this rule? Instead, it's the former point - that the smaller form factor was suitable for this rule - that makes sense when you compare the front of the Cooke rule with the Model 4071-3, which despite its smaller rule height has some tradeable assets.

Because the maker's mark, patent information, and "Made in USA" label occupy a large part of the top stator, their removal provides sufficient room to add another scale, which the Cooke does by moving the "L" scale (base 10 log) up to where the labels were and then adding a folded $ 2\pi$ scale to the bottom of the rule (more on that scale shortly). The removed text would be relocated to the right side of the rule's front. The same considerations could not be made with the 4081 rule that is already packed with scales and no extra room to add a specialty scale. And furthermore, there is no candidate scale, like the K scale, that could be replaced on the 4081. The 4071 format rule was able to make a sacrifice for the Cooke rule, subtracting the K scale on the rule's back and adding the LC scale from the 4091-3 Spec rule.

With this configuration, the scale set is as follows:

Front Side: L, DF [CF, CIF, CI, C] D, $ 2\pi$
Back Side: LC, A [B, T, ST, S] D, DI

If you noted that the non-inverted, folded $ 2\pi$ scale is not the inverted and folded $ 2\pi$ "F" scale that I described earlier, then hopefully you also noticed the DI on the bottom stator of the back of the rule? If the "F" scale on the "Radio Special" was read from the D scale, then here on the Cooke the $ 2\pi$ scale would work with the DI scale. So in practice, the user simply has to flip the rule over to read between $ 2\pi$ and DI. This allows similar use to compute the reactance formulas discussed earlier.

Despite appearing first in the 1942 price list, the rule was designed and entered production much sooner than that, likely as early as 1938, as I have an original variant Cooke Radio sample with a 1938 serial number (686538). Now this prompts conflict in my mind for a few reasons. First, as we said, the Model 4071-3 Deci-Trig on which it's based did not debut until 1939, so it would seem strange that the Cooke rule could make it into production a year earlier. This could be explained by the notion that the way we date K&E rules by their serial numbers has some degree of imprecision, certainly within +/- 1 calendar year of error; and, in fact, McCoy tends to place the serial number of my rule to early 1939. Second, with this first principle in mind, just because the 1939 catalog marked the first appearance of the base 4071-3 model, we shouldn't be inclined to believe the rule didn't begin production until 1939. There are 4070-3 and 4071-3 models with serial numbers closely approximate with the Cooke rules.

And my third item of conflict; even with a 1939 production debut, the Cooke rule somehow managed to miss the 1939, 1941 and 1942 product catalogs. I have no explanation for this, other than that, perhaps, earlier years of the production rule weren't yet offered to the public. But solace can be found in the fact that the Morrison Model 4138 was the only "radio" rule to be listed in a catalog prior to 1943, meaning that the Cooke wouldn't have been the only radio rule to not be in a catalog while it was in production.

The Cooke rule was most certainly the latest of all radio rules based on it being a fusion of the other two radio "special" rules. But having its inception as early as 1938 makes complete sense given K&Es association with the US Navy during most all of the late 1930s. I could see K&E sitting down with US Naval Academy instructors as early as 1935 discussing ideas for as many as five future slide rules. Five? Interestingly, the three radio rules - all except the Morrison - are not the only slide rules impacted by math professors at the US Naval Academy. They too are responsible for the best selling Model 4080/4081 Log Log Duplex and the Model 4110 Power Trig slide rule (see Chapter 6) produced around 1941, also a short-lived rule that never saw a K&E catalog.

As for the Cooke, the slide rule seems successful, as it's not scarce, especially after so many years of production. Its evolution can be easily traced over the nearly three decades of production, appearing in as many as six variants over its life-span (Journal of the Oughtred Society, Vol. 24, No. 2, 2015, p. 36). Four of these variants have covered celluloid edges and two have the inlayed celluloid, exposed mahogany edges beginning around 1952.

The first four variants were created by alterations and movement of the texts and emblems on the rule. For example, the <4139> model indication would not appear on the first variation of the rule, which included all samples up to ~852,000 in serial number, dating to somewhere in 1941. Likewise, the circular "Cooke Radio" badge is on the back of the side with the earliest model, unlike most of the later variants. I will spare the reader the tedium of each variant, but a few other notes are worth stating:

The ST scale would be replaced with an SRT scale around 1956, just as with all other Deci-Trig slide rule models.
The circular K&E logo, which was dropped from other slide rules around 1945, persisted from beginning to end with the Cooke Radio Rule.
Horizontal lines separate both scales on each rail up until 1951 when the rule converted to inlayed edges.
The familiar red, capitalized K&E logo, which appeared on the majority of rules after 1945, only appeared on the Cooke rule for one variation around 1945.

As I indicated, Cooke Radio rules are not rare, but they aren't as common as any of the K&E general-mathematics slide rules. They can be found on eBay at a frequency (pun-intended) of 1 or 2 entries per month, but as of this writing there are none. Average price for the collector will be around $50 in good condition with a case, and double that amount if box and documentation is included.

For more of the type of problems that can be done with the Model 4139 rule, enjoy this visit to Miguel Ramirez's My Rules page. Here Miguel offers a illustrated tutorial for use of the Cooke Radio rule. Additionally, interested students may also use the manual written by Cooke which came with each slide rule, as linked below. The manual notes the accuracy of the rule is more than sufficient for any calculation required, as most discrete components used in electrical circuits, such as resistors, have a tolerance of +/- 10%. It also provides 62 pages worth of problems for using the Model 4139.

Cooke Radio Slide Rule - A Supplementary Manual by Nelson M. Cooke
File Size:	4575 kb
File Type:	pdf

Download File

- image courtesy of Richard Smith Hughes, "Specialized Slide Rules for Electronic Engineers," The Oughtred Society.

National Union Radio Rule

Note: This slide rule might be better talked about later in Chapter 7 where we discuss the "Brick-o-Meter," as many parallels can be drawn from it. This rule is not listed in a K&E catalog and in fact doesn't have the symbol K&E or the words Keuffel and Esser anywhere on the rule. But its application makes fit better here.

Often referred to as the National Union Radio Reactance Computer, I have shortened it to the name found on the 4 page instruction pamphlet that came with this slide rule. We will talk about dating this rule shortly, but we should note here that the instructions state that the rule was "designed and Copyright 1937 by National Union Radio Corporation." This business, based in Newark, New Jersey, and formed in 1929, constructed radio (vacuum) tubes prior to researching and developing some transistors in the 50s prior to closing down in 1954. Keeping these dates in mind, let's look at the slide rule itself (see right).

The scale set is simple:

Front side: 4 [3, 2] 1
Back of slide: [extended 2 and 3 scales from front]

The purpose of the rule from these scales is clear...only provide scales that allow for the computation of inductive reactance, capacitive reactance, and resonant frequency without any other math operation.

For inductive reactance, the input frequency on the 3 scale (in cycles) is aligned to the index of the 4 scale. The hairline is moved to the value of the inductor on the 2 scale (in henries) and the total inductance is read under the hairline on the 4 scale (in ohms). For capacitive reactance, the value of the capacitor on the 1 scale (in farads) is aligned with the index of the 2 scale. The hairline is moved to the frequency value on the 3 scale (in cycles) and capacitance is read off the 4 scale (in ohms). For resonant frequency, the value of the capacitor on the 1 scale is aligned directly over the value of the inductor on the 2 scale, whereas the frequency can read off of the back window of the slide rule. Of course, all of these instructions are shown on the back of the rule. As indicated with the other radio rules, the engineer would have a good understanding of the decimal placement for these computations, as the domains for such a wide-variety of inductor and capacitor sizes makes for wild swings in results.

Back to the dating of this rule. Most people who have referenced this slide rule pinpoint the date somewhere in the 1950s. I believe this to be in error. For a business that closed down in 1954 and spent most of the 50s working with transistors, a date in this range would be too late. Likewise, if the National Union Radio rule was indeed modeled after the 1958W, then this rule would have been the N4058W by that time with an all-plastic cursor. The metal rimmed cursor of the National Union Radio rule is more of a match to the late 1930s versions. This would also align well with the 1937 date in the instruction pamphlet and the production date for the aforementioned Brick-O-Meter, which is identical except for the printed parts of the rule. This would also date the rule with all of the other radio rules discussed in this section. A local electronics company coming to its neighbor, K&E, and asking for a solution because they couldn't afford to give their employees a Cooke Radio rule? That sounds logical to me.

As such, I could see this simple rule as a cheap alternative for K&E to provide to National Union Radio Corporation for in-house use to provide the functions of the radio rule - and only those functions - in the least amount of money possible. Because there is no suggestion that the rule was sold publicly, not being published to a product catalog, and due to the rarity of the slide rule, it's my opinion that the rule is custom-made and commissioned for National Union only.

This slide rule comes up rarely on eBay, not even yearly. Cost will be variable. It will be sold cheap if the seller sets a Buy-it-Now price - which a seller is likely to do - simply because the slide rule doesn't look like it should have value. It is, after all, constructed like the cheap student rule of the 4058W series. But if this rule was auctioned, it would potentially go for several hundreds of US dollars.

The Sector Family

From 1881 to 1915, K&E catalogs illustrated sector rules for sale, in both 6" and 12" varieties, and in both ivory and boxwood construction. But in the interest of full disclosure, there are three statements to be made.

- Sector rules are not slide rules, nor do they operate on any of the mathematical principles contained in this very length document.
- There are no known sector rules, neither in ivory or boxwood, with the words K&E on them...anywhere...ever.
- There is zero information on the internet about K&E sector rules...anywhere...or from anybody.

So, why talk about them?

That is 34 years in which K&E offered sector rules in their product catalogs. Operating on the theory that K&E sold some slide rules during that time, the question becomes why no historical samples have been identified?

Conrad Schur, in an article about the Scofield-Thatcher slide rule, put it this way:

"The Scofield-Thatcher slide rule falls into a category of intrigue, which several collectors have been pondering for some time. This category also includes such other interesting examples as the Charpentier, and surveyor's and stadia slide rules, to name just a few. All of these were offered for sale for many years; they were inexpensive; and there was a large potential market (at least for the engineer's and surveyor's slide rules). The question then, is what has become of all these slide rules, or were they in fact shunned by their intended users, in favor of the more general-purpose models available? In the latter event, why did companies like Dietzgen, K&E, etc. persist in offering these models over and over for so many years? If any readers have any special insight (or even plausible theories) to explain this puzzlement the authors (and other collectors) would be pleased to hear about them."
- Conrad Schur, Journal of the Oughtred Society, Vol. 3, No.1, March 1994, p. 24.

I think such questions are more easily answered in the case of "K&E" Sector rules, with the simple answer that a) K&E likely didn't label identify these outsourced rules with their own identifying marks and b) sector rules would have fallen out of favor for much of those 34 years, if not all of them.

But even so, we will discuss them because they are intrigue me. And because they were always listed with the rest of the slide rule in K&E product catalogs.

4175 Ivory Sector

4176 Boxwood Sector

Demonstration Rules

Most slide rules makers produced larger versions of their popular slide rules for demonstration use. These rules are typically measured in feet, not inches, and are usually large enough for a room full of people to see. Such rules were often supplied to schools with the purchase of classroom sets of the regular sized rules or they could have been purchased separately. They also have a dual-purpose as marketing displays.

Such demonstration rules were surprisingly inexpensive, or least much less that you might otherwise imagine. An example is the 1958 price of the Polyphase N4053-3 at $13.50 and the 100 Mannheim Demonstration rule at $22. This is mostly because production techniques for the demonstration rules were not as involved as the real item. For example, the Deci-Lon Demonstration Rule in my collection is made of redwood, milled down to two rails and a slide in a 6.5 foot length. The rails are grooved and the slide is rabbeted, allowing them to slide within each other, with easy to fabricate metal brackets - or in this case wood made to look like metal - placed on the ends. They are painted white and marked with painted-on scales and other labeling to simulate the real rule. The cursors are typically two sheets of plexiglass, held together with wooden cursor rails and marked with a center-line. Gravity serves as the cursor spring.

As such, K&E demonstration rules were not intended to be exact-scale models, but merely functional duplicates of the real thing. No engine-divided celluloid facings or expensive wooden stock. They would not have taken much more time or material expense to build over the actual item.

K&E demonstration rules appeared for the first time in the 1933 Educational Catalog with the 100 and 101 models, the origin before that is not known, but there is though that they'd been around for a while prior, but not listed in the full line product catalogs. The Model N105 would follow, appearing first in the 1952 educational product catalog, but soon making its appearance in the full line 1954 catalog along with the Model 100. The Model 101 was dropped at some point between the 1936 and 1954 educational catalogs.

Another form of demonstration rule were those made for overhead projection. Thus, slide rules models could be made completely from a transparent or translucent material, marked with opaque scales, and placed on a 10 x10 inch "overhead projector," - probably from Dukane, 3M, or Bell and Howell - as commonly found in most classrooms of the era. These would appear as the Models 68-1960 and 68-1955 in 1962, supporting the Deci-Trig (4080/81) and Deci-Log (68-1100) respectively.

For the collector, these rules are obviously display pieces. They are rare to find in the wild unless posted for local/regional sale on Craigslist or other online market-place sites. They also pop up with some frequency on eBay, but buyers will always pay upwards of $100 extra for shipping, which reduces the number of actual sales of the large rules unless people finally change the price to something that makes more sense to buyer. Typical pricing of such rules can be between $50 to $2000 (or more), plus any relevant shipping charges, depending on the condition of the rule and demand.

Model 100 Mannheim Demonstration Rule

The earlier Model 101 made of thick, light-colored softwood. Later Model 100 types would be of much thinner redwood. -photo courtesy of eBay

Created to demonstrate the Mannheim, Polyphase Mannheim, and Beginner's series single-sided rules, the Model 100 Demonstration Rule was likely K&E's first such rule. Its placement in the first educational catalog of 1933 tells us its purpose. The Model 4041, 4053 and 4058 rules were those supported, which were the K&E rules for the most basic of general math use, likely finding themselves in many classrooms. The catalog description tells us, "It's appearance will add to that of any classroom in which it may be placed."

The rule was approximately 7 ft. long and could be supported at each end with an eye screw. It had an indicator that suspended via gravity, so no spring was used. It was likely made of clear celluloid, though the catalog did not specify. Earlier pictures make the window look tinted, which is typical of celluloid as it ages...or in some cases, could be made tinted or might not have been made to be entirely transparent. Plexiglass was invented in 1933, the same year in which the rule first appeared in a catalog, but it's unlikely that plexiglass would be used with this product, at least not at first, unlike it would be used for later demonstration models. The wood was lightweight, but it is a type unknown to this writer as I've never seen this rule in the wild, although plenty do exist. Since later demonstrator models would be made primarily of redwood, it could be expected that it would have also been used with these earlier rules. Yet, many of these Model 100 samples use a light-weight, light colored wood such as spruce or fir. Samples of these rules do appear occasionally via sources like eBay, Craigslist, and Facebook marketplace.

To my best counting, eleven of these rules have been sold on eBay over the last 24 years with an average bid price of $151. This would not have included the cost of any shipping, which could double the price of some of those rules sold. As I write this in July 2024, one is available on eBay with an asking price of $525, incomplete as it's missing the indicator.

Model 101 Polyphase Duplex Demonstration Rule

When the Model 4088 Polyphase Duplex was introduced in 1913, it represented a slightly different scale set than the typical Mannheim rules. So a demonstrator was needed to train groups using the new Polyphase scale set, particular in its duplex form. Thus, the Model 101 Polyphase Duplex demonstrator rule was born.

Model N105 Log Log Duplex Deci-Trig Demonstration Rule

8 foot 4081-3 demonstration slide rule

Demonstration rules from the 1962 slide rule catalog.

Model 68-1944 Mannheim Demonstrator

(100) 7 foot 4053 and N4058W demonstration slide rule

Model 68-1923 Log Log Duplex Deci-Trig Demonstrator

(N105) 8 foot 4081-3 demonstration slide rule

Model 68-1929 Deci-Lon Classroom/Demonstrator

Found on Craigslist in Austin, Texas, I am proud to have this massive slide rule in my collection. Most demonstrator rules are indeed not built of the same material as those from which they are modeled, so this giant Deci-Lon is not made of Ivorite ABS plastic, awesome as that would be. Instead, this Model 68-1929 Deci-Lon Demonstrator is made of big chunk of redwood about 6.5 feet long, with faces painted in white, and then screen-painted with scales. The cursor is two plates of plexiglass attached to wooden cursor rails, with a black cursor line affixed on both sides. Gravity keeps the large cursor in place, so no spring is required. The scales are painted black, with all inverse scales colored in red, and with red maker's marks, Deci-Lon 10 logo, and model/copyright numbers also printed in red. All scales from the actual Deci-Lon 10 slide rule are present on the demonstrator.

To hang this fully duplex slide rule on a wall, as I do in my classroom, it must be hung in a fixture that allows use of both sides. As such, K&E originally sold an optional trunnion (metal brackets) for mounting the slide rule to the wall, or free standing on a desk, allowing it to pivot to front and back positions. I mostly use mine as a display within my classroom, but I sometimes use it to demonstrate its use to my students. I also allow some of my students to use it, as I have extra credit for students who make a paper slide rule and are willing to demonstrate how a slide rule works. They enjoy using the enormous slide rule on my wall to demonstrate knowledge!

As a educator, perhaps one of the most satisfying aspects of having a demonstration rule comes from understanding its historical provenance. My sample came from Mineola ISD, as known by the metal ID asset inventory tag still attached to the rule. I find it exciting to know that a very small school district approximately 120 miles east of my residence in the Dallas/Fort Worth metroplex had K&E's most powerful demonstration rule hanging in a classroom. The implication is that their high school likely had an advanced math class, similar to my own, which not only used such a slide rule, but might have had a classroom set of Deci-Lon 10s for their students to use. Certainly this meant that some serious math computation was happening in Mineola, Texas, whereas a more simple demonstrator just wouldn't have sufficed!

Originally introduced in 1962 with the actual Deci-Lon family of slide rules, the 68-1929 demonstrator would have set you back $60, a very reasonable price considering its size. Remember, the actual Deci-Lon 10 slide rule was $25 itself. The 68-1929 would appear one last time in the 1967 catalog, disappearing after that.

(Afterword: Note that the 1968 price of the demonstration rule remained $60, while the actual Deci-Lon 10 rule rose to $28.50. Nevertheless, the 4081-5 (68-1200) Log Log Deci-Trig was $70 with the lower price case in 1968. As such, it shows the level of workmanship that went into K&E's "highest quality" rule at that time.

Model 68-1960 Log Log Duplex Deci-Trig Overhead Projection Rule

Log Log Duplex Decitrig Overhead Projector Slide Rule - Only shown in 1967 catalog

Model 68-1955 Deci-Lon Overhead Projection Rule

Deci-Lon Overhead Projector Slide Rule for demonstration

Beseler Projection Slide Rule

Coming soon

Chapter 5: Miscellaneous K&E Rules

There exists a number of products over the history of K&E that were highly specialized devices distributed by the company. All of these are known items with descriptions listed in their product catalogs. I deem them as "miscellaneous" because they are either unattached to a "family" or theme like we saw in the previous section OR they are unlike the traditional slide rules offered throughout K&E's long history . Some of these items were made in healthy quantities and aren't necessarily rare, but still have value due to age and uniqueness. Others were more expensive, produced in short supply, or existed for a very short duration of time.

Many such devices were listed for decades in company catalogs, but yet, for some reason, remain exceedingly rare - or even non-existent. As we saw when discussing the Sector Family of rules in the last chapter, it is this aspect of these rules that provokes so many questions.

As we talk about many of these devices throughout this section, I will not hesitate in an attempt to answer speculate with my own theories, plausible or not!
Yet, on the whole, all of these devices will be hard to find in good condition today, and all of them have excellent collector value.

As you will note, many of these are not linear slide rules, but rather cylindrical, circular, or pocket-watch style. In fact, one is completely mechanical. The term "computing device" might be a more apt title for many of the rules in this chapter. As such, there are some distinct categories in which they can be described. We have Cylindrical and Circular Rules, Pocket Watch Rules, Rare Linear Rules, and Mechanical Calculators/Charts.

My own collection does not contain many of the following analog computing devices, but I'll provide descriptions of them nonetheless. Of the devices I do have, I will be able to speak about them in a much more detailed fashion.

Cylindrical and Circular Rules

In a previous section, we described the Webb's Stadia Rule as a cylindrical-type of slide rule. And while it could have been discussed in this section as well, it just made more sense to me to discuss it with the other rules of the Stadia Family. However, the Webb's device is a good model for the two other rules that K&E offered during its history. The features will be the same: an internal drum marked with logarithmic scales and an outer frame of scales in which the inner drum can be translated and rotated. These two calculators are some of the most famous and well-known analog computing devices ever known.

Likewise, while other companies like Concise, Gilson, and Fearns are best known for their circular styles of rules, the origins of that design go back to a rule that K&E imported and sold during the earlier years of the company. It is here where we learn some of that history.

This is the Model 4012 Thacher Calculator in my collection, serial number 2758 which places its production date right around 1910 to 1912. It's a really good sample, with only a light amount of bleeding/spotting on the triangular scales covering the brass rails. This version was purchased through the F. Weber Co. in Philadelphia, which often distributed K&E products, as well as produced the occasional slide rule of their own. Note that the Weber badge on the box uses the "Thatcher" spelling as well.

Thacher Calculator (Models 1740/41 and 4012/13)

We start with this device because of its importance to history - and not just K&E history. The Thacher Calculator was first patented by Edwin Thacher in 1881 and manufactured by Stanley Ltd. in England. K&E arranged to have it distributed in the USA beginning around 1884, which is the copyright date of the first instruction manual produced by K&E. The device would first appear in the 1887 Catalog, known as the Model 1740. K&E likely imported the first Thacher Calculators, and shortly thereafter worked in partnership with Stanley to build them, with Stanley producing the scales and K&E building the rest, including the lovely box-jointed carry cases in which they came. There are slight variations in the construction of the earliest samples, enough to make me believe that there was indeed a transition between the earliest Stanley-made Thachers and those that would be made almost entirely by K&E.

Regardless, I believe the scales, indicated by the "divided by Stanley" text on the device itself, were always supplied by Stanley regardless of who made the remainder of the device.

At this time the main competition for such a high precision calculating machine came from another Stanley cylindrical rule, known as the Fuller Calculator (later carried by K&E as well, as discussed below) and the monstrous, heavy mechanical calculators of the day. Incidentally, the scales of the Fuller (made entirely by Stanley) look like they could have been divided by a similar machine to the Thacher, lending credence to the idea that K&E always sourced the scales from Stanley.

The Thacher features a 4" diameter drum 18" in length. The drum (also known as the slide) is covered with a single continuous "A" scale amounting to the equivalent of an 80" linear slide rule, rotating in a brass cage, with a secondary "B" and "C" paper scale on each of the triangular rail covered with shellac. Resolution is 4 to 5 significant figures, depending on the computation. Instructions for operating the rule were always affixed to the front of the Thacher's base, with serial number and maker-marks engraved on the back of the base. Likewise, the end of the drum contained patent information and the name of Edwin "Thatcher" curiously misspelled - note that the actual patent name differed from the name on the instruction plate as well as that in K&E Catalogs. "Thacher" is the correct name.

To use the Thacher, numbers are computed similarly to a normal slide rule. For example, to multiply something like 4.56 x 3.24, the number 456 on the drum (A scale) is aligned with the index of the B scale (there are indices at both the edge and the center of a triangular rail for convenience). Spin or slide the drum to the appropriate scale. Then, by spinning the cage to find the number 324 on the B scale, the product can be read off of the A scale. Like any slide rule, supply your own decimal! For division, bring the divisor to the dividend instead, and then read the quotient opposite the index on the B scale. Other computations, like cubes, can be computed by reversing the drum. Again, instructions for doing any such computations are affixed to the front of the device's base. These instructions, like the scales on the triangular rails, will often be deteriorated on many of these devices due to age.

By the turn of the century, it is estimated that around 1000 Thacher's were produced, with a rate that would increase greatly over the next couple of decades. These Thacher's are known as the Type I today, which includes the Model 1740 as well as a Model 1741 that came with a brass rail with attached 3" magnifying glass. These first appeared in the 1887 Product Catalog, priced at $30 for the Model 1740 and $40 for the Model 1741. The Model 1740 (equating to $970 in today's money) was mostly purchased by research and science facilities within university or government settings, as well as within industry for heavier number-crunching in engineering fields. It is widely known that Lowell Observatory used (and still displays) a Thacher Calculator for computing ephemeris data for celestial bodies, including Pluto, as discovered in 1930 by Clyde Tombaugh at that very observatory.

This Thacher Calculator and several K&E product manuals adorn my high school mathematics classroom.

In 1901, with the introduction of the new production line of slide rules built in-house, K&E changed the model number to the 4012, all of which are known today as the Type II version of the machine. It would remain known as the Model 4012 until the end of its production. The magnifying version was called the Model 4013 and is definitely considered the more rare of the two Thachers. These versions represented a significant change from the original models. The changes included a more open design of the ends of the brass cage where it is mounted to the base, as well as a possible change to the 4" drum from a metal to wood. This newer version would use only 2 screws on each end to mount the cage, whereby the Type I model used 3 screws per side. Bob Otnes (J.O.S, Fall 2001, p. 39), who owns both Type I and II Thachers, is of the opinion that K&E took over production entirely at this point, making the numerous changes because Thacher's original copyright ended in 1898 and was not renewed. This would have given K&E the liberty to make the device themselves without royalties to Thacher (or Stanley) and could build the units as they see fit.

The Type II would exist from 1901 to 1927 with an annual production rate increase to around 164 units. According to the September 1994 Journal of the Oughtred Society (Feely and Schure), the last known Model 4012 has a serial number of 5432. I own a Type II variant myself, serial number 2758, which places it maybe a little later than 1910 for a production date. It still uses scales with the "divided by Stanley" text on the drum, so it's clear to me that Stanley still had something to do with the Type II, even if Thacher himself might have been kicked to the curb, so to speak.

In 1927, along with many aesthetic changes that K&E was making to their slide rules, a new Thacher Type III variant began, changed enough to take on a "N-" prefix. As such, the new Model N4012 and Model N4013 added a single pedestal attachment to the cage ends, as well as knurled rings and white number bands at the ends of the cage. The Model N4013 sales ended right at the start of World War II in the early 1940s, with the Model N4012 lasting until the early 1950s, appearing in the 1952 catalog one last time. Priced at $130, it's easy to see why the Thacher would no longer be a viable option during that "golden-age" of nice, less expensive, yet powerful slide rules, not to mention the ingenious Curta Calculator (see sidebar below) introduced in the 1950s. And traditional usage of the Thacher could be replaced entirely with vacuum tube and transistorized mainframe computers into the 50s and 60s.

This scaling off of production was precipitous after the war, with the last known serial number ending just

Table of Contents