Circular and Combination Slide Rule Devices

ALCO Addiator with Slide Rule

Here's an unusual item for you! This all-metal gizmo contains a basic slide rule on the back, and an addiator for addition and subtraction on the front. You even get inch and centimeter rulers, if you pull out the slider piece. The included stylus is used to help you do you basic sums on the addiator front ... it's actually quite efficient once you get the hang of it. Included with this rule was a simple one page instruction sheet on how to use the slide rule portion. Either the other pages got lost, or they thought the addiator was self-explanatory (it isn't, but you can figure it out pretty quickly). What I'm really puzzled about is who really made this thing! The addiator clearly says "Made in Japan", but I know from Peter Hopp's book that Kingston in Hong Kong made an identical model. And the early Faber Castell addiators look virtually identical, as you can see from this page of a fellow slide rule enthusiast showing a Faber addiator. The Faber slide rule portion is infinitely more advanced than this crude thing, but aside from a more rakish-looking stylus, the only difference on the addiator face is the Faber name, black colour scheme, and the lack of the "Made in Japan" label. According to Dieter von Jezierski's book, the addiator was actually invented by Carl Kubler, who received a German patent on it in 1937 and made it available for Faber rules. Although originally made out of brass, later rules used sheet metal (like this one?). Judging from Dieter's Faber catalog listings, I'd guess that the closest thing Faber offered was a 67/87R Rietz model (although I'm sure the slide rule part was much nicer). Interesting rip-off ... if anyone out there has any more information about this nefarious business, I would love to hear it!

Update: Ron Manley's excellent website now includes detailed instructions on how to use an addiator. Check it out if you want to learn more about these calculating devices.

ASA E6-B Flight Computer

A glaring omission in my collection, finally rectified with this inexpensive offering from ASA. The E6-B Flight Computer (commonly referred to as the Dalton Flight Computer) is one of the few true slide rules still in general manufacture today. Although its slide rule features are limited to the unlabelled C/D scales on the circular front portion, this low-tech device is commonly used by pilots for flight path calculations, wind correction, fuel consumption, etc. (the back of the rule has a useful graphical method of doing vector calculations involving trigonometry). Of course, hand-held electronic versions are also available to do all these functions and more, but I suspect a lot of flight enthusiasts don't want to trust their lives to the charge storage capacity of your average alkaline battery. E6-B models come in a variety of sizes and materials (including aluminum, plexiglass, and the cheap fiberboard model shown above), and are easily obtainable at any of the online pilot-gear stores. This particular rule was made by ASA of Seattle, Washington, and comes with a simple little 30 page introductory manual and plastic see-through case. Both the rule and manual are copyright 1987. A very nice little device, although I'd recommend you splurge and get yourself the more durable aluminum version.

Update: The origins of this ingenious device (and flight computers in general) appears to be a little more contentious than I originally thought. Based on some suggestions from the folks at the Yahoo Slide Rule Discussion Group, and some miscellaneous online resources, it seems the standard accepted theory is that a US Navy pilot named Philip Dalton developed a series of flight computers in the 1930s, including the ever-popular E6-B somewhere around 1932 (originally called the Dalton Dead Reckoning Computer). However I've been contacted by one flight enthusiast who claims to have a 6B flight computer of British manufacture from the 1920s, so I'm unclear as to the exact origins of this device. Regardless of which, the Dalton E6-B has survived to this day in slightly modified form through the hands of a large number of civilian manufacturers (you can even get 6 foot tall classroom demonstration models from one of them!).

Concise No. 28

This was my first circular slide rule, picked up on the cheap in a local antique shop. A fairly common basic circular model, this one is in excellent condition with the original manual. In terms of precision, it's roughly equivalent to a 10 inch Mannheim rule, but is obviously a lot smaller. This is the key advantage of circular rules, in that you can squeeze much more onto them by arranging the scales in a circle (you can calculate exactly how much more using one of these!). The disadvantage is that a small discrepancy in lining up the hairline with the inner cursor scales could result in a potentially major error in reading the outer scales. Concise was the largest maker of these, many of which you'll find imprinted with some company name or logo, as they were often used as little give-away promotional items. This one, conveniently made to fit nicely in a shirt pocket, has conversion tables on the back and a nice faux leather case (i.e. vinyl). A good example of a basic circular rule that still looks brand new.

Update: It turns out Concise is still in the slide rule manufacturing business! Check out their website to see current models available for purchase. I haven't ordered from them directly yet, but I did take part in the first group purchase by the International Slide Rule Discussion group (see below).

Concise No. 300

This is one of Concise's higher end circular slide rules, made in a special run for the International Slide Rule Discussion Group at Yahoo in the fall of 2002. First thing you'll probably notice is the inclusion of the group's logo in the center of the rule. Everyone involved did an incredible job organizing the production of these rules, most especially Ted Hume who did most of the leg work. A much more substantial rule than the common No. 28, in every sense of the word. It's considerably larger (about four and half inches in diameter and a lot heavier), duplex in nature, with 15 scales including basic trigonometry. The way it works is that the inner scales rotate independently on each side, while the outer scales are completely fixed. The case is identical to the earlier model No. 28 above, although the manual is a slightly disappointing photocopy now. A detailed copy of the instruction sheet is presented on my Slide Rule Manuals page. All in all, a very nice rule and a happy addition to the collection given that Concise will still manufacture these on demand.

Otis King - Model L

A gift from a retired Engineer, this marks my first cylindrical rule, also known as a tubular or helical slide rule. This design features spiral scales that can be read by roating and extending the scale portion from the holder base, and reading off the marker indicators on the opaque midsection cursor (which can be moved up and down the scale portions). The advantage of this design is that allows you to put much longer scales into a small area, thus providing greater precision (up to 4 or 5 sig figs in this case). Otis King of England was perhaps the best known maker of this type of rule, which came in two major models, type K and type L. The type L model shown above features a log scale in addition to standard multiplcation/division scales. Since the scanner "flattened" out the look of metal holder, I thought I'd include a digital camera pic reduced to approximately the same resolution for comparison. Both pics show the black hard cardboard box the rule came in (the rule telescopes down to fit snuggly inside). Interestingly, each individual Otis King rule seems to have been marked with a unique serial number (R.3170 in this case), which is matched to the hand label on the box it came in. I'm also fortunate to have a copy of the common manual for Otis King rules, which I've included in its entirety on my Slide Rule Manuals page. A scarce and beautiful rule, in excellent condition, and a welcome addition to my collection.

Update: Dick Lyon maintains a page dedicated to Otis King rules, which can be found at his website. It's a great resource for those hoping to date their rules, or simply compare them to others out there. The range of known models is considerable.

Sterling Silver Tie Clip

A gift from a retired engineer, this is an actual working slide rule tie clip. Although relatively rare, you still see these items come up for auction from time to time (they were obviously popular novelty items with slide rule users). This particular clip seems to be of higher quality than most, and features 3 scales, a movable slide, and magnifying cursor with cursor line. I haven't included more detailed scans, as I figure the somewhat blown-up images above give you a fairly representative view of what it looks like. The rule is actually a lot shinier than it looks here, but I captured images in fairly low light conditions so that glare wouldn't obscure the features. I suppose it is a bit of a stretch to consider it a combination device, but it is a functioning slide rule and tie clip, and a welcome addition to the collection.

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Page last updated on December 12, 2002 -
All material © 1999, 2002 by Eric Marcotte, Ph.D.