My Slide Rule Collection

My Own Concoctions

Here are some of the slide rules I've made for my own use. There have been others, but I don't know where they went.

The scales for these were usually designed on a computer and printed out with a laser printer. The bodies were formed with layers of cardboard glued together, and the prints were then glued to the top. Very much like the way celluloid scales are glued to mahogany or bamboo to form a "real" slide rule. Just think of the fun I could have if I had a CNC engraving machine ... I'd make a circular slide rule out of AOL CDs.

Navigation Calculator, 3" Circular Duplex

On the front this slide rule has A|B|Sin scales on two wheels. On the back, it has linear scales, 0-360|0-360|0-180-0 (Double Naysmith Director). I made this for playing Silent Hunter, and sank a LOT of Japanese merchantmen with it. I hope none of them were carrying slide rules...

Two-Slide 6" Simplex

The scales, from top to bottom, are: Log10, r(1-x^2), x^2[x^2, arcsin(x), arctan(x), x]x, exp .00x, exp .0x, exp .x, (pi)x[(pi)x, sin & tan, 1/x, x]x, x^3, x^4. This was an experiment; I've never really used it for anything.

Chain Maille Link Aspect Ratio Calculator

This slide rule is for calculating the aspect ratio (ratio of wire diameter to ring diameter) of the links used in making chain maille - another of my many, many hobbies. I've posted this on the Maille Artisan International League website here, where it can be downloaded, printed, and made.

Weightwatchers POINTSfinder slide, 3.5"

I gave the original back to my wife - this is a 1/2 scale version I made by reducing the original on a photocopier. It works just like the original does, only it fits in my pocket.

Books, Manuals, Instructions, etc

My Wish List

These are slide rules or adding machines I don't have, but wish I did. Y'know, just in case Santa Claus stumbles across this site... These images were stolen without remorse from eBay. If they're yours and you want me to remove them, contact me and I will be happy to accomodate you.



The Curta is a very cool machine, designed by Curt Herzstark while incarcerated in a Nazi concentration camp! This amazing little machine can add, subtract, divide, multiply, fit in your pocket, grind pepper, predict the weather, and unlike those boring electronic calculators, the Curta has ten billion moving parts!

Otis King Cylindrical Slide Rule

Otis King Cylindrical slide rule

The advantage of a cylindrical (actually helical) rule is the great scale length, and the precision that goes with it, in a small package. In the case of the Otis, the scale was almost sixty-six inches long! This let the scales be read to more significant figures than usual. Or at least they would be if I had any clue how to use it.

Thacher's Calculating Instrument

Designed by Edwin Thacher in the late 1870's, and marketed by K&E as the model 4012 and 4013, the Thacher had a scale length of sixty feet and could be read to as many as five significant figures. The scales - the longest ever made at the time - are broken into a fourty small sections, which are arranged in parallel and cylindrically. The inner cylinder can be slid back and forth, and rotated, to align the inner scale with the outer scale. An amazing and clever design.

Thacher's Calculating Instrument

When it first came out in 1887, this baby went for a whopping $30 - equivalent to $550 in Y2K money. It was available until 1952, when it went for an even more whopping $130 - $840 in Y2K money! They go for more than that on eBay.

Since I'll never be able to afford a real one, I built a virtual one in Blender:

My Virtual Thacher's Calculating Instrument

This uses the scales that can be found on the Slide Rule Museum website, and can actually be used within Blender. I didn't get the ratio of the length to the diameter quite right the first time, so I used this virtual model to re-calculate the proper length. There's something really cool about the idea of a virtual slide rule recalculating it's own length...

Five digits of virtual precision!

[paragraph about replica builds here.]

Monster Teaching Slide Rule

7' Teaching Rule

Teaching slide rules were designed to be big enough to be seen from the back of the classroom. (Actually, they were designed to sell slide rules by exposing students to a particular brand.) This one is a Pickett, as are most of the ones I've seen going on eBay for hundreds of dollars, but there are also other brands. Since I'm sure I'll never find one I could afford, I'll probably end up making myself one. I'll hang it up in my shop and use it to design even more useless stuff!


© 2010 W. E. Johns