Midget Slide Rule
published 2025-12-29
by Christopher Howard
So far as I can tell, my sister doesn't have any personal interest in slide rules. Nevertheless, sensing my own interest in the subject, she bought me a vintage slide rule for Christmas. The model I received was a Gilson Midget, a circular slide rule four inches in diameter. Some documentation of the Midget is available here:
It came with the original instruction manual. The slide rule disc is marked MADE IN U.S.A., copyright 1919, and patent 1-17-1922. One interesting feature of the scales, which jumps out, are three trigonometric scales on the back covering, respectively, angles from 0 to 30, 30 to 60, and 60 to 90 degrees, with each of the three scales listing degrees, sines, and tangents, in combination. There is another set of scales on that side providing conversions between common fractional values (9/16, 5/8, etc) to their decimal values. The other side is mostly common place scales such as C and CI scales, an A scale, and Log and Log-Log scales. There is, though, an interesting drill and thread system toward the center of that side.
The most unusual characteristic of the slide rule, however, is that it is a rare example of a "duplex circular rule". Instead of having two rotating dials and a cursor, as you would expect, there is one dial and two cursors. The basic idea is that you set the L (long) dial to 1(10), and then you set the S (short) dial to the number you want to multiply or divide by, using the appropriate scale (C or CI). Then you rotate them both so that the S dial points to the number being multiplied or divided. The answer then would be on the L scale.
The tricky part of this whole idea is that the cursor must be able to allow two seemingly contradictory feats: you must be able to turn the S cursor separately from the L cursor, to set the ratio, and then you must be able to turn the S and L cursors simultaneously while exactly preserving the angle between them. The magic that allows this seems to be based entirely in the design and arrangement of the bolts and washers that hold the cursors in place.
Unfortunately, this functionality is not working correctly, and I have not been able to fix the problem yet. It seems that either (1) the bolts/washers/cursors were incorrectly (re)installed by somebody, or (2) the washers are slightly damaged, or both. I have tried taking it apart and reassembling it several times, but I have not yet had success in getting it to work correctly, at least for all angles. Practically speaking, I can't really learn about and play around with the scales until I get the cursors working correctly. Hopefully I can get that figured out .
I tried looking up patent 1-17-1922, thinking that it might containing an assembly drawing, but that patent doesn't seem to be searchable on the U.S. patent database.
Copyright
This work © 2025 by Christopher Howard is licensed under Attribution-ShareAlike 4.0 International.