Midget Slide Rule Part 2
published 2025-12-30
by Christopher Howard
I disassembled and reassembled the cursor assembly a few more times, and now my Midget slide rule is functioning at a satisfactory level — at least well enough to proceed with investigating the scales. My current configuration left one unused washer, which I have placed in a ziploc bag. Now if I turn the L cursor, both the S and L cursors turn and the angle is preserved. I can also change the angle by holding down the L cursor while turning the S cursor. According to the instructions, I am supposed to be able to simply move the S cursor freely, i.e., without having to hold down the L cursor. But if I try to do that, sometimes the L cursor catches and they start turning together — so something is still not quite right.
I observe that there is also some slight inaccuracy introduced depending on where the cursors are around the dial, as though a cursor is not perfectly centered, or some similar problem. Generally, if I am expecting the answer to be x (some whole number) the answer might actually be somewhere between x and x+0.1.
I tested some basic multiplication and division using the C and CI scales. These are fundamentally straightforward, but working with two cursors and one dial does require some extra thought, if you are used to working with two rulers and one cursor.
I also played around a little with the log scale. The basic idea is that you can set a cursor to any number of the C scale, and the log scale will show the logarithm for that number. A bonus feature of this slide rule is that, having two cursors, you can use the log scale for addition and subtraction — the log scale being linear. This is a feature I have not seen on any of my other slide rules, which would require having two log scales — one for each ruler. I have sometimes wondered if slide rules could, or should, be used for addition, but this is the first slide rule I've seen that implements it. Of course, this method of addition is rather limited, since the two numbers have to be within one or two orders of magnitude of each other. So you couldn't repeatedly add many small numbers into a large number.
A nice part of working with a circular slide rule is that, unlike the straight version, you don't have to give any thought to keeping your answer on the ruler, i.e., you don't have to switch indexes to keep your answer from falling off the end of the ruler. I think this would allow for rapid multiple calculations on a circular slide rule with two dials. That benefit is offset, however, on a duplex slide rule like this one. Since both cursors must be set up for a new calculation, and there is no information contained in the relative offset of two dials, then there is not a way — on the slide rule itself — to preserve the result of a calculation as the starting point for the next one.
Copyright
This work © 2025 by Christopher Howard is licensed under Attribution-ShareAlike 4.0 International.