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The Slide Rule

© Copyright 2000, Jim Loy

Just a few years ago, before electronic calculators, slide rules were very popular. Now they have almost disappeared. On the left is a simple slide rule, simpler than the ones that students used to use because this one adds. Here I am adding 4+5=9. We are just adding lengths. That is all that a slide rule did, add lengths. But the scales were actually adjusted so you could multiply (and take powers and roots). This was done by using Logarithms (logs). As you may recall, you can multiply by adding logs. Well, if you use logs on a slide rule, then you can multiply by adding lengths. The slide rule above right uses logs, and shows 4x6=24 (1 is the left edge of both scales of this slide rule, and 64 is on the right edge). Like most real slide rules, here there is no linear scale. So this slide rule cannot be used to add.

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