What’s a slide rule and how are they so useful?
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It’s basically a manual calculator. It lets you do division and multiplication (and sometimes square and cube roots depending on the design) before digital calculators were widely available.
If you think of a regular ruler, you an see how you could use two rulers to add…put a 2″ ruler with a 4″ ruler and you’ve got 6″. 2 + 4 = 6.
Slide rules do the same idea, except they’re marked in a particular way (not every interval is the same length) so that “adding” lengths on a slide rule works out to multiplication. “Subtraction” does division. This uses a branch of math called logarithms…there are other good ELI5 posts on that if you want to know more.
They’re useful because, if you’re proficient, they’re very fast and they don’t need any batteries or anything. And, when they were in common use, regular calculators either didn’t exist or were ludicrously expensive. A slide rule, even a really good one, is comparatively cheap.
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