## Wednesday, February 13, 2008

### Wednesday Math, Vol. 15:slide rules

As a math teacher, I don't like to admit that any part of my subject is obsolete, but you have to be pretty damn old school to still be using a slide rule. The idea is that the numbers are put in a logarithmic scale on scales A and B (click on picture for larger version) which means that the distance from the number 1 to the number 3 is the same as the distance from 2 to 6 or 3 to 9 or 4 to 12. By sliding Scale B over so that the 1 on scale A is next to the 3 on scale B, you can easily multiply by 3. You should be able to do this in your head, of course, but it does let you get good approximations of multiplying large numbers together, and using scale B and scale C together, you can get approximations of square roots.

There's other stuff you can do, but it's been ages since I used one, and I can only remember the basics. Some old guys when I was a kid were really wizards with these things.

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