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As a math teacher, I don't like to admit that any part of my subject is obsolete, but you have to be pretty damn old school to still be using a slide rule.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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ken said...

Fun slide rule simulator at

commander other said...

at the risk of dating myself, growing up, the only slide rule i was actively aware of was the one where you wait for the moron ahead of you to get the hell off the slide before you come down.

unless, of course, you thought it might be funny to plow into them at the bottom.

especially if they had already face-planted into the sand.