|Re: Slide rule|
Message #21 Posted by Ernie Malaga on 2 Dec 2002, 8:07 p.m.,
in response to message #19 by Michel Beaulieu
Using a slide rule is tricky at best. The absolute most basic concept is, as Dave has pointed out, that the scales are logarithmic. Of all the scales, C and D are the "main" ones. If you want to multiply 1.2 times 5, you move the "1" of the C scale so it is on top of the 1.2 of the D scale. Then you travel along the C scale until you reach 5, and read the answer in the D scale: 6.
Things begin getting complicated when you work with several multi-digit numbers, for then an answer (on the C or D scale) that reads "2" may, in fact, be 0.2, 20, or 2 x 10^-12. There are guidelines for placing the decimal point correctly, and I used to know them 30 years ago, but not anymore.
Also tricky is estimating numbers. You get at least 8-digit precision when working with calculators, but slide rules -- even the best ones -- rarely give you more than 3. Very large slide rules may give you 4-digit precision. Plus, the movements one makes when sliding the rule are never precise, and the errors tend to accumulate.
And then there are the _other_ scales (the typical slide rule has a couple dozen), such as CI, DI, CF, DF, A, B, S, T, ST, L, LLn, LL0n, and so on ad nauseam.
I keep my old Faber Castell slide rule as a memento, but whenever I need to calculate something, I pick up my HP-41CX.