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
Michel:
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 multidigit 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 8digit 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 4digit 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 HP41CX.
Ernie
