What is a slide rule?

   Slide rule is an instrument composed of various scales, the positions of which in relation to each other may be altered, used for the rapid performance of certain arithmetical operations. The scales may be engraved on straight rods, disks, or on the surface of a cylinder. In its simplest form it consists of two rules, arranged to slide on each other, and so divided into scales that by sliding the rules backward or forward until a selected number on one scale is made to coincide with a selected number on the other, the desired result is read off directly on a third scale. By means of a duplex slide rule, where the rule may be set for four factors instead of two, more complicated problems may be solved. Revolving slide rules are employed to increase the virtual length of the scales and the number of decimal places to which results may be read. Circular slide rules, resembling watches, are also made. The various slide rules proper depend on the mechanical use of logarithms, and the scales are graduated on a logarithmic basis. The slide rule used by research engineers and scientists has many special scales for the evaluation of trigonometric functions and angles, roots and powers, calculations involving ir, and hyperbolic sines and tangents. The size of the slide rule determines the accuracy of the answer. Larger rules are subdivided so that the anwer may be read to more significant figures than can be estimated on the smaller rule.