A mechanical device formerly used by engineers and
scientists for rapid and approximate multiplication, division, extraction of
roots, raising to powers, and other simple computations. The slide rule has
been almost totally superseded by the small hand-held electronic calculator.
The principle of the slide rule is the translation of all computations to
equivalent additions or subtractions that can be carried out on a set of scales
sliding over each other. Thus, two uniformly graduated marked scales can be
used for addition or subtraction, multiplication, division, powers, and
logarithms. Other scales, such as for sine, cosine, and tangent, and logarithm
and for calculations involving p (pi) are also found on the usual rectilinear
slide rule. A glass runner or cursor with a finely engraved vertical line is
provided for easier alignment of the scales.
The computational accuracy possible depends on the size of
the slide rule and on the care with which the scales are printed. The commonly
used 10-in. slide rule permits multiplications and divisions to be made with an
accuracy of about 1/10th percent, which suffices for many engineering
calculations. Both the rectilinear and the less commonly used circular slide
rule were invented by the English mathematician William Oughtred shortly after
the discovery of logarithms. Various special slide rules have been devised for
the solution of widely applicable engineering formulas, or for business
calculations, such as the determination of interest, compound interest
accumulation, and depreciation.
(Adapted from Encarta Encyclopedia)