slide rule

Close-up of part of a slide rule showing cursor

The slide rule works by the addition and subtraction of the logarithms of numbers. (A) To multiply a by b, the 1 mark on the sliding logarithmic scale (blue) is set opposite the a mark on the fixed logarithmic scale (red). The product a.b is then read off the fixed scale opposite the b mark on the sliding scale. (B) To divide a by b, the b mark on the sliding scale is set opposite the a mark on the fixed scale. The quotient a/b is then read of the fixed scale opposite the 1 mark on the sliding scale. A standard 250mm long slide rule will give answers correct to about 3 significant figures.

A device based on logarithms which, before the advent of electronic calculators, was used for rapid, though approximate, calculations.

Two scales are calibrated identically so that, on each, the distance from the "1" point to any point on the scale is proportional to the logarithm of the number represented by that point. Since log (a.b) = log a + log b, the multiplication a.b can be performed by setting the "1" point of the scale (1) against a on scale (2), then reading off the number of scale (2) opposite b on scale (1). Division is performed by revering the procedure.

In practice, slide rules have several different scales for different kinds of calculation, and a runner (cursor) to permit more accurate readings. scales.

Related category

• COMPUTERS, ARTIFICIAL INTELLIGENCE, AND CYBERNETICS

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