A slide rule is a device used for rapid numberical calculations that involve mainly multiplication and division. It typically consists of a ruler with a sliding middle section. Both the ruler and the slide have similar logarithmic scales printed on their corresponding edges. An indicator made of transparent material, with a vertical line down the middle, is used to fix corresponding points on the scales.

Slide rules were first devised in the seventeenth century, following the invention of logarithms by John Napier. The integration of logarithms into the existing repertoire of calculating devices was sealed by the English mathematician Edmund Gunter, who published his account of a calculating instrument using logarithms in 1623. Gunter described a set of single scales. To use these a pair of dividers or compasses was required, with which the quantities were stepped out along the length of the scale. The straight slide rule (such as the one shown above) was invented by English mathematician William Oughtred.

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John Napier
William Oughtred

THE ROBINSON LIBRARY
Science.--Mathematics.--Instruments and Machines.

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