For more than three centuries, they were essential to scientists, engineers, even businessmen. Slide rules could multiply, divide, and find square roots. Later in their history, they could perform trigonometric functions, too - without batteries or solar power. But one needed training to use a slide rule accurately, and they could not be used to add or subtract.
Pocket calculators quickly made slide rules obsolete. In 1972, Hewlett-Packard introduced the first scientific pocket calculator, the HP 35 (cost: $395). By 1975, Keuffel & Esser, America's premier slide-rule maker, had stopped producing them. High-school math-club members who used to have slide-rule cases clipped to their belts soon sported calculator holsters.
Slide rules consist of a carefully calibrated ruler with a central sliding mechanism. The "cursor," a glass with an indicator line on it so that answers can be read more easily and accurately, was added in 1859.
The discovery of logarithms in 1614 by mathematician John Napier made slide rules possible. The first known one was made in 1654 by Robert Bissaker.
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