Description
Developed during the seventeenth century, the modern slide rule is based upon the design by William Oughtred (circa 1630). It is one of many calculation devices that is based on the logarithmic scale, a calculation method invented in 1614 by John Napier.

Before the rise of the pocket electronic calculator in the 1970s, the slide rule was the most common tool for calculation used in science and engineering. It was used for multiplication and division, and in some cases also for ‘scientific’ functions like trigonometry, roots and logs, but not usually for addition and subtraction.

Calculations are determined by aligning a point along the central movable strip with a mark on one of the scales on one of the two fixed strips, and then observing the relative positions of other points along the scales. A logarithm transforms the operations of multiplication and division to addition and subtraction according to the rules log(xy) = log(x) + log(y), and log(x/y) = log(x) - log(y). Moving the top scale to the right by a distance of log(x) aligns each number y, at position log(y) on the top scale, with the number at position log(x) + log(y) on the bottom scale. Because log(x) + log(y) = log(xy), this position on the bottom scale gives xy, the product of x and y.