General Science, Mathematics, and Technology


What is a slide rule, and who invented it?

Up until about 1974, most engineering and design calculations for buildings, bridges, automobiles, airplanes, and roads were done on a slide rule. A slide rule is an apparatus with moveable scales based on logarithms, which were invented by John Napier, Baron of Merchiston, and published in 1614. The slide rule can, among other things, quickly multiply, divide, square root, or find the logarithm of a number. In 1620, Edmund Gunter (1581–1626) of Gresham College, London, England, described an immediate forerunner of the slide rule, his “logarithmic line of numbers.” William Oughtred (1574–1660), rector of Aldbury, England, made the first rectilinear slide rule in 1621. This slide rule consisted of two logarithmic scales that could be manipulated together for calculation. His former pupil, Richard Delamain, published a description of a circular slide rule in 1630 (and received a patent about that time for it), three years before Oughtred published a description of his invention (at least one source says that Delamain published in 1620). Oughtred accused Delamain of stealing his idea, but evidence indicates that the inventions were probably arrived at independently.

The earliest existing straight slide rule using the modern design of a slider moving in a fixed stock dates from 1654. A wide variety of specialized slide rules were developed by the end of the seventeeth century for trades such as masonry, carpentry, and excise tax collecting. Peter Mark Roget (1779–1869), best known for his Thesaurus of English Words and Phrases, invented a log-log slide rule for calculating the roots and powers of numbers in 1814. In 1967, Hewlett-Packard produced the first pocket calculators. Within a decade, slide rules became the subject of science trivia and collector’s books. Interestingly, slide rules were carried on five of the Apollo space missions, including a trip to the moon. They were known to be accurate and efficient in the event of a computer malfunction.


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