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History of the Slide Rule

A slide rule is a mechanical device once used to perform complex calculations for chemistry equations to structural engineering that are now done electronically. The slide rule’s popularity increased over the years, and by the 20th century they were utilized by scientists, engineers and school children. The Oughtred Society says researchers have estimated that, in the 20th century alone, 40 million slide rules were produced.

John Napier
- The slide rule’s history begins with John Napier, a Scottish mathematician. In 1614, Napier discovered logarithms. The logarithms allowed mathematicians to perform the functions of multiplication and division through adding and subtracting. In 1617, Napier turned this into an invention referred to as Napier’s Bones—a set of two sticks used for calculations.
Edmund Gunter
- Edmund Gunter expanded on Napier’s idea by creating a number line or logarithmic scale. Gunter took the scale and placed it on a ruler using a pair of calipers as dividers to assist in performing mathematical calculations according to “The Oxford Companion to the History of Modern Science.” Using the scale and a divider, numbers were multiplied by measuring the distance that existed between each number and the beginning of the scale and then finding the number where the two distances combined.
William Oughtred
- William Oughtred was an Anglican minister who simplified Gunter’s idea by placing two of his scales side by side and eliminating the need for dividers. Oughtred is thought of as the original inventor of the slide rule, but many others throughout the years refined the instrument. By the 17th century the slide rule was a common instrument among scientists and mathematicians.
Formats
- Slide rules have been made in a variety of formats from the original straight rules to circles, cylinders and spirals. Round and cylindrical slide rules allow for longer and more complicated computations. According to the Museum of HP Calculators, some cylindrical rules, when stretched out, were more than 40 feet long. Regardless of the format, the one unifying idea for all rules is Gunter’s logarithmic scale.
Modern Usage
- The invention of the calculator in the 1970s significantly reduced slide rule usage, but slide rules are still in use today for a variety of functions. The “Wired” website reported on an exhibit at Purdue University that put slide rules of scientific celebrities on display along with a wide range of slide rules made from a variety of materials. While calculators do all the functions of a slide rule more efficiently, slide rules are still effective for teaching students basic concepts of mathematics and for simple computations.

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