History of the Slide Rule

The Slide Rule is almost 400 years old

The slide rule appeared around 1625, soon after the propagation of the logarithms by the Scottish nobleman Napier in 1614.

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Gunter scale

Edmund Gunter designed around 1624 the first logarithmic scale, for multiplication and division by constructing sum and subtraction of scale distances with a pair of dividers.

William Oughtred, mathematician and parson in Oxfordshire, was the first to describe in 1632 a circular slide rule in his paper "The Circles of Proportion and the Horizontal Instrument", but by then the instrument had already been applied for at least 10 years. During the 17th and 18th century, many specialized slide rules have been designed by mathematicians and instrument makers for most diverse applications. They were used for navigation at sea, land surveying, artillery ballistics, alcohol gauging, and carpenters calculations. Next to the usual scales for multiplication and division, the goniometrical scales were included not much later. Late 19th century the dual log scales were added. Different materials (ivory, wood, metal, and later plastics) and various constructions were used.

The users eventually appeared to favour a fixed body, a sliding tongue, and a moving cursor with a hairline. Next to the linear slide rule, also circular and cylindrical shapes were used. On a cylinder one can achieve a very long effective scale, by applying spiralling or parallel subscales. A longer scale on a slide rule results in a higher accuracy.
From the second half of the 19th century, the industrial production of slide rules made the instrument available to all engineers.
When electronic pocket calculators started to rise around 1970 (and especially the "scientific calculators", like the Hewlett Packard 35 in 1972), the less accurate and slower slide rule was soon defeated.

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HP 35