Slide Rules

Cylindrical slide-rule made by Keuffel & Esser about in 1885, on patent by Edwin Thatcher in 1881

Loose pebbles were the first objects used to aid humans in calculating numbers. Later they were used as counters on ruled boards before a range of materials were used to suspend counters on wire frames. This fixed wire structure known as the 'abacus' was developed in India and later adopted in China and Europe.


In 1614 John Napier invented a system of logarithms that simplified some of the more complex calculations needed for navigation. This table of 'logs' was converted into a series of engraved numerals on wooden rods which were arranged and read in a specially made box. Known as 'Napier's Bones' this early calculator was improved on by Edmund Gunter who simplified Napier's logarithms making his 'bones' an essential tool for navigators throughout the seventeenth and eighteenth centuries.

Gunter had also contributed to the development of another navigational instrument, the 'sector' which used a pair of dividers to measure distances that were then aligned against the 'sectors' scales. Gunter found he could speed up the calculation of a pair of logarithms by engraving a scale onto a piece of wood and then use the dividers to add the values. 

This piece of wood, known as 'Gunter's Line of Numbers', was quickly adopted in Europe and around 1621 William Oughtred decided to use two of these 'lines' to slide against each other to remove the need for dividers. This was the prototype of many slide rules developed over the next 400 years although the first real slide rule with a fixed stock was invented by Robert Bissaker in 1654. 

By the eighteenth century the timber measuring rule designed by Henry Coggeshall and the gauging rule of Thomas Everard had set the standards by which instrument makers fashioned slide rules. Changes and modifications were made over the next hundred years to suit the many needs people had for using the slide rule: circular slide rules appeared in the early 1700s, in the late 1700s Boulton and Watt developed a special engineering rule known as the 'Soho rule' and in 1851 Amedee Mannheim popularised a simple slide rule that included a moving cursor. 

by Geoff Barker

References
Baxandall, D., 'Calculators Machines and Instruments, Science Museum, 1975
Wynter, H., and Turner, A., 'Scientific Instruments', Studio Vista,1975
Kidwell, P., and Ceruzzi, P., 'Landmarks in Digital Computing; a Simthsonian Pictorial History', Smithsonian Institution Press, 1994
Williams, M., 'A History of Computing Technology', Whitehall Books, Wellington, New Zealand, 1985
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