Otis King's Patent Calculator believed to be a model 'L' circa 1921
Cylindrical Slide Rule with Helical Scales 429 and 430.

The model L adds a linearly divided scale section on the top, instead of two decades of logarithmically divided scale. A linear scale on a slide rule is used to find logs and anti-logs, which can be used along with multiplication and division to do arbitrary powers and roots.

The bottom scale for model L is usually scale 429, with 430 on top, but other pairs of scale numbers are sometimes found on older units.

It can can perform multiplication and division, logs, anti-logs, square roots and arbitrary exponential powers.

Otis Carter Formby King (1876–1944) was a grocer and engineer in London who invented and produced a cylindrical slide rule with helical scales, primarily for business uses initially. The product was named Otis King's Patent Calculator, and was manufactured and sold by Carbic Ltd. in London from about 1922 to about 1972.

With a log-scale decade length of 66 inches, the Otis King calculator should be about a full digit more accurate than a 6-inch pocket slide rule. But due to inaccuracies in tic-mark placement, some portions of its scales will read off by more than they should. For example, a reading of 4.630 might represent an answer of 4.632, or almost one part in 2000 error, when it should be accurate to one part in 6000 (66"/6000 = 0.011" estimated interpolation accuracy).

The Geniac brand cylindrical slide rule sold by Oliver Garfield Company in New York was initially a relabelled Otis King; Garfield later made his own, probably unauthorized version of the Otis King (around 1959). The UK patents covering the mechanical device(s) would have expired in about 1941–2 (i.e. 20 years after filing of the patent) but copyright in the drawings would typically only expire 70 years after the author's death.

See http://www.svpal.org/~dickel/OK/OtisKing.html for further details.

This Otis King calculator was very kindly donated by Jim Odell

Manufacturer: Otis King
Date: 1921

This exhibit has a reference ID of CH31221. Please quote this reference ID in any communication with the Centre for Computing History.

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