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Otis King Slide Rule

Otis Carter Formby King
Online Manual

Standard slide rules (Slide Rule) had limited precision. The Curator's father (Arthur Roswell) always had a 6" slide rule in his engineers pocket protector. If he needed more precision, he used a standard slide rule (about 12") and then a 3' long slide rule for even higher accuracy.

The Otis King Calculator packaged a huge slide rule in a small size by making the scales 66" long and winding them around a cylinder like a barber shop pole.

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

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