Cylindrical pocket slide rule with packaging, Otis King Model K, metal / paper / metal / cardboard, made by Carbic Limited, London, England, 1950s
The Otis King pocket cylindrical calculator is an extendable tube having two slide rule scales, one that is fixed and an extendable one that can be rotated against the fixed one for calculations. The cursor is a brass or blacked metal outer tube that rotates against the two scale tubes. The scales are graduated spiral scales of log-log divisions. The length of the spiral improves the accuracy of the calculation over the standard linear slide rule. Scales on this model are: fixed: no.414, extended: no.423.