Pocket cylindrical slide rule, Otis King Model L, metal / paper-coated metal, made by Carbic Ltd, Londin, England, 1921-1972
The Otis King pocket cylindrical calculator consists in a pair of concentric tubes having two sliderule scales, one that is fixed and an inner 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.429: logarithmic, extended: no.430: linear and log.