Callipers - Outside
Similar to dividers, spring callipers are typically constructed from two lengths (or legs) of curved steel that taper to a fine point to form an arc on either side of the calliper. The wider end is fitted with a loop of spring steel and a peg to help pivot the scribe on one leg. The distance between the leg points is adjusted by a screw and nut that is positioned one quarter to half way down the legs. The screw also helps keep the legs in place.
Firm point callipers keep their setting by friction between the legs at the fixed end. They can be adjusted with the application of light finger pressure or gentle taps.