dial caliper

A dial caliper is a precision measuring tool that takes outside, inside and depth measurements and reads them on a round dial rather than on a ruler. In a shop question it is the instrument for the outside diameter of a shaft, the inside of a bore, or the depth of a hole, to a thousandth of an inch.

Its parts are worth naming: two large jaws for outside work, two smaller ones for inside work, a depth rod at the far end, a sliding body, and the dial, whose needle usually sweeps once per tenth of an inch with each small mark worth one thousandth. Close the jaws to zero the dial before you start.

Handle it like the precision tool it is — a light touch, no dropping, and never as a wrench or a scribe — and store it with the jaws just open. Its close relative in a shop is the micrometer, which takes very accurate outside measurements of one dimension.