Dial caliper

A further refinement to the vernier caliper is the dial caliper.

In this instrument, a small gear rack drives a pointer on a circular dial. Typically, the pointer rotates once every inch, tenth of an inch, or 1 millimeter, allowing for a direct reading without the need to read a vernier scale (although one still needs to add the basic inches or tens of millimeters value read from the slide of the caliper). The dial is usually arranged to be rotatable beneath the pointer, allowing for "differential" measurements (the measuring of the difference in size between two objects, or the setting of the dial using a master object and subsequently being able to read directly the plus-or-minus variance in size of subsequent objects relative to the master object).

The slide of a dial caliper can usually be locked at a setting using a small lever or screw; this allows simple go/no-go checks of part sizes.
http://en.wikipedia.org/wiki/Caliper