A vernier micrometer.
A micrometer is an instrument for measuring accurately small dimensions or separations. Its basis is that when a screw is turned once, it advances or retreats a distance equal to its pitch. The hairlines in telescope and microscope eyepieces are adjusted by means of a precision micrometer screw to measure separations.
The micrometer caliper has a G-shaped frame on whose "leg" is a scale; inside the leg runs a screw (of pitch usually 0.5 mm) attached to a thimble, calibrated for fractions of a turn, which runs over the scale. An object is placed between the screw's spindle and an anvil at the far side of the G's opening, and the screw turned until the object is just held.
For greater accuracy (of the order of 1 μm) a vernier scale may be used.
A micrometer is also a unit of length, equal to one millionth of a meter (μm). (British spelling: micrometre.) Known in the CGS system of units as the micron.