A variety of micrometers exist for different applications. The Three major types of micrometers are outside, inside, and depth. All micrometers operate based on the principle that an accurately made screw will advance a specified distance with each complete turn. Micrometers graduated in the inch system advance 0.025mm for each turn.
Micrometers have both a linear and circumferential scale. The linear scale measures the axial advance of the spindle. It is generally graduated in increments identical to the pitch of the micrometers screw. The circumferential scale indicates the amount of partial rotation that has occurred since the last complete revolution.