What is the least count of a standard metric micrometer depth gauge?
0.01 mm
The least count of any micrometer is fixed by two design figures: the pitch of its spindle screw and the number of divisions on the thimble.
In a standard metric micrometer, including the depth-gauge type, the spindle screw has a pitch of 0.5 mm, so one full turn of the thimble advances the spindle by exactly 0.5 mm along the barrel.
The thimble is divided into 50 equal parts, so each single division represents a fraction of that one-turn travel.
Dividing the pitch by the number of divisions gives \(\dfrac{0.5}{50} = 0.01\ \text{mm}\), which is the smallest length the instrument can read directly.
A least count of 0.1 mm or 1 mm is far too coarse for a precision micrometer and matches a steel rule or coarse scale instead.
A reading of 0.001 mm needs an added vernier scale on the barrel (a vernier micrometer), which the standard instrument does not have.
Hence, the answer is 0.01 mm.
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