Glossary
micrometer
A micrometer is a precision measuring tool that closes a spindle onto a part and reads the gap on a scale, to the nearest thousandth of an inch.
An outside micrometer measures small thicknesses and diameters rather than lengths along a rule. A C-shaped frame holds a fixed anvil on one side and a spindle that closes on the part from the other. The thimble at the far end turns the spindle on a fine screw thread, so a large turn becomes a very small movement.
The reading has two scales. The sleeve is marked off in tenths of an inch and every 0.025 inch — one full turn of the thimble; the beveled edge of the thimble is divided into 25 marks, each one thousandth of an inch. Read the sleeve for the last whole division, then add the thimble mark that lines up.
It is not a caliper. A dial caliper reads inside, outside and depth with one tool, quickly, and is good to a thousandth at best; a micrometer measures one dimension and measures it harder. That is why bearing journals, shim stock and wire are checked with a micrometer and layout work is done with a caliper and a rule.
Taught in
Related terms
- dial caliper — A dial caliper measures outside, inside and depth dimensions, and reads them on a round dial instead of a ruler — commonly to a thousandth of an inch.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)