A micrometer is a tool for measuring, not a unit of length. You hold it in your hand. People in workshops and labs use it when a normal ruler is too rough — for a thin wire, a metal part, a sheet of paper. It looks like a metal C with a screw. You turn the screw until it just touches both sides of the thing. Each little turn moves the screw a known tiny distance, so you can read a length as small as a thousandth of a millimetre, or even smaller. That is why mechanics and engineers like it. Though actually, in American English the same spelling, micrometer, is also used for the unit (a millionth of a metre). In British English the tool is the micrometer and the unit is the micrometre. In casual talk people mix them up all the time.

千分尺（工廠、機械科也常叫分厘卡）