What does a micrometer measure?

A micrometer is a precise measuring instrument that is specifically designed to measure small lengths or thicknesses with a high degree of accuracy. It typically consists of a calibrated screw mechanism that allows the user to measure dimensions down to the micrometer level, which is one-millionth of a meter. This makes it particularly useful in engineering and manufacturing environments where precision is crucial, such as in machining and mechanical design.

The design of a micrometer enables it to measure external dimensions, internal diameters, and depths, making it versatile for various applications where accurate length measurement is required. This precision can be critical in ensuring parts fit together correctly within tight tolerances.

The other options relate to measurements that are not within the micrometer's capabilities. Fluid pressure is measured using manometers or pressure gauges, temperature requires thermometers or thermocouples, and electrical current is measured with ammeters or multimeters. Thus, the micrometer's primary function of measuring length distinctly categorizes it as the correct choice for this question.