Micrometers are divided into mechanical micrometer and electronic micrometer. ① Mechanical micrometer. Micrometer, short for micrometer, is a portable universal length measuring tool that uses the principle of precision thread pair to measure length. 1848, J.L. Palmer of France obtained the patent for the outside micrometer. 1869, J.R. Brown and L.Sharp of the United States made the outside micrometer into a commodity to measure the outside diameter of metal wire and the thickness of plate. There are many kinds of micrometer. Micrometers with different purposes can be made by changing the shape of measuring surface and ruler frame, such as micrometer for measuring inner diameter, pitch diameter of thread, common normal of gear or depth. ② Electronic micrometer. Also known as digital micrometer, grating length measurement technology and integrated circuit are applied in the measurement system. Electronic micrometer appeared in the mid-1970s, and was used to measure the outside diameter.