Spiral micrometer, also known as micrometer, spiral micrometer and centicaliber, is a more precise tool for measuring length than vernier caliper. It can measure length accurately to 0.01mm, and the measurement range is several centimeter. Part of it is processed into a thread with a pitch of 0.5mm. When it rotates in the threaded sleeve of the fixed sleeve B, it will move forward or backward. The movable sleeve C and the screw are integrated into one, and its periphery is divided into 50 equal divisions. The number of complete turns of the screw is measured by the engraved lines spaced 0.5mm apart on the fixed sleeve. The part that is less than one turn is measured by the engraved lines around the movable sleeve. The final measurement result needs to be estimated to one decimal place.