Definition
A device attached to a TELESCOPE or microscope, consisting of a wire thread (filament) connected with a screw in such manner that as the screw is turned, the wire moves through a continuous succession of parallel positions, all in the FOCAL PLANE of the instrument. Any small angle may be measured by noting the number of turns and fractions of a turn of the screw which are made in moving the wire from one bisection to another, and multiplying this by the value of one turn. See MICROMETER: OCULAR, MICROMETER MICROSCOPE, and MICROMETER COMB.