Collier's New Encyclopedia (1921)/Micrometer

MICROMETER (-krom′-), an instrument used with a telescope or microscope to measure small distances, or the apparent diameters of objects which subtend very small angles. Micrometers are variously constructed. The field of the telescope may be provided with a graduated scale, or a metallic ring, or a diaphragm having parallel and intersecting spider-lines or fine wires. The micrometer with a graduated scale is used for measuring distances by direct comparison.