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in astronomy, an instrument for measuring small distances in the focal plane of an astronomical telescope or measuring microscope. The measurement is usually accomplished with the aid of a precision micrometer screw, whose angle of rotation is proportional to the linear displacement, in the instrument’s field of view, of a frame with measuring wires, with the frame being driven by the motion of the screw.
This principle is the basis for the construction of the filar micrometer, which was first used by the French astronomers and geodesists A. Auzout and J. Picard in the second half of the 17th century. Filar micrometers are widely used in optic tubes and measuring microscopes of astronomical and geodetic instruments. A micrometer in which the frame can be rotated in the focal plane so that it is possible to measure not only the distances between the images of celestial bodies in the focal plane but also the position angles of the line joining them is called a position micrometer. Astronomers make use of the registering micrometer, invented by the German instrument maker A. Repsold at the end of the 19th century; this instrument makes it possible to register the moments for some positions of the micrometer wire as it moves across the telescope’s field of view. For good micrometers the errors do not exceed 0.002–0.003 of a screw revolution, and the accuracy of the reading is about 0.5 jam. A spiral micrometer is used when more precise scale readings are required; in this instrument a fine-pitch spiral of Archimedes is visible in the field of view of the eyepiece. By rotating the spiral so that it coincides with the marks on a scale, one can obtain a reading with an accuracy of about 0.1 jam. Some use is made of a micrometer in which the measurements are made by superimposing the two images of an object that are obtained in special prisms made of ordinary or birefringent optical material.
REFERENCESBlazhko, S. N. Kurs prakticheskoi astronomii, 3rd ed. Moscow, 1951.
V. V. PODOBED
ii. A unit of linear measurement equal to one millionth (10−6) of a meter. Also called a micrometer.
iii. The amount of pressure exerted by a column of mercury one-micrometer (one millionth of a meter) high under standard conditions. A micron of pressure is equal to .001 mm of mercury.
micrometer(1) One millionth of a meter. Also known as a "micron." See metric system and micron.
(2) A mechanical instrument that is used to measure diameters and small distances.