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(mīkrŏm`ətər, mī`krōmē'tər). 1 Instrument used for measuring extremely small distances. Typical examples are devices used in astronomical telescopes to measure the apparent diameter of celestial objects and similar devices used in microscopes. In both of these devices a fine hair or filament is moved from one extremity to the other of the image of an object and the distance read on a calibrated scale. Another typical micrometer is the micrometer caliper, a device in which an object to be measured is enclosed between two jaws, one fixed and the other movable by means of a fine screw. When the jaws are just touching the object, the distance between the jaws can be read on an associated scale, often to an accuracy of 10−4 (one ten-thousandth) in., or 10−6 (one millionth) m. 2 Unit of linear distance equal to 10−6 (one millionth) m. It was formerly known as a micron.



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.


Blazhko, S. N. Kurs prakticheskoi astronomii, 3rd ed. Moscow, 1951.



An instrument attached to a telescope or microscope for measuring small distances or angles.
A caliper for making precise measurements; a spindle is moved by a screw thread so that it touches the object to be measured; the dimension can then be read on a scale. Also known as micrometer caliper.
A unit of length equal to one-millionth of a meter. Abbreviated μm. Also known as micron (μ).


i. A unit used to measure the effectiveness of filters. One micron is a micrometer, or one millionth of a meter (10−6).
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.


1. any of various instruments or devices for the accurate measurement of distances or angles
2. a type of gauge for the accurate measurement of small distances, thicknesses, diameters, etc. The gap between its measuring faces is adjusted by a fine screw, the rotation of the screw giving a sensitive measure of the distance moved by the face


(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.