English

An auxiliary device to provide measurement of very small angles or dimensions, by an instrument such as a TELESCOPE. See MICROMETER: FILAR, MICROME­TER: TRANSIT.

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.

A form of registering MICROMETER with its movable wire placed in the FOCAL PLANE of an astronomic TRANSIT and at right angles to the direction of motion of the IMAGE of a STAR which is observed at or near CULMINATION. The transit micrometer is also termed an impersonal micrometer, because it almost completely eliminates the effect of the PERSONAL EQUATION on time observations made with it.

A notched SCALE placed at right angles to the movable wire of a MICROMETER and so designed that one turn of the MICROME­TER SCREW will move the micrometer wire across one notch of the comb; the central notch of the comb in conjunction with the zero of the microme­ter head furnishes a fiducial point from which all micrometer readings are reckoned. The comb is used for keeping count of whole turns of the MICRO­METER SCREW, parts of turns being read on the gradua­ted micrometer head.