MICROMETER ( from isafph small, and a measure), the term generally applied to contrivances for measuring small spaces or angles with great accuracy or convenience.
The word is not applied to some artifices for subdividing the gradua tions of an astronomical instrument (for these see VERNIER), nor when a magnified portion of a subsidiary are is used, which may be beet con sidered under the heads of SECTOU, and Zeerrn Secret', though they come properly under the definition. We shall follow the usual mean ing of the term 1. 117re Mieronitter. —When the rays from any bright object fall upon a convex lens, an inverted Image of the object is formed, which may be viewed by the eye:piece as if it were a material body. If a fine wire or spider's web be stretched across the telescope tube at the place where the image Is formed, this too will be seen distinctly through the eye-pieee. Instead of fixing the wire to the telescope tube, it is stretched across a sliding-piece, which is moved by a screw perpen dicularly to the length of the telescope, and can thus be imule to immure the image in terma of the revolutions and parts of the screw. The head of the screw in divided, and there is an index by which the parts are read off. A little tongue passing over the notches of a plate notes the whole number of revolutions, An English gentleman named Gascoigne seems first to have applied this principle to practice, but he unfortunately lost his life, in 1644, in the great civil war ; and though hie telescope fell into Townley's hands, and was used by him, the construction does not seem to have been generally known until it was re-Invented by Auzout in 1666. Different improvements were gradually made nearly up to the present time.
The plate carrying the wire is drawn by the screw, and held back by springs, which prevent any lost time. A micrometer of this kind is now generally applied to circles, transits, and theodolites, in addition to the fixed wires, which of course arc always necessary. There are two verifications : first, the ascertaining the value of a revolution of the screw ; and secondly, determining the reading of the screw-head when the moveable wire coincides with the fixed wire. In a circle or theodo lite the micrometer wire is placed upon a sharp distant object, and the divided limb read off. The screw is turned through several revolu tions, and the object is again bisected by moving the whole instru ment by its tangent screw, and the divided limb read off a second time. We have then the same angle measured in revolutions of the screw and in the divisions of the instrument, and by a simple proportion have the value of a revolution and of a part. With a transit, the pas sage of Polaris over the micrometer wire is observed after successive revolutions of the screw. The angular motion of Polaris for the inter vals is computed from the polar distance, and thus the value in arc obtained for a revolution of the screw. To determine the zero position
of the micrometer wire, the moveable wire is brought to touch the fixed wire, first on one side and then on the other, and the screw-head read off each time. The mean of the two readings will be that when the two wires are exactly superimposed.
The position wire micrometer is much used for observations of double stars, and is the wire micrometer proper for equatorials. In this con struction there are two wires parallel to each other, each moveable by its own screw : the whole apparatus can also be turned round in the plane of the wires, so as to place the wires in any direction, the angle round which it is turned being read off by two verniers upon a small circle called the position circle. In measuring a double star the wires are brought near each other, and the apparatus turned round until the two stars are either threaded on one of the wires, or, being placed between them, are judged to lie in the name direction. The division of the micrometer circle is then read off, and the observation in posi tion is made. Now, by the divided circle of the micrometer turn the apparatus round 90°, and the wires will be at right angles to the line joining the two stars. By moving the equatorial, place one wire A on one of the stars, and place the other wire a, by its screw, on the second star. Read off the screw-head of u, and then place A on the second star by moving the equatorial, ands on the first by moving its screw, and read off the revolutions and parts of B. The difference of the two readings of B will give, in revolutions and parts of the screw, twice the angle between the two stars. The process may be repeated, keeping is fixed and moving A. Before or after a series of observations the zero or index error of the position circle should be ascertained. Place the instrument nearly in the meridian, and make a star run along one of the wires from end to end. Read and note the position circle, which should mark 90° and 270°, and the difference from this is the correction to be applied to all -the angles of position observed during the evening. The value of a revolution of the screw may be deter mined by separating the two wires a given number of revolutions, and observing a series of transits of known stars over them. As large equatorials are always carried by a clock movement, we should recom mend fixing the position micrometer upon a slipping-piece, by which a small motion up or down or to the right or left can be given to the wires without meddling with the clock or the equatorial. With this apparatus the measurement of double stars is perfectly easy. The wire micrometer requires illumination for seeing the wires, and the light thus admitted often obliterates faint and ill-defined objects.