MI'CROM'ETER (from Gk. pit ,6{, mikros, small + LLE7por, nu iron, measure). Any device by 11.4 of which it is possible to make a linear ineasorement more accurately by using levers, serews, or magnifying glasses than by using a simple rule or settle. Fig. I shows a shnple form of lever mm•rnn•1er adapted to the measurement of thieknesses, diameters, and the like. The movable lever AR turns on a pivot at C. and since the arm Cit is five times as long as the arm CA, the pointer at the end of 11 Will move over the scale D five times as far as the points are opened at A; and conse quently the measurement is about five times as ,o curate as if a scale were applied directly. Fig. 2 illustrates a form of simple screw mi crometer. The screw has ten threads to the inch, and consequently one complete revolution will remove the point of the screw, A. from the plate, C. one-tenth of an inch. The head, 11, of the screw has its rim divided into one hundred equal parts; hence a rotation of the screw through one of these parts means one one-hundredth of a complete revolution, and such a motion would remove the point from the plate by a distance of 1/100 of 1/10, or 1/1000 inch. A very common form of screw mi crometer, described and il lustrated Under CALIPERS, has forty threads to the inch, and the head is di vided into twenty-five parts, making the accuracy 1/25 of 1/40, or again 1/1000 of an inch.
In working with the tele scope and the microscope it becomes necessary to make measurements upon the image formed by the ob jective, and for this purpose a micrometer ocular is employed. The simplest form of this device is a fine scale ruled upon. glass in hundredths of an inch, or tenths of a millimeter, and so inminted in the draw tube that it will be soon distinctly by means of the eyepiece, and hence trill lie in the plane of the image formed by the objective. The scale appears to lie upon the object. and it is only necessary to read off the dimensions. A revolution of the draw tube makes measurements in different directions possible without moving the object.
A more accurate and satisfactory micrometer ocular is that. devised by Ilamsden, and illustrat ed in Fig. 3. 11 is the divided head of a mi crometer screw, S, reading to a hundredth of a screw revolution, 1/200 millimeter for example.
The screw is so arranged that it Will cause a rec tangular frame, AA, to move backward and for ward as the screw revolve. -\eloss t he middle of the frame 1 \ .are stretched two line spider lines, at right angles to the axis of the screw, and quite close together. The whole device is so attached to the draw tithe of the microscope telescope that the spider-lines lie in the focal plane of the objeetive, and hence are distinctly seen magnified by the ocular. In making meas urementr with this instrument the screw S is turned nntil the eoiderslines straddle one point. and then a rending is made of the position of the head. 11. Next the screw is again turned un til the lines straddle the other point, another reading is made, and the difference of the two readings the distance between the points upon the image. By placing a known scale, for example a tenth of a millimeter, upon the stage of the microscope, and measuring the image as above. the magnifying power of the microscope objective is obtained, and it is possible to cal culate what distance upon the stage, or in the object, corresponds to one revolution of the mi crometer screw. The whole number of revolu tions of the screw• is sometimes read by means of a second wheel, so geared to H that it makes one revolution for twenty or thirty revolutions of the screw S. In other cases a strip of metal with small teeth like saw teeth, and as far apart as the threads of the screw• 5, is placed across the side of the opening so that the double spider line appears to move over it from tooth to tooth, each tooth corresponding to one complete revo lution of the screw. Such micrometers are used in measuring objects under the microscope, in most accurate linear and angular determinations, and in telescopes for obtaining star distances, and for a great variety of measurements. A very elaborate and delicate micrometer attached to the eve end of the telescope and used in star work is called a position micrometer. A special form of micrometer is used for measuring the star distances on the photographic plates that are taken of star groups and clusters. See MI CROSCOPE: TELESCOPE.