COKER'S INSTRUMENTS. 213 



by the deflection of the needle. The number of divisions 

 through which the screw has been turned affords a measure 

 of the angular twist of the bar. The actual value of the 

 angle corresponding to a division of the micrometer screw 

 is determined in the first place by means of the graduations 

 on the circle and the vernier. It will be noticed that, as 

 the micrometer screw moves tangentially, its readings are not 

 strictly proportional to the angles, but to the tangents of 

 these angles ; but for small angles the errors involved are very 

 small, and may be neglected as compared with the quantities 

 under measurement. In the case of the particular instru- 

 ment described by Mr. Coker, the mean of the calibration 

 tests gave 18'6 divisions of the micrometer screw as 

 corresponding to one minute of arc, or, in other words, 

 one division of the micrometer screw is equivalent to about 

 one-thousandth of a degree. It will be seen that the 

 accuracy of the readings is dependent upon the accuracy 

 of the making and calibration of the micrometer screw. 



A second instrument of greater precision is shown on 

 Fig. 103. Here the micrometer screw and electrical 

 apparatus are replaced by a reading microscope, which is 

 made use of much in the same way as the one in Professor 

 Ewing's extensometer (see p. 118 ante). As in the last 

 instrument, one end of the measured portion of the bar 

 carries a graduated circle, whose attachment is made 

 through a self-centering chuck of somewhat novel form, 

 by means of which the centre of the graduated circle is' 

 made to automatically coincide with the centre of the test- 

 bar. In this instrument the graduated circle is at A, the 

 chuck B, the vernier plate N carrying a radial arm O, 

 from which projects an arm carrying the wire P upon 

 which the readings are taken. 



At the other end of the bar is attached an arm S, by 

 means of three centering screws ; and, at the extremity of 

 this arm is carried a reading microscope, R, by means of 

 which the readings are taken. For convenience in read- 

 ing, the eye-piece of this microscope is placed at right 

 angles to the main tube, and a right-angled prism is inter- 

 posed between the objective and the eye-piece. A 

 graduated glass scale is placed in the focus of the eye- 

 piece. The calibration ot the instrument is effected by 

 moving the wire upon which the readings are taken 

 through a definite angle of ten minutes, and noting the 

 equivalent reading of the micrometer eye-piece. In the 

 particular instrument described by Mr. Coker it was found 



