Small Strains in Bars subjected to Twist. 521 



again completed as indicated by the galvanometer-needle. 

 The number of divisions through which the screw has been 

 turned affords a measure of the angular displacement, and it 

 only remains to calibrate these readings in terms of angular 

 measurement. 



Fig. 1. 



The calibration is effected by reference to the graduated 

 circle and vernier-plate. 



The micrometer-screw is sej; tangentially to the radial arm, 

 and therefore its indications are nearly proportional to the 

 tangent of the angle moved through ; but if the faced end of 

 the screw is always maintained at or about its central position, 

 the error introduced by taking the readings as directly pro- 

 portional to the angular displacement is relatively small 

 compared with the quantity under measurement, and may be 

 neglected. 



