286 Measuring-Instruments used in Mechanical Testing. 



ported on the test-bar by a screw d ; the upper clip is sup- 

 ported on the lower by a stay-bar with knife-edges, e. At r 



Fig. % Fig. 3. 



s *w 



and m are the roller and mirror, the axis of these being at the 

 same distance from the knife-edge of the stay-bar as the set 

 screw of the clip. A touch-piece or finger, /, attached to the 

 lower clip presses on the roller. If the bar extends, the roller 

 approaches the lower clip by an equal amount ; it turns 

 against the finger/; and the amount of rotation is read by a 

 telescope and scale. This instrument will easily read to 

 J th of an inch. The roller being at the centre of the 



clip, its movement is the mean of the elongations on the two 

 sides of the test-bar. 



The author showed a third instrument on the same principle, 

 for obtaining the compression of small blocks of stone. 



