212 



OOKERS INSTRUMENTS. 



whose torsional strains are to be measured is marked S. 

 On one end of the length to be measured is attached the 

 graduated circle A, through a chuck plate B, which is 

 itself attached to the specimen by means of the three 

 centering screws adjustable by hand. At the other end of 

 the measuring portion is attached the arm E, which works 

 on a swivel joint carried by the ring C. This ring has also 

 adjusting screws with which to fix it to the shaft. The arm 

 E terminates in a contact ball F, which can be brought into 

 contact with the end of the micrometer screw H, which is 

 mounted on a vernier plate J. This can be clamped in 

 any position on the graduated circle, the fine adjustment 



FIG. 103. Coker's Microscope Torsional Measuring Instrument. 



being made with the tangent screw N. A cell and galva- 

 nometer, M and L, are joined in circuit through the bar, so 

 that when the ball F is in contact with the screw H the 

 circuit is completed, and the galvanometer needle is 

 deflected. 



The instrument is used in the following way : The 

 contacting pieces being supposed to be touching, a twisting 

 moment applied to the bar will separate these pieces, and 

 the circuit be broken. The micrometer screw must now 

 be advanced until the circuit is again completed, as shown 





