COKER'S INSTRUMENTS. 211 



to the other, and, as the shaft twists under the stress 

 imposed, these two move with respect to one another ; that 

 is to say, the pointer moves over the graduated arc and 

 traces out the angle of twist. The larger the diameter of 

 the graduated circle, of course, the more precise will be the 

 readings which it is possible to obtain. Suppose that the 

 smallest linear measurement which can be conveniently 

 read by direct means to be y^t-h of an inch, and the radius 

 of the pointer to be 20 in.; this one-hundredth of an inch 

 at the circumference corresponds to an angle of 0'03 of a 

 degree. 



FIG. 102. Coker's Screw Micrometer Instrument for Measuring 

 Torsional Strains. 



In their experiments on the torsional strength of shafts, 

 Messrs. Platt and Hayward made use of beams of light 

 reflected from two mirrors, instead of material arms. 

 Unfortunately the details of the apparatus employed are 

 not given in the paper describing the experiments.* 



1 18. Coker's Instruments for Measuring Small Torsional 

 Strains. Two instruments of greater delicacy than those 

 described have been designed by Mr. Coker, of Mc.Gill 

 University. 



The former of these is shown on Fig. 102. The shaft 



* Proceedings of Institution Civil Engineers, vol. xc 



