354 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



db 



T 



Figure 1. 



the coil of 2956 turns 

 wound on the spools 

 shown in the diagram. 

 The yoke was furnished 

 with a number of pairs 

 of pole pieces or jaws, 

 to receive specimens 

 of different lengths and 

 shapes. To measure the 

 amount of the induced 

 current in a test coil 

 wound closely upon the 

 piece to be examined, 

 a ballistic galvanometer 

 (V), described in a for- 

 mer paper, 3 was used. 

 The period of this in- 

 strument was so long 

 that the throw due to a 

 reversal of the exciting 

 current of the yoke did 

 not appreciably differ 

 from the throw which 

 the same quantity of 

 electricity would have 

 caused if it had been 

 sent instantaneously 

 through the circuit. 



The specimens used in 

 the work here described 

 were of two forms. The 

 first form (C, Figure 2) 

 was a cylinder about 1.27 

 centimeters in diameter 

 and about 15 centime- 

 ters long over all, with 

 tapered ends to fit tightly 

 in sockets in the ends of 

 the conical pole pieces of 

 the yoke. The sockets 



3 These Proceedings, 44 (1909). 



