308 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



main current. The scale of the oscillograph in the secondary circuit was 

 different from that used before, but the general shape of the induced 

 current is shown by the boundary of the shaded area v. Curves C and 



F of Figure I show the forms of in- 

 duced currents in the testing coil in 

 the case of a very large magnet the 

 cross section of the solid core of which 

 had an area of about 500 square cen- 

 timeters. A and D show the corre- 

 sponding currents in the main circuit : 

 in the first case the generator was a 

 battery of 40 storage cells, and a con- 

 siderable amount of extra resistance 

 was used in the circuit ; in the second 

 case the same final current was caused 

 by a battery of 10 cells, and very little 

 extra resistance was needed. This 

 particular engraving, which was made 

 by the "Wax Process," does not repro- 

 duce the original exactly, for the upper 

 portions of A and D are here too 

 nearly horizontal. 



A very uncommon form of second- 

 ary current is shown in Figure J. 

 Curve 1 represents the form of the 

 main current of a very large electro- 

 magnet with massive core. At the 

 axis of a portion of the core was a 

 longitudinal hole about an inch in 

 diameter, and in this hole was inserted 

 an iron rod around which a layer of 

 insulated wire was wound to serve as 

 a test coil. Curve 2 shows the form 

 of the induced current in this coil 

 when the main circuit was closed ; the 

 dotted curve gives the form of the 

 induced current when the main circuit 

 was suddenly broken. The crest of 

 the curve 2 does not come until fourteen seconds after the main current 

 starts. 



Figure K shows the manner of growth of a current of final intensity 

 2.3 amperes, under a voltage of perhaps 60, in a coil of 1388 turns 



!N3BUnO 



