126 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



current became too small to measure. In some cases it seemed better 

 to omit the second part of the process. 



The Establishment of a Steady Current in the Coil of an 



Electromagnet. 



If the circuit of the exciting coil of an electromagnet contains a 

 battery of storage cells of constant voltage E, and if this circuit be 

 suddenly closed, the strength of the current will rise more or less 

 gradually from its initial zero value to E/7- amperes, where r is the 

 whole resistance of the circuit in ohms. In the case of a given magnet, 

 with a given electromotive force in the coil circuit, the manner of 

 growth of the current depends very largely, as we have seen, upon the 



SECONDS. 



Figure 19. 



Currents from a battery of 20 storage cells in the circuit of a coil of 2788 

 turns belonging to the magnet Q. Before the middle curve was taken, the core 

 was carefully demagnetized. The upper and lower curves represent direct and 

 reverse currents, respectively. The areas Fand IF are equal. 



magnetic state of the core when the circuit was closed. The three 

 curves of Figure 19, which are carefully made reproductions of the 

 photographed records of an oscillograph, show the march of the current 

 from a battery of 20 storage cells in the circuit of a coil of 2788 turns 

 belonging* to the magnet Q under three different sets of conditions. If 

 after the core had been demagnetized as thoroughly as possible, by the 

 method already described, the circuit was suddenly closed, the current 

 followed the middle curve of the three. If the current was allowed 

 practically to attain its maximum value, and if then a commutator in 

 the circuit was reversed and, at intervals of a few seconds, reversed 

 again and again, and if finally the circuit was broken, it was possible 

 by closing the commutator again in the proper direction, to make the 

 new current follow either the upper or the lower curve of the diagram. 

 If this current coincided in direction with the last current through the 



