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PR0CEEDINGS OF THE AMERICAN ACADEMY. 



2788 turns were used in the exciting coil and a battery of 40 storage 

 cells with a voltage of about 82 furnished the electromotive force. 

 Curve C evidently corresponds to a case where the total resistance 

 in the circuit is about twice as great as in the case represented by A, 

 but for every value of k the building-up time is greater for C than 

 for A, though the difference becomes very small at the end. A com- 

 parison between A and D shows the same fact. Before each of the 

 curves A, B, C, D, was taken the core of the magnet was carefully de- 

 magnetized. Figure 26 exhibits current curves taken for different 

 values of r with the same coil of the magnet Q and with the same elec- 

 tromotive force as the curves just mentioned. In each of the cases 



Figure 26. 



Currents in the coil of the electromagnet Q for four different values of r when 

 E and the number of magnetizing turns were fixed. At the starting of each 

 current the core was magnetically neutral. 



shown in Figure 26 the core was put several times through a cycle 

 before the direct and reverse oscillograms were taken. The records are 

 reproduced as accurately as possible ; B, C, and D run together in a 

 complicated manner, and the same tendency is shown in the reverse 

 curves G, H, I, but in general the longer building-up times belong to 

 the lower currents. 



If in an inductive circuit without iron r and L are fixed, the build- 

 ing-up time will be independent of the value of E, but this is not the 

 fact if the circuit contains an electromagnet. Figures 27 and 28 show 

 current curves obtained from the coil of 2788 turns belonging to the 

 magnet Q. In all the curves of each diagram the value of r was the 

 same, but the voltage of the battery in the coil circuit had three differ- 



