130 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



If Ejr and L are given, different values of E may be used by giving 

 properly corresponding values to the non-inductive resistance, and if 

 the " building-up time " of the current under given initial conditions 

 in the core be defined as the number of seconds required for the current 

 to attain any arbitrarily chosen fractional part of its final value, this 

 time will be inversely proportional to E. In the case of a circuit 

 which has one or more iron cores the phenomenon is much less simple, 

 and if the cores be of solid metal, the effects of eddy currents may 

 complicate the problem seriously; but although under these circum- 

 stances the law of proportionality no longer holds, it is almost univer- 

 sally true that the establishment of a current of given final intensity 



Figure 23. 



Direct and reverse current curves for the maj^net Q with a given final excita- 

 tion of 2650 ampere turns, under applied voltages of 82, 41, and 20.5, nearly. 



in the coil of a given electromagnet can be accelerated by increasing 

 very much the applied electromotive force and then introducing a 

 sufficient amount of non-inductive resistance to make Ejr the same as 

 before. 



Figure 22 shows current curves for the magnet Q under a fixed final 

 excitation of 2650 ampere-turns. In curves A, B, C, the currents 

 were caused by 40 cells, 20 cells, and 10 cells, respectively, and these 

 currents were made equal by adding to the circuit in each case a 

 suitable non-inductive resistance. Before each of these curves was 

 taken, the core of the magnet was carefully demagnetized by the 

 elaborate process described above. After the magnet Q had been put 

 a good many times through a cycle with a given maximum excitation 



