566 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



A circuit which contains an electromagnet has, of course, no definite 

 inductance in the sense of the ratio of the flux of magnetic induction 

 linked with the circuit to the intensity of the current, for this ratio is 

 different for different current strengths, and for a given electromagnet, 

 and a given current depends upon the previous magnetic history of 

 the core. 



Figure 26. Two parallel coils of resistances 20 ohms and 30 ohms and of 

 inductances 2 and 3 respectiveljf, connect, in parallel, the terminals of a 

 storage battery of 1 ohm internal resistance. At the beginning there is no 

 mutual inductance between the branch circuits and the currents follow the 

 curves OU, OK. At the time OC, the conformation of the circuit is sud- 

 denly changed so as to introduce a mutual inductance of 2, and as a result, 

 the courses of the two currents are altered: the first follows henceforth the 

 line UDA, the second the line KCFB. The inductions linked with the parallel 

 branches are shown by the dotted curves. 



In the case of a single circuit without iron the magnetic flux which 

 accompanies a changing current is at every instant the same as it would 

 be under a steady current of the intensity which the changing current 

 then has. If, however, a second circuit closed on itself is brought into 

 such a position that the two circuits have a mutual inductance, a 

 changing current in the first circuit induces a current in the other 

 which contributes to the flux through the first. If, therefore, an elec- 

 tromagnet has a solid core, the eddy currents induced in it while a 

 current is growing or decreasing in the exciting coil aflect the amount 

 of the flux through the core, and it is not possible to obtain a hysteresis 



