PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 109 



strength in the exciting coil, usually depends in some slight degree upon 

 the manner of growth of the current. If after a large core has been 

 magnetized in one direction by the steady application of a given elec- 

 tromotive force until the current has reached its full value, the excit- 

 ing circuit be broken, and, after the direction of the electromotive force 

 has been reversed, closed again, 

 it sometimes happens that the 

 magnetic flux after the new cur- 

 rent has attained its maximum 

 value is slightly less when the 

 current follows the course of 

 the curve V than when it grows 

 by short stages in the manner 

 indicated by the curve U. If, 

 however, there are but two or 

 three steps, the difference is, as 

 a rule, of no practical impor- 

 tance, and if one has a suitable 

 oscillograph or other recording 

 instrument, it is possible to get 

 a set of current curves for any 

 given maximum value of the 

 current from which an extremely 

 good statical hysteresis diagram 

 may be obtained for the core. 



If while a steady current from 

 a constant storage battery of 

 voltage E is passing through 

 the coil of an electromagnet, the 

 resistance of the coil circuit be 

 suddenly increased to a new 

 value 7\, so that the current (i) 

 will ultimately fall to a lower 

 value represented by ON in 



Figure 7, the current curve, which has been a horizontal line, sinks in 

 such a manner as to become asymptotic to the horizontal line JYB. At 

 any instant after the change, 



Figure 



When a direct current curve is taken, 

 tlie core of the electromagnet makes a 

 magnetic journey represented approxi- 

 mately by the arc PFM ; in the case of a 

 reverse curve the core follows the line 

 QUZM. 



E 



dN 



dt 



= m, 



(10) 



in absolute units, so that in volts, ohms, amperes, and maxwells, 



