PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 107 



electromotive force in it unchanged in intensity, but in some instances 

 changed in direction, the circuit was then closed again and a current 

 curve obtained. If the electromotive force has its old direction, such a 

 curve is said to be " direct " ; if the new direction is the opposite of the 

 old, the curve is called "reverse." In one case the magnetic journey 

 of the core during the rise of the current is represented approximately 

 by the portion Pi^J/of the corresponding hysteresis diagram (Figure 6) ; 

 in the other case the journey follows the arc QUZM. Lines 1, 2, 

 and 4 in Figure 5 are reverse lines, while 3 and 5 are direct. 



In Figure 4 the line (9 1" corresponds to the final value (/^) of the 

 current, and if its length in centimeters is in i^ and if A is the area in 

 square centimeters shut in by Y, YJl, and V, it is evident that in the 



TIME. 



W^ 



p5= 



ps=- 



W 



SECONDS. 



Figure 4. 



Curves which represent the growth of the current in the exciting coil of an 

 electromagnet when {V), the circuit which has the resistance r, is closed and 

 left to itself; and when (U), the circuit, is closed when it has a comparatively- 

 large resistance, which is then reduced to r by steps. 



case represented by T" the whole change in induction flux through the 

 turns of the coil due to the current is 



10' -E- A 

 lOY ' 



In the case represented by the line U, (10^ B/l) times the sum of the 

 terms formed by dividing each of the small shaded areas by the ordi- 

 nate, expressed in centimeters, of its upper straight boundary, gives 

 the change in the induction flux through the turns of the coil due to 

 the current when it grows in the manner indicated. Of course if the 



