PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 131 



of 2650 ampere turns, under one of the voltages just named, direct and 

 reverse curves were taken with the help of the oscillograph. Careful 

 reproductions of these curves are given in Figure 23 : to avoid con- 

 fusion the reverse curves are drawn from a separate time origin. 



If in a circuit which contains no iron, E and r be kept constant, 

 while L is changed, the building-up time as defined by equation (17) 

 will be proportional to L. Of course no such simple relation holds 

 when the circuit includes the magnet Q; Figure 24 shows current 

 curves for the same final value of 2.60 amperes, under an applied elec- 



FlGUEE 24. 



The manner of establishment of a current of final strength 2.60 amperes, in the 

 coil circuit of the magnet Q, under a voltage of 82, when the number of active 

 turns was 407, 823, 1394, or 2788. 



tromotive force of about 82 volts, for exciting coils of 407 turns, 823 

 turns, 1394 turns, and 2788 turns. For convenience, the curves are 

 drawn from different time origins. The dotted line which crosses curve 

 Q calls attention to the fact that if curves P and Q were drawn from 

 the same origin, the former would cross the latter. 



If in a circuit without iron E and L were kept constant while r was 

 varied, the building-up time {LIr) would be inversely proportional to 

 the resistance of the circuit, or, since the electromotive force is fixed, 

 directly proportional to the current strength. There is no approxima- 

 tion to this in a circuit which contains iron. The current curves 

 shown in Figure 25 were obtained from the electromagnet Q when 



