PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 101 



Bois-Reymond showed that if the secondary circuit contained no 

 battery, and if, after the primary current had been fully established, 

 its circuit were suddenly broken, the current induced in the secondary 

 circuit would have a form like that of the dotted curve (P) in Fig- 

 ure 2 ; if after a few seconds the primary circuit were again closed, 

 the secondary current when plotted against the time would yield 

 a curve either like Q or like 8 in the same diagram. The lines in 

 this familiar figure have been drawn to scale for a certain pair of 

 circuits the self-inductances of which are equal, fixed quantities 

 and the resistances also fixed. Q, B, 8 correspond to three different 

 values of the mutual inductance (31), which are respectively half 

 as great, nine tenths as great, and equal to the self-inductance (L) 



TIME 



FlGLKE 1. 



If the current is expressed in absolute units (absamperes) and the time in 

 seconds, the shaded area represents the change in the total flux of magnetic 

 induction through the circuit, during the time OP. 



of either circuit. These curves show the currents induced in the 

 secondary circuit when the primary is made ; the crest of any such 

 curve comes earlier the larger the value of 31. The curve P, which 

 represents a current induced in the secondary circuit when the 

 primary circuit is broken, is drawn for the case 31 = \L, and there- 

 fore corresponds to the curve Q ; E. du Bois-Reyrnond called atten- 

 tion to the fact that in such problems as this the areas V and W 

 must be equal. The curves like P corresponding to R and 8 could 

 be found merely by exaggerating all the ordinates of P in the ratio 

 9/5 or the ratio 2. 



From the early days of induction coils, iron cores had been used 

 to increase the mutual inductance of the circuits, and, soon after 

 Helmholtz had given the equations for the currents in neighboring 



