Prof. E. Bouty's Studies on Magnetism. 89 



tion of the soft iron. The inequality of duration of the extra 

 currents authorizes us to affirm that the demagnetization of soft 

 iron is more rapid than its magnetization*. 



Analogy has led physicists to regard the coil, which gives rise 

 to the extra currents, as the seat of temporary electromotive forces 

 the same in direction as, or contrary in direction to the principal 

 current ; but it does not authorize us to identify in all points 

 the interior of a coil with that of an element of a pile and, con- 

 sequently, to regard the extra currents as having the same effects 

 inside the coil as in the rest of the circuit. We shall see that the 

 extra currents are without effect within the coil from which they 

 emanate. 



A condenser C (fig. 3) placed in a Fig- 3. 



voltaic circuit, on a derivation, is, like 

 an induction-coil, the seat of tempo- 

 rary electromotive forces at the time 

 of the opening or closing of the 

 circuit. We will examine only the 

 two following cases. 



1st. The condenser C is placed on 

 a derivation without resistance, on 

 which the interruptions are made. 

 The points of bifurcation, A and B, 

 have the same potential when the 

 current passes, since the derivation 

 is destitute of resistance and consequently the condenser is not 

 charged. When the current is interrupted the difference of po- 

 tential in A and B becomes equal to the electromotive force of 

 the pile P. Thus the condenser discharges itself during the 

 period of the establishment of the current, and becomes charged 

 at the time of the interruption, whence there result in the 

 principal circuit B N P A an inverse and a direct extra current. 



2nd. The condenser C is placed on a derivation the resistance 

 of which is sufficiently great for that of the rest of the circuit, 

 including the pile, to be negligible. The interruption is made at 

 N, apart from the derivation. In this case the points of bifur- 

 cation are at the same potential when the current is interrupted ; 

 but when it is passing, the difference of their potentials is sen- 

 sibly equal to the electromotive force. The extra currents are 



* Villari (Pogg. Ann. 18/3) has determined directly the time taken by 

 flint, a diamagnetic substance, to gain or lose its magnetic rotatory power, 

 the correlative of its magnetization. He found that the demagnetization 

 is more rapid than the magnetization, and assigns to this latter a duration 

 of 00024" (see Journal de Physique, vol. ii. p. 422). This is the only ex- 

 perimental determination 1 know of in regard to the duration of mag- 

 netization. 



