76 Intelligence and Miscellaneous Articles. 



when it has ceased, leaving in the apparatus the excess of the vis 

 viva which has brought them over that which they carry away with 

 them — that is to say, heat, like an elastic body which rebounds with- 

 out rising to the height from which it feil. 



This heat will be proportional to the square of the mean intensity 

 of the extra current, and will measure it. We shall presently see 

 that it will be feeble if = 0, that it will increase to a maximum for 

 0=go (that is, if the bobbin is open). This is in fact what is proved 

 by experiment. 



The extra current is produced iD the inducing coil as if it were a 

 battery ; in returning from the condenser it traverses afresh the wire 

 of this coil. We must ask whether it produces heat. It is easy to 

 answer this question. Suppose the two coils enclosed in the same 

 vessel ; they will be traversed by the inducing and the induced 

 currents, the intensities of which are I and I', and which develope 

 quantities of heat 



Adding them together, and subtracting the sum from the heat w 

 observed in this coil, the rest will be due to the extra current. Now 

 we see from the Table that this remainder is virtually nothing. 



Soft Iron. — We must imagine to ourselves the iron core slowly 

 magnetized as the inducing current increases, very quickly and pow- 

 erfully during the extra current, that then it becomes slowly demag- 

 netized during the break. It is natural to assume that magnetiza- 

 tion creates heat, and demagnetization cold, that these actions have 

 just compensated each other if they have been of equal duration, 

 and that there is a gain of heat the more considerable the shorter 

 has been the first action. If there is any foundation for this view, 

 the extra current should produce in the soft iron a quantity of heat 

 which will increase with it at the same time as 6. Experiment 

 proves that this is the case. This heat is represented by the ordi- 

 nates of an hyperbola; at the limit it attains a maximum. 



We have now a very precise idea of induction. The inductor 

 acts both on itself and on the induced wire ; it developes with the 

 same facility in both coils an inverse current while it is being 

 established. This is not the case with terminating or direct currents. 

 That of the inducing coil, the extra current, does not pass, or 

 passes with difficulty, in the open circuit ; it is compelled to rush into 

 and accumulate in the condenser, which opposes a definite resistance 

 to it. The terminating current of induction, on the contrary, circu- 

 lates readily through the induced coil if it is closed without any ex- 

 ternal resistance ; the inducing force is thus entirely carried in this 

 direction ; there is no extra current, and therefore no heat in the con- 

 denser or in the soft iron. If 6 increases (that is, if a resistance be 

 added to the induced coil and it increases) to infinity, the termi- 

 nating induced current diminishes gradually until it becomes zero. 

 As the inductive force finds on this side an increasing resistance, it 

 bears more and more upon the inducing coil, and is ultimately en- 

 tirely expended there. Thus the extra current will increase with 



