III.] ELECTRIC AND MAGNETIC ENERGY. 35 



If this interpretation is true, (and it appears to me the only 

 possible one,) it follows that during the flow of the current of a, 

 and between the flow of the two momentary currents of B, a 

 definite quantity of energy has become static in b, due to the 

 tension of its molecules. And, as ought to be the case on this 

 hypothesis, the longer are the wires, the greater will be the force 

 of the current of b ; ^ for, supposing the force of the tension to 

 be given, the quantity of energy due to the tension will ob- 

 viously be in proportion to the length of the wire. In other 

 words, the greater is the length of "^vire that is thrown into a 

 state of tension, the greater will be the quantity of energy due 

 to its tension. 



But as no process can either create or destroy energy, the 

 energy that has become static in b must have come from some- 

 where ; and its only assignable source is the current of a. This 

 is corroborated by the following facts : — 



The current of a is the inducing current, and those of B the 

 induced currents. It is possible to leave out B, and to obtain 

 the induced currents in a : in other words, it is possible to 

 obtain the induced currents in the same wire with the inducing 

 current. Let a be sufficiently long, and let B be left out ; let Expevi- 

 the current along a be stopped by breaking contact suddenly; in ™^"'' ^''• 

 the act of breaking contact there wiU be a momentary increase 

 of the force of the current, producing a spark. This increase 

 of the current is evidently the same as the current that would 

 have been induced in b, had b been there, in the same direction 

 as that of a. The two currents, the inducing and the induced, 

 flow for the moment along the same wire and in the same 

 direction, and the spark is due to the sum of their effects. 



But the current induced in b, at the commencement of that 

 of A, is opposite in direction to that of a : so that if the inducing 

 and the induced currents can be caused to exist at once in the 

 same wire, the resultant current will be due to their difference. 

 This may be experimentally shown. If b is left out, and a long Expei-i- 

 enough, the current will be sensibly diminished in force at the ^^^^ ^^ ■ 

 moment of its commencing to flow along a.^ 



1 De la Rive on Electricity, English translation, vol. i. p. 358. These facts 

 were discovered hy Faraday, who at first gave an explanation of them 

 with which mine is substantially identical. He called the state of ten- 

 sion into which wire B is thrown the electro-tonic state. He afterwards, 

 however, changed his opinion on grounds which I do not understand. 



2 Ibid. pp. 359, 360. 



D 2 



