HI. J ELECTKIC AND MAGNETIC ENERGY. 37 



of molecular tension, constituting the magnetised state. And 

 when the iron is taken out of the spiral, the energy that was 

 static in the iron becomes kinetic in the wire, and increases for 

 the moment the force of the current. 



Thus the facts of electro-dynamic and of electro-magnetic 

 induction are not only themselves parallel, but admit of a 

 parallel interpretation ; and this tends to confirm it for both. 



Besides this, there is other and more direct proof that soft 

 iron, when temporarily magnetised by the passage of an electric 

 current near it, is in a state of tension. 



Instead of moving the iron bar into and out of the spiral 

 while the electric current is flowing, let the iron remain in the 

 spiral while the current is turned on and off ; the act of turning Experi- 

 the current on will magnetise the iron, and the act of turning it a^"\rjjj ' 

 off will demagnetise the iron. 



It has been ascertained by Professor Joule, that at the moment Experi- 

 of turning the current on so as to magnetise the iron bar, the Elonea- ' 

 bar will be elongated by about a 720,000th of its length ] which tion of 

 increase of length it will lose in the moment of turning the (jm-jm,. 

 current off so as to demagnetise it.^ This appears to be con- magnetisa- 

 clusive evidence that the magnetised iron is in a state of 

 molecular tension. 



This is still further confirmed by the very remarkable fact, E.xperi- 

 that when the iron bar and the wire spii'al are placed as in the 

 last experiment, and the current alternately turned on and off 

 at the rate of several times in a second (which is easily done 

 by means of a mechanical arrangement), sounds are heard pro- 

 ceeding from the iron.^ By turning the current alternately on 

 and off, the iron is alternately thrown into and out of a state of 

 tension. 



If a magnet is put into a wire spiral like that used in the Sounds 

 former experiments, while there is no current flowing along the Y,y m&s,- 

 wire, the approximation of the magnet will induce a momentary netisation. 

 current in the wire ; and if the magnet is taken out again, 

 another momentary current will be induced in the wire, but in 

 the opposite direction. It is on this principle, although the The 

 mechanical arrangements are different, that the magneto-electric electric 

 apparatus is constructed for supplying the electric light. machine 



Whence comes the energy of the induced currents in these 



"• De la Rivp on Electricity, English translation, vol. i. p. 306. 

 2 Ibid. ji. 303. 



