426 FKAGMENTS OF SCIENCE. 



of the battery is not only carried away, but concen- 

 trated, so as to produce, at any distance from its origin, 

 a heat next in order to that of the sun. The current 

 might therefore be defined as the swift carrier of heat. 

 Loading itself here with invisible power, by a process 

 of transmutation which outstrips the dreams of the 

 alchemist, it can discharge its load, in the fraction of a 

 second, as light and heat, at the opposite side of the 

 world. 



Thus, the light and heat produced outside the bat- 

 tery are derived from the metallic fuel burnt within 

 the battery ; and, as zinc happens to be an expensive 

 fuel, though we have possessed the electric light for 

 more than seventy years, it has been too costly to come 

 into general use. But within these walls, in the 

 autumn of 1831, Faraday discovered a new source of 

 electricity, which we have now to investigate. On the 

 table before me lies a coil of covered copper wire, with 

 its ends disunited. I lift one side of the coil from the 

 table, and in doing so exert the muscular effort neces- 

 sary to overcome the simple weight of the coil. I unite 

 its two ends and repeat the experiment. The effort 

 now required, if accurately measured, would be found 

 greater than before. In lifting the coil I cut the lines 

 of the earth's magnetic force, such cutting, as proved 

 by Faraday, being always accompanied, in a closed con- 

 ductor, by the production of an ' induced ' electric 

 current which, as long as the ends of the coil remained 

 separate, had no circuit through which it could pass. 

 The current here evoked subsides immediately as heat ; 

 this heat being the exact equivalent of the excess of 

 effort just referred to as over and above that necessary 

 to overcome the simple weight of the coil. When the 

 coil is liberated it falls back to the table, and when its 

 ends are united it encounters a resistance over and 



