THE ELECTRIC LIGHT 445 



side. One of the fundamental truths to be borne in mind 

 is that the sum of the foreign and domestic of the ex- 

 ternal and internal heats is fixed and invariable. Hence, 

 to have heat outside, you must draw upon the heat with- 

 in. These remarks apply to the electric light. By the 

 intermediation of the electric current the moderate warmth 

 of the battery is not only carried away, but concentrated, 

 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. Load- 

 ing itself here with invisible power, by a process of trans- 

 mutation 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 battery 

 are derived from the metallic fuel burned within the bat- 

 tery; and, as zinc happens to be an expensive fuel, though 

 we have possessed the electric light for more than sev- 

 enty years, it has been too costly to come into general 

 use. But within these walls, in the autumn of 1831, Fara- 

 day 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 necessary to overcome the simple weight 

 of the coil. I unite its two ends and repeat the experi- 

 ment. 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 cut- 

 ting, as proved by Faraday, being always accompanied, 

 in a closed conductor, by the production of an "induced" 

 electric current which, as long as the ends of the coil le- 



