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 1881, 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 re- 



