664 FHA QMENTK F SCIENCE. 



copper wire, while into the circuit of the other a thin 

 platinum wire is introduced. The platinum glows with a 

 white heat, while the copper wire is not sensibly warmed. 



Now an ounce of zinc, like an ounce of coal, produces, 

 by its complete combustion in air a constant quantity of 

 heat. The total heat developed by an ounce of zinc 

 through its union with oxygen in the battery is also 

 absolutely invariable. Let our two batteries, then, continue 

 in action until an ounce of zinc in each of them is con- 

 sumed. In the one case the heat generated is purely 

 domestic, being liberated on the hearth where the fuel is 

 burned, that is to say in the cells of the battery itself. In 

 the other case, the heat is in part domestic and in part 

 foreign in part within the battery and in part outside. 

 One of the fundamental truths to be borne in mind is that 

 the sum of the foreign and domestic of the external and 

 internal heats is fixed and invariable. Hence, to have 

 heat outside, you must draw upon the heat within. 

 These remarks apply to the electric light. By the inter- 

 mediation 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. 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 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 seventy 

 years, it has been too costly to come into general use. But 

 within these walls, in the autumn of 1831, Faraday dis- 

 covered 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 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 



