556 THE POPULAR SCIENCE MONTHLY. 



electric current tbe 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 invisi- 

 ble 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 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 investi- 

 gate. 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 by Farada}', being always accompanied, 

 in a closed 'conductor, by the production of an " induced " electric cur- 

 rent 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 above that of the air. It generates an electric current opposed 

 in direction to the first, and reaches the table with a diminished shock. 

 The amount of the diminution is accurately represented by tbe warmth 

 which the momentary current develops in the coil. Various devices 

 were employed to exalt these induced currents, among which the in- 

 struments of Pixii, Clarke, and Saxton were long conspicuous. Fara- 

 day, indeed, foresaw that such attempts were sure to be made ; but he 

 chose to leave them in the hands of the mechanician, while he himself 

 pursued the deeper study of facts and principles, " I have rather," he 

 writes in 1831, "been desirous of discovering new facts and new rela- 

 tions dependent on magneto-electric induction than of exalting the 

 force of those already obtained ; being assured that the latter would 

 find their full development hereafter." 



For more than twenty years magneto-electricity had subserved its 

 first and noblest purpose of augmenting our knowledge of the powers 

 of nature. It had been discovered and applied to intellectual ends, 

 its application to practical ends being still unrealized. The Drum- 

 mond light had raised thoughts and hopes of vast improvements in pub- 

 lic illumination. Many inventors tried to obtain it cheaply; and in 



