4 Professor Tyndall [Jan. 17, 



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 consumed. In the one case the heat generated is purely 

 domestic, being liberated on the hearth where the fuel is burnt, 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 which our late 

 Secretary would wish you to bear in mind is that the sum of the 

 foreign and domestic — of the external and internal — heats is fixed 

 and invariable. To have heat outside you must draw upon the 

 heat within. 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. 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 burnt within the battery ; and, as zinc haj^pens 

 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 simjde 

 weight of the coil. I unite its two ends and repeat the experiment. 

 The eff'ort 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 conductor, 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 al)Ove 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 ojiposcd in direction to the first, and reaches the table ^vitli a 

 diminished shock. The amount of the diminution is accurately reiirc- 

 Rcntcd by tlio warmth which the momentary current develops in tlic 

 coil. Various devices have been employed to exalt these induced 

 currents. Faraday, indeed, foresaw that such attempts were sure to be 

 made ; but he chose to leave them in tlie hands of the mechanician, 

 wliile he himself pursued the deeper study of facts and principles. " I 

 have rather," ho writes in 1831, " been desirous of discovering new 



