556 Prof. Thomson on the Applications of Mechanical Effect 



initial working cxtcraal electro-motive force, depends on the pri- 

 mitive state of the platinized silver plate. It could never be 

 greater than to make the initial working force be 



J X 1670 X e, or 5441500, 

 corresponding to the combination of zinc with gaseous oxygen, 

 and of the oxide with sulphuric acid. It might possibly reach 

 this limit if the platinized surface had been carefully cleaned, 

 and kept in oxygen gas until the instant of immersion, or if it 

 had been used as the positive electrode of an apparatus for de- 

 composing water, immediately before being connected with the 

 zinc plate ; and then it could only reach it if the whole chemical 

 action were electrically efficient, and if there were no " chemical 

 resistance'' due to the affinity of the platinized surface for 

 oxygen. 



9. It is also to be remarked, that the permanent working elec- 

 tro-motive force of a galvanic element, consisting of zinc and a 

 less oxidizable metal immersed in sulphuric acid, can never exceed 

 the number 2056200, derived above from the full thermal equi- 

 valent for the single cell of Smee's, since the chemical action is 

 identical in all such cases, and the mechanical value of the ex- 

 ternal effijct can never exceed that of the chemical action. In a 

 pair consisting of zinc and tin, the electro-motive force has been 

 found by Poggendorff * to be only about half that of a pair con- 

 sisting of zinc and copper, and consequently less than half that 

 of a single cell of Smee's. There is therefore an immense loss 

 of mechanical effect in the external working of a galvanic battery 

 composed of such elements ; which must be compensated by heat 

 produced within the cells. I believe with Joule, that this com- 

 pensating heat is produced at the surface of the tin in con- 

 sequence of hydrogen being forced to bubble up from it, instead 

 of the metal itself being allowed to combine with the oxygen of 

 the water in contact with it. A most curious result of this 

 theory of "chemical resistance" is, that in experiments (such 

 as those of Faraday, Exp. Researches, 1027, 1028) in which an 

 electrical current passing through a trough containing dilute 

 sulphuric acid, is made to traverse a diaphragm of an oxidizable 

 metal (zinc or tin), dissolving it on one side and evolving bub- 

 bles of hydrogen on the other; part (if not all) of the heat of com- 

 bination will be evolved, not on the side on which the metal is 

 eaten away, but on the side at which the bubbles of hydrogen 

 appear. It will be very interesting to verify this conclusion, by 

 comparing the quantities of heat evolved in two equal and similar 

 electrolytic cells, in the same circuit, each with zinc for the posi- 



♦ " Berl. Acb. 46, 242," Pogg. Ann., Ixx. 60. Dove's Repertorium, 

 vol. viii. p. 341. 



