to the Measurement of Electro-motive Forces, 555 



as the equivalent of zinc, instead of 32*3 which I used in my 

 former paper). According to Dr. Andrews^ experiments, the 

 combination of this with gaseous oxygen would evolve 



3^!^ X 33808, or 1039-3 units of heat. 



Hence the thermal equivalent of the whole chemical action cor- 

 responding to the consumption of a grain of zinc in Smee^s 

 battery is 



1301 +369~1039-3, or 630-7 (I.) 



The equivalent of that part which consists of the oxidation of 

 zinc and the deoxidation of hydrogen is 



1301-1039-3, or 261*7 (XL) 



Hence (I.) if the whole chemical action be efficient in producing 

 the current, the electro-motive force is 2056200. 



(II.) If only the oxidation and deoxidation be efficient, the 

 electro-motive force is 853190. 



The external electro-motive force (or the electro-motive force 

 with which the battery operates on a very long thin wire connect- 

 ing its plates), according to either hypothesis, would be found by 

 subtracting the "chemical resistance *^^ due to the evolution of 

 hydrogen at the platinized silver, from the whole electro-motive 

 force : but, on account of the feeble affinity of the platinized sur- 

 face for oxygen, it is probable that this opposing electro-motive 

 force, if it exist at all, is but very slight. 



(III.) The external electro-motive force of a single cell of 

 Smee's battery is, according to Joule^s experimentsf, -65 of that 

 of a single cell of DanielPs ; and therefore if we take the prece- 

 ding number (III.), derived from his own experiments, as the 

 true external electro-motive force of a single cell of DanielPs, 

 that of a single cell of Smee^s is 



1,629,600. 

 This number is nearly double that which was found for the elec- 

 tro-motive force on the supposition that the oxidation and de- 

 oxidation alone are electrically efficient ; but it falls considerably 

 short of what was found on the suppositions that the whole che- 

 mical action is efficient, and that there is no " chemical resist- 

 ance.^^ 



8. It is to be remarked that the external electro -motive force 

 determined for a single cell of Smee's, according to the pre- 

 ceding principles, by subtracting the " chemical resistance '' from 

 the value of J^e, is the permanent working extevuBl electro- motive 

 force. The electro-statical tension, which will determine the 



* See foot-note on § 6 of my paper on the Mechanical Theory of Elec- 

 trolysis. 



t Phil. Mag. 1844, xxiv. p. 115, and Dove's Rep. vol. viii. p. 341. 



