266 Mr. J. P. Joule on the Heat evolved by Metallic 



24. Ten feet of copper wire, f 024 of an inch thick, were 

 formed into a coil in the manner described in (7.) ; its resist- 

 ance to conduction was called unity. Three experiments 

 were made in order to ascertain its heating power. 



25. 1st. Ajar was filled with two pounds of water, and a 

 current which produced a mean deviation of the needle of 

 the galvanometer (3.) equal to 57^° = 2 0, 54 Q of current 

 electricity, was urged through the coil for twenty-seven mi- 

 nutes, by means of a zinc-iron* battery of ten pairs. The 

 heat thus acquired by the water, after Cor. A, and that part 

 of Cor. B which relates to the caloric of the jar, had been 

 applied, was 6°*22. 



26. 2nd. The battery was now charged with a weaker so- 

 lution of sulphuric acid. In this case it passed the mean cur- 

 rent 2 o, 085 Q during forty-five minutes. The heat thus pro- 

 duced, when corrected, was 7 0, 04. 



27. 3rd, A battery of five pairs (three of which had pla- 

 tinized silver ; one silver, and one copper, for their nega- 

 tive plates,) passed the mean current 1 0, 88 Q during one 

 hour, in which time 7°'47 were generated. 



28. "When the first two experiments are reduced, in order 

 to compare them with the third, we have, in accordance with 



the principles laid down in (17.) } , / 2 x -^ x 6 0, 22 



= 7°'57, and £**F 9 x ^ x 7°'04 = 7°'63. Thus we have 



7°-57 + 7°-63 + 7°-47 . 



= 7°*56, the mean and total quantity of 



3 



heat produced per hour by the passage of 1 0, 88 Q of current 



electricity, against the unit of resistance. 



29. Before I proceed to give an account of some experi- 

 ments on heat evolved in the cells of voltaic pairs, it is im- 

 portant to observe that every kind of action not essentially 

 electrolytic must be eliminated. For instance, the dissolution 

 of metallic oxides in acid menstrua, which has been proved 

 by Dr. Faraday to be no cause of the current, is the occasion 

 of a very considerable quantity of heat, which, if not accounted 

 for in the experiments, would altogether disturb the results. 

 I have taken the oxide of zinc, prepared either by igniting 

 the nitrate, or by burning the metal, and have repeatedly dis- 

 solved it in sulphuric acid of various specific gravities; and 

 on taking the mean of many experiments, none of which dif- 



* Whenever an iron battery was used, it was of course placed at a di- 

 stance from the galvanometer sufficiently great to render its action on the 

 needle altogether inappreciable. 



