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



the thermometer, immersed in the dilute sulphuric acid stood 

 at 56 0, 7 } indicating a rise of 4°*75. Another trial now gave 



-j — — — = -; , whence r' = 0*413. The mean resist- 



i J + 2-26 r' +0-06 



ance of the pair was therefore 0*427. 



42. 4°*75 + 0°*1 (for Cor. A), and — 0°*4 (for Cor. B, 



which in this case includes the capacity for heat of the porous 



cell) = 4°*45. The heat generated by the dissolution of 



3°*44 10 



oxide of zinc was in this case 40*3 x 4*77 X - x — r 



100 60 



= 1°*1, which, subtracted from 4 0, 45, leaves the correct vol- 

 taic heat 3°*35. 



f4*77 N ) 2 10' 



43. The theoretical result is ). ' ( a X 0*427 x 7°*56 x — ^~ 



(l'oo) ' 60 



= 3°*46. 



44. Exp. 6 was made with a pair in every respect similar 

 to the last : the circuit, however, was completed by means of 

 a thin copper wire, in order to reduce the intensity of the 

 current. At the end of one hour, during which the needle of 

 the galvanometer advanced gradually from 41° to 42°, the 

 correct voltaic heat that was generated was 1°*7. The theo- 

 retical result was 1°*82. 



45. I was desirous of knowing how far the same principles 

 would apply to the heat generated in Prof. Daniell's constant 

 battery. But in this battery considerable cold is produced, in 

 consequence of the separation of oxide of copper from the 

 sulphuric acid to which it is combined. This is altogether a 

 secondary effect, and should be eliminated as decidedly as 

 the heat produced by the dissolution of oxide of zinc. I have 

 not yet been able to obtain accurate data for the correction 

 thus needed, and shall therefore content myself with remark- 

 ing, that my results with Mr. Daniell's arrangements are, as 

 far as they go, quite consistent with the theory of resistances. 



46. Experiments, such as I have related, were varied in 

 many ways; and sometimes a number of pairs were arranged 

 so as to form a battery. Still the results were similar, and 

 established the fact, that the heat which is generated in a given 

 time in any pair, by true voltaic action, is proportional to the 

 resistance to conduction of that ■pair, multiplied by the square 

 of the intensity of the current. 



47. I now made some experiments on the heat consequent 

 to the passage of voltaic electricity through electrolytes. 



48. Exp. 7. — Two pieces of platinum foil, each of which 

 was an inch long, and a quarter of an inch broad, were her- 



