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



3 0, 44 

 quantity by — — (see (29.)) ; the quantity of the oxide be- 



ing obtained by multiplying the equivalent of oxide of zinc 

 by the mean quantity of current electricity. We have then 



3 0, 44 

 4 0-3 x 1'8 x '-y^r = 2°*5 : this, when subtracted from 4 0, 6, 



leaves 2°*1, the correct voltaic heat. 



33. Assuming in this case, as well as in that of a metallic 

 conductor, that the heat evolved is proportional to the resist- 

 ance multiplied by the square of the electric intensity, we 



have, from the data in (28.) and (31.), \ ; . 2 x 0'293 X 7°'56 



(1*88 ) 



as 2 0, 03, which is very near 2 0, 1, the heat deduced from ex- 

 periment. 



34. Exp. 2. — I now constructed another pair, consisting 

 of plates precisely similar to those used in Exp. 1, but half 

 an inch only asunder : it was also immersed in two pounds of 

 sulphuric acid, sp. gr. 1137. The circuit was closed for one 

 hour, during which the needle of the galvanometer advanced 

 gradually from 47^° to 50^°, the mean deviation being 

 49° 35' = 1 0, 84 Q. The liquid had then gained 4°-8 : this, 

 + 0°-l (for Cor. A) and _0°'5 (for Cor. B), = 4°-4. The 

 heat due to the dissolution of oxide of zinc is in this case 



3°*44 

 4 0'3xl*84x'-rTT-= 2 0, 55, which, when subtracted from 



4 0, 4, leaves the correct voltaic heat 1 0, 85. 



35. The resistance of the pair was ascertained in this, as in 

 every other instance, at the beginning and at the end of the 



1*714 

 experiment. The equations thus obtained were — — 



0-432 . 191 0-446 . _ 6 , , .. /_ 



as and -j — 7-77;= 1 — 7^r„ 5 whence r = 0*31 1, and 



r + 0-06, r'+ 1-16 r'+O'Oe 



i J = 0*275 : the mean resistance was therefore 0-293. Now, 

 calculating as before (33.), on the basis of the heat produced 

 by the passage of electricity against the standard of resistance, 



('1-84) 2 

 we have \ ' 4 X 0-293 x7°'56 = 2°-12. 



(l-ss^ 



36. Exp. 3. — I formed another pair on Mr. Smee's plan ; 

 it was similar to the last, with the exception that the plates 

 were only one inch broad. When the circuit was closed, a 

 current of the mean intensity l°-46 Q passed through the 

 apparatus during one half hour. The heat thereby produced, 



