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



cities of different degrees of tension, 

 in the following table : — 



My results are arranged 



16. The differences between the numbers in columns three 

 and five, and those in columns four and six, are very inconsi- 

 derable, taking into account the nature of the experiments, 

 and are principally owing to the difficulty which exists in 

 keeping the air of the room in the same state of quiet, of hy- 

 grometry, &c. during the different days on which the ex- 

 periments were made. They are much less when a larger 

 quantity of water is used, so as to reduce the cooling effects 

 (28.). 



17. We see, therefore, that "when a current of voltaic electri- 

 city is propagated along a metallic conductor, the heat evolved 

 in a given time is proportional to the resistance of the conductor 

 multiplied by the square * of the electric intensity. 



18. The above law is of great importance. It teaches us the 

 right use of those instruments which are intended to measure 

 electric currents by the quantities of heat which they evolve. 

 If such instruments be employed (though in their present 

 state they are far inferior in point of accuracy to many other 

 forms of the galvanometer), it is obvious that the square roots 

 of their indications are alone proportional to the intensities 

 which they are intended to measure. 



19. By another important application of the law, we are 

 now enabled to compare the frictionalf and voltaic electri- 



* The experiments of De la Rive show that the calorific effect of the 

 voltaic current increases in a much greater proportion than the simple 

 ratio of the intensities. — Ann. de Chimie,\%36, parti, p. 193. See also 

 Peltier's results, Ann. de Chimie, 1836, part ii. p. 249. 



+ The experiments of Brooke, Cuthbertson and others, prove that the 

 quantity of wire melted by common electricity is as the square of the 

 battery's charge. Harris, however, arrived at the conclusion, that the 

 heating power of electricity is simply as the charge, Phil. Trans., 1834, 



