492 Mr. J. P. Joule on the Heat disengaged 



Table III. 



The resistance of the mercury wire in comparison with that of 

 the silver wire, was found by ascertaining the intensity of the 

 current produced by a pile of five cells : — 1st, when the pile was 

 in direct communication with the galvanometer ; 2nd, when the 

 resistance of the circuit was increased by the addition to it of the 

 coil of silver wire ; and 3rd, when the mercury wire was substi- 

 tuted in the circuit for the silver wire. Calling the intensity of 

 the current in the first instance A, and the resistance y ; in the 

 second instance B, and the resistance 1+y; and in the third 

 instance C, and the resistance x + y, we have, by the laws of Ohm 

 and Pouillet, 



_ B(A-C) 

 X ~ C(A-B)' 



The observations from which I have deduced the constant 

 quantities of the above formula are arranged in the following 

 table. In these experiments the precaution was taken that the 

 temperature of the water in which the coils of mercury and silver 

 were immersed should be as nearly as possible the same as in 

 the experiments of Table III., in order to obviate the possibility 

 of an alteration of the resistance arising from an alteration of 

 the temperature of the metals. I may mention also, that each 

 of the recorded deflections is the mean of two observations, one 

 on one side, and the other (by reversing the direction of the cur- 

 rent) on the other side of the magnetic meridian. The effect of 

 any change in the intensity of the pile during any of the expe- 



