LIQUID RESISTANCES. 391 



In the case of absolute silence, the second equation gives 



dy ,dy' , m 



m > or = 



and the two latter 



/A-j^*\^ + /_^*\ y _^c = o. 



\M M' m'J dt \M M' m'J M' 



As this equation must continuously be satisfied, it follows that 



C = 0, 

 (47) p A. Mm 



The induction balance combined with a telephone is an 

 apparatus of extreme sensitiveness. It renders evident the smallest 

 differences of a weight, nature, degree of purity, or temperature of 

 two conductors of the same dimensions, such as two coins placed 

 in identical conditions in respect of the two systems of coils. 



It enables us to detect very small masses of metal* in a 

 badly conducting body, and may be employed with much advan- 

 tage in verifying the insulation of the different windings of a coil 

 the ends of which are open, f It is thus a very valuable instrument 

 for qualitative researches, but lends itself less well to quantitative 

 determinations ; it is in general impossible to obtain complete 

 extinction of sound, and, whatever be the mode of correction, it 

 is difficult to interpret rigorously the results obtained. 



988. LIQUID RESISTANCES. The measurement of the resistance 

 of an electrolysable liquid, presents special difficulties owing to the 

 variable polarization of the electrodes. A given quantity of elec- 

 tricity cannot traverse the liquid without decomposing a propor- 

 tional weight, and transferring the products of decomposition to 

 the electrodes. Those which are identical at the origin when in 

 contact with the liquid present falls of potential which are no 

 longer equal and of opposite sign ; if H and H' are the values of 

 these falls for each of them, I the intensity of the current, and R 



* GRAHAM BELT,. American Journ. of Sciences. August, 1882. 



t Lord RAYLEIGH and Mrs. SIDGWICK. Phil. Trans., p. 411. 1884. 



