﻿Mr. H. Mance on the Measurement of Electrical Resistance, 315 



produced on G by a current arising in L will probably be consider- 

 ably changed. I say probably, because, by sliding the leakage r along 



Fig. 1. 



r 



- 



the whole length of the shunt, we shall at last find a point Z at which 

 the needle will return to its original deflection ; the position of Z 

 being ascertained, any resistance, varying from infinity to "dead 

 earth," may be applied without causing any change in the deflection 

 of the needle. 



It is evident that, although the total resistance of the circuit be- 

 tween x and y has been lessened by the insertion of the leakage, a 

 proportionately larger amount of current is diverted from the galva- 

 nometer by that part of the shunt between L and the leakage at Z. 



Presuming the electromotive E in L to remain constant, and 

 taking r=0, we have the intensity of the current passing through 

 G represented by the equation 



E . 



| L+ G+(A+B) +E JT A+B } 

 but after r is connected, the equation becomes 



A+G+ S 



A 



As the condition that the galvanometer-deflection remains unchanged, 

 the first of these equations must be equal to the second, from which 

 we obtain the formula 



L=R. A , 



the resistance Gr being immaterial. It will therefore be seen that R 

 always bears the same proportion to L that B does to A, the latter 

 branches bearing some analogy to the proportion-coils of a Wheat- 

 stone testing-bridge. 



Under certain circumstances a test might be taken without any 

 battery at all. In a submerged cable there is frequently sufficient 

 earth- current to supply the electromotive force in the branch L ; if 

 not, a small battery can be inserted to maintain a steady current, and 

 the internal resistance of the cells afterwards deducted. The polari- 



