250 



Mr. H. Mance on the Measurement 



[Jan. 19, 



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 polarization-current from a leakage of 

 low resistance in a cable would enable us to find the resistance from either 

 side through the fault without the application of a battery. And, lastly, 

 this method may be used to ascertain the internal resistance of a battery. 



The above method occurred to me about two years since during some 

 experiments made to determine the resistance of the bridge-circuit and 

 the exact proportion of current traversing each branch of the Wheatstone 

 balance when the potentials at W and Z are unequal. 



Fig. 2. 



If I equals the intensity of the current at % or y, and i v i 2i i B , i Ai i. the 

 intensities in the sections G, R, A, r, B, then 



G.(B + R + r) + BR T _ I 

 A . (B+ R + r) + Br i x 



A. (B + R-f r)+Br 1 = I 

 G.(B + R + r) + BR * 8 \ 



R. (A-f B + G)+BG 1== I_ 

 r . (A + B + G) + AB i 2 



r.(A + B + G) + BA I 

 R.(A + B + G) + BG i 4 



B. (R4-r) + (B + R + y).(A + G) _ I 



Gr-AR L 



(1) 

 (2) 

 (3) 

 (4) 

 (5) 



Or if the current in the branch B passes from W to Z, 



AR-Gr 



should be substituted for the denominator of the last equation. 



Equations (1), (2), (3), and (4) give the shunt-coefficient of the respec- 

 tive branches A, G, r, R ; thus if G were a galvanometer, the strength of 

 the deflection recorded multiplied by equation (1) would give the value of 

 intensity I. 



If, then, we consider G a galvanometer and the resistance r a leakage 

 applied at Z, we have a similar diagram to that given in fig. 1 ; and the 

 first of the five equations given above will enable us to determine the shunt- 

 coefficient for the part A which lies between L and the leakage at Z. 



