THE LOCALISATION OF BREAKS AND FAULTS. 



545 



same kind, and a8 nearly as possible equal E.M.F., the voltage 

 being as low as practicable. If an eartii current is present the 

 observed deflections should be corrected as usual in deflection 

 tests. 



Having carefully compared times to a second, both stations 

 arrange to observe their deflections at, say, every minute for 

 10 minutes. This allows time to dissipate the charge and dis- 

 charge at each application of current and to observe the earth 

 current before and after each reading, the mean of which is 

 used for correcting the deflections in the usual way. When a 

 set of deflections has been obtained both stations take the con- 

 stant of their galvanometers through a high resistance, and work 

 out by its means the resistance equivalent to each deflection. 



^|l|lh 



Fig. 311. — Potextials in Betts' Method. 



Mr, Betts recommends obtaining the resistance values this 

 way from a well-established constant rather than by repro- 

 duced deflection. 



Let each pair of equivalent resistances taken at the same 

 moment by A and B be represented by a and h. Eeferring to 

 the diagram of potentials (Fig. 311) the current flowing to cable 



when A tests is 



E 



and a is the resistance which when 



acted upon by the total battery voltage E allows the same 

 current to flow — that is 



E-c_E 

 X a 

 Similarly, on the B side, 



E-e E 



(1) 



h-x b 



(2) 



