THE LOCALISATION OF BKEAKS AND FAULTS. 45B 



A B 



which is practically x— — - + ohms 



^ * 100 10,000 



, B + a; , 



and ?/= ohms. 



■^ 100 



If the metallic circuit (L) is measured, we have 



A + L 



which is practically 

 and 



y- 



In J. J. Allen's modification of the Murray loop the usual 

 places of the galvanometer and battery are interchanged, and 

 a ratio of 10/1,000 is used {The Electrician, Vol. XV., page 

 350). With the cable connected as in the illustration (Fig. 254) 

 we have 



X + 10 y 



T^OOO^A' 



and on reversing the cable ends we have 



y+10 X, 



whence x = 



and ?/ = 



1,000 ~B' 



10 B(A4- 1.000) 

 1,000,000 - AB ' 



( 10 + a;) A 

 1,000 



Free Overlap. — In cases where the fault makes only partial 

 earth, the observers at either end can speak to each other, 

 arranging times for testing, and then compare results. Id 

 this test each station frees in turn while the other tests. The 

 whole of the testing current passes through the fault, and if 

 the fault is very small and variable it is difficult to follow the 

 changes on the bridge, as the resistance to balance may run 

 up thousands of ohms in a second or two. The test is hardly 

 practicable on very small and variable faults, but cases occur 

 of fairly steady faults, when reliable readings can be 

 obtained by this method and a useful check thus obtained 



