THE LOCALISATION OF BREAKS AND FAULTS. 507 



Let _JL^=P 



71 + 1 



Then (A-B)P = 



c 



and by substituting this value of - in the first equation, we have 



a'=A-(A-B)P (2) 



which is the formula to use for the reversals test. 



The currents c and nc in the cable do not differ much and 

 the ratio n between them is very little higher than unity. 

 Table IV. gives the values of P for different ratios. 



Table IV. — Co-efficients in Reversals Test, 



(The ratio of the currents in this test is 1 when there is no earth current 



present.) 



Eatios of currents (n). Values of co-efficient P= 



n+1 



100 0-5 



1-05 0512 



I'lO 0'524 



1-15 0-535 



1-20 0-545 



1-25 0-555 



1-3 ! 0-565 



A useful modification of this test has been devised by3Mr. 

 R. R. Black {The Electrician, January 13, 1899), in which the 

 current to line is maintained constant in two observations by 

 adjustment of a resistance in the line. 



The third arm (d) of the bridge (Fig. 282) which is usually 

 adjusted to balance is in this method made a fixed quantity and 

 balance is obtained to true zero by adjustment of a resistance 

 R in the line. The testing current is then reversed and the 

 second observation taken with the line resistance readjusted to 

 E^. We then have 



First balance R + x + - = d-- 



c h 



Second ,, 'R. + oi--=d^ 

 c b 



