508 



STJBMAEINE CABLE LAYING AND REPAIRING. 



Where x= the true CR of the line. 

 e= earth current potential. 



R= resistance added when earth current opposes the 

 testing current. 

 Rj= resistance added when earth current assists the 

 testing current. 



R Cable 



L|i|i|i|i|i|h - 



Fig. 282.— Black's Reversals Method. 



Rj will be the greater added resistance whether taken with 

 zinc or carbon testing current. 

 By addition 



h 2 

 that is, the true CR is the ordinary bridge result less the 

 arithmetical mean of the two added resistances. 

 With an even bridge this becomes 



