THE LOCALISATION OF BREAKS AND FAULTS. 367 



high. If the zinc current only is used, and the balance taken 

 to false zero, as in Kennelly's two-current test, the false zero 

 must be observed as soon as possible after the battery is dis- 

 connected, in order that it may represent the polarisation 

 potential active during the test. The polarisation potential is 

 always in opposition to the testing current, and consequently 

 is equivalent to a resistance added to the fault or break. In 

 the Schaefer break test it is treated as part of the break resis- 

 tance, and the function of the testing current expressing the 

 variation in resistance of the break with strength of testing 

 current includes the resistance effect of the polarisation 

 potential. 



Balancing to False Zero. — What is termed "false zero" is 

 the position of the spot of light when the testing battery is dis- 

 connected, and the galvanometer short-circuit key open. The 

 deflection is due to any current or combined currents that may 

 be passing in the cable. By balancing the bridge to this deflec- 

 tion as zero, the effect of the currents in the cable is eliminated. 

 The results obtained are also true for either direction of testing 

 current ; and, therefore, the false zero is well suited for tests of 

 faulty cables, as readings can be taken by the zinc current only, 

 which opens and cleans the fault. In observing the false zero 

 it should be borne in mind that the position of the spot should 

 as nearly as possible represent the strength of cable current at 

 the time of the test. The false zero of course changes with each 

 adjustment of the bridge, and therefore must be observed again 

 after each balance. The difference between the false zero and 

 the position of the spot at balance gradually becomes less and 

 less as the resistance is adjusted nearer, until finally they both 

 coincide, at which point the balance is correct. When the cable 

 current is simply an earth current (as in a good cable) it does 

 not matter whether the false zero is observed before or after 

 each fresh adjustment, because an earth current is not affected 

 by testing current ; but when one component of the cable 

 current is due to polarisation of a fault the false zero should be 

 observed as soon after each adjustment as possible, as the polari- 

 sation current rapidly falls after the battery is disconnected. 



With the testing current on, and the resistance in the third 

 arm of the bridge adjusted to bring the spot of light to the 



