THE LOCALISATION OF BREAKS AND FAULTS, SO*? 



the false zero until that has taken place. All that can be 

 done is to estimate by experience the point at which the 

 electrostatic effect has disappeared, and to take that point as 

 the false zero. It Is neither theoretically nor practically 

 correct, but the error for small currents (not exceeding 

 25 milliamperes) is generally small, except when the length of 

 cable in circuit is under 50 or over 5,000 ohms. It is the 

 best approximation that can be made. 



" The balance is therefore made while the battery current is 

 OD, to that point taken as zero, determined as soon as possible 

 after the battery is taken off. The period of time elapsing 

 before fixing upon the false zero is much greater with a long 

 than with a short cable. Fortunately the error produced is 

 not great within the limits named." 



The battery and battery switch or commutator should be 

 well insulated. The number of cells to give about 20 milli- 

 amperes to line must be regulated by the milammeter. The 

 current may fall off slightly, due to polarisation of the cells, and 

 therefore the milammeter readings should be noted down after 

 each pair of observations. 



Preliminary bridge tests taken when first communication 

 is interrupted will show whether the fracture polarises with 

 the zinc or carbon current. Generally, of course, it polarises 

 with positive current to line and breaks down with nega- 

 tive, signifying an exposure of copper at the break. It 

 happens sometimes, however, that the bare conductor at the 

 fracture is buried in mud, thus preventing the gas evolved 

 by the zinc current from escaping and stopping any deposition 

 of salts with the positive current. Under these circum- 

 stances the break is very variable, and sometimes polarises 

 with zinc and not with carbon current. The direction of 

 current used in testing is always that which breaks down 

 the resistance of a fault or fracture, or does not allow 

 it to rise. This direction is almost always zinc to line ; 

 but if the break polarises with zinc as described (which, 

 however, is rare) the test can be taken with positive 

 current, which, under these circumstances, gives the lowest 

 results and follows the law with currents up to 25 milliamperes. 



A diagram of the connections is given in Fig. 235, in which 

 it will be noticed that the milammeter is shov/n connected in the 



