THE LOCALISATION OF BREAKS AND FAULTS. 



369 



measure the resistance with reversals of current, noticing the 

 rate and amount of polarisation with each current. If the 

 wires of the battery are reversed and a carbon current sent 

 into the insulated wire, the bubWes cease and the resistance 

 goes up. The galvanometer will also show the direction of the 

 current of polarisation, and its rate of fall after the testing 

 current has been taken off, 



A good insight can be obtained into false zero testing with 

 such an artificial fault, especially as there is no interference 

 from electrostatic discharge, as in a cable. 



With a carbon current the action is to deposit chloride 

 of copper on the exposed copper at the fault, and thus "seal " 

 it up. In other words, the deposit of salts which almost 

 instantaneously forms on the copper increases the resistance of 

 the fault, because it covers it up from contact with the sea. 



Fig. 218.— Artificial Fault. 



In low-resistance faults, or those which have comparatively 

 large exposed area, the action is not so rapid as to prevent a 

 reading being obtained ; but, with a fault of small exposure and 

 high resistance, it is risky to touch it with positive current, as 

 it runs up immediately into hundreds and thousands of ohms, 

 and sometimes the zinc current will not break it down again. 

 The fault generally falls in resistance when the zinc current is 

 put on, but not always. Sometimes it polarises up, and this 

 happens with small exposures of conductor at the fault, or 

 exposures obstructed by mud or in other ways so that the 

 hydrogen gas cannot easily escape. The gas then collects on 

 the surface of the exposure and insulates it from the water. 

 When this happens it is best to test with reversals. 



The Milammeter. — The adoption of the dead-beat moving 

 coil milammeter in cable testing has very greatly simplified 

 methods and calculations in fault and break localisations. The 



