THE LOCALISATION OF BREAKS AND FAULTS, 42S 



from zero. At the moment before the evolution of gas occur 

 the resistance unplugged is that required, namely, the resist- 

 ance up to fault plus the lowest resistance of fault obtainable. 



The test is repeated several times, each time cleaning the 

 fault, first by the application of zinc current, then sealing it by 

 a short duration of positive current, then applying zinc again^ 

 and following the fall in resistance by the plugs on the bridge. 

 Each time the test is repeated the application of zinc may be 

 of shorter duration, as the exposure, having once been well 

 cleaned, responds better to the current. 



When the break or fault is at a considerable distance the fall 

 in resistance due to negative current is gradual and easily fol- 

 lowed on the bridge by the plugs, but when it is situated near 

 to the testing end of the cable the fall is rapid, and there is 

 some diflBculty in shifting the plugs quickly enough. In that 

 case Kempe recommends inserting a resistance in the line to 

 slow down the action. This resistance is, of course, deducted 

 afterwards from the bridge reading. The result on the bridge 

 iocludes the resistance of the fault or break exposure, and this 

 must be estimated and deducted in order to arrive at the 

 fault's distance. The rate of polarisation is the chief means 

 by which the fault's resistance is estimated. If the rate of 

 polarisation when touching it with positive current is slow it 

 indicates a large exposure and small resistance, if rapid a 

 smaller exposure and larger resistance. 



Quick Reversals to True Zero. — This test is taken with a 

 dead-beat reflecting galvanometer, such as Sullivan's, and is 

 applicable to breaks or faults with large exposures. Bridge 

 connections are employed with the usual battery reversing key. 

 The zinc current is first put to line and the key kept clamped 

 down while the bridge plugs are adjusted to the fall in resistance 

 due to the cleaning of the exposure by this direction of current. 

 After a time the resistance will cease to drop further and will 

 become fairly steady, and this, being the lowest result attain- 

 able, is accepted as the zinc reading. The carbon or positive 

 current reading is then taken very quickly in order to prevent 

 the fault sealing up, the proper manipulation for which requires 

 some practice. This current must only be applied for the 

 shortest period of time necessary to' obtain balance. The 



