THE LOCALISATION OF BREAKS AND FAULTS. 425 



then be deducted from the result to give the distance in ohms 

 from the testing end. 



It sometimes happens, though rarely, that polarisation 

 occurs with the zinc current and the fault breaks down with 

 carbon, in which case the positive current must be kept on and 

 the negative applied only so long as to obtain the balance in 

 each pair. 



Rymer-Jones High-Resistance Break Test. — Breaks in which 

 the conductor is buried in mud or partially sealed by the draw- 

 ing over of the insulator cannot be localised by the methods 

 applicable to exposed ends, and being generally very variable 

 are difficult to locate. The resistance of ends in breaks of this 



Fig. 245. 

 Kelvin's Astatic Reflecting Galvanometer as a Differential. 



kind frequently runs up from thousands to 50,000 ohms. Mr. 

 Rymer-Jones met with some variable breaks of 20,000 ohms 

 on the Amazon River cable, and to meet the difficulty in localis- 

 ing these, devised the test described in the Electrical Bevieiv, 

 June 7 andl4and July 5, 1901. This has now been revised and 

 simplified. The only special instrument required is a differential 

 galvanometer, and for this purpose the Thomson astatic reflecting 

 galvanometer can be readily adapted for shore tests by using the 

 two half coils on either side of the suspension in parallel, as in 

 Fig. 245. Universal shunts are employed on each side, as in 

 Fig. 246, Gi and Ga being the two halves of the differential 

 galvanometer with their respective shunts. In this diagram 

 commutators are shown for making the changes required for 

 the different observations, but these are only conventional and 

 for the purpose of assisting the explanation. The changes 



