502 



SUBBIARINE CABLE LAYING AND REPAIRING. 



battery and galvanometer and at the other end wrapped closely 

 two or three times round the middle of the tapered end 

 of the percha, as in Fig. 280. By this device the conductor 

 and the core surface close to it are at the same potential, and 

 therefore no leakage can take place between them. As regards 

 the rest of the core — namely, the surface between the guard 

 wire and the sheath — it may not always he practicable in ex- 



Paraffin Wax 



Fig. 279. — End Paraffined to Prevent Surface Leakage. 



ceptionally moist surroundings to keep this part of the surface 

 quite free from a slight film of moisture, but if the resistance^ 

 of this film is large in comparison with the galvanometer re- 

 sistance no leakage will take place over the core surface to 

 earth. 



The guard wire, although bare where attached to the core, 

 must be otherwise well insulated throughout its whole length 

 from the core to the testing table. Should it make earth or 



Fig. 280. — Price's Guard-wii'e in Insulation Test. 



partial earth anywhere along its length there will, of course, be 

 leakage to earth at the weak spot. This will have the efifect of 

 slightly lowering the potential of the battery when on the 

 cable, and to be exact in the comparative observations in D.R. 

 tests the same condition should hold while the constant is taken. 

 It Is therefore advisable to retain the guard wire on the core 

 when taking the galvanometer deflection through the high 



