THE LOCALISATION OF BREAKS AND FAULTS. 



473 



When the fault is large, and its potential too low to obtain 

 a satisfactory reading, a positive current can be put to line for 

 a few minutes to seal it up and raise the potential, the test 

 being independent of the actual resistance of the fault. The 

 resistance of the fault is neatly eliminated in this test, but the 

 potentials observed will be affected by the variation in its 

 resistance. For this reason it is better for the shore station to 

 put z'nc to line to keep the fault clean. If the fault resistance 

 increases the potentials will all increase, and vice versa, the 

 variation being shown most on the smaller potentials pi and 

 I), but very little on the larger one P. It is advisable, there- 

 fore, that the slide reading on board {p-^ and the throw (or 

 balance) on shore {f) should be repeated until the results are 

 steady and reliable. 



Balance Key 



Battery ^q^ g^^^g j^^^^ 



Plug 

 or Key 



Fig. 200. — Clark's Potential Test. Rymer- Jones' Method for Ship 

 Observations. 



Mr. J. Eymer-Jones has devised a convenient method for 

 observing the single potential reading ( i^-^ on board ship {Elec- 

 trical Review, September 7, 1894). This is an ingenious adapta- 

 tion of the Kelvin mixed charge test (see page 480), and being 

 a null method, the readings at sea can be taken with greater 

 convenience and accuracy than condenser throws. Two stan- 

 dard condensers are connected as in Fig. 260 to the mixing key 

 (either Lambert's or Silvertown pattern, described above), the 

 lower contacts of which are in connection with the free end of 

 the cable and the travelling contact of the slides respectively. 

 The most suitable capacity of these condensers will depend upon 

 the resistance of the fault, and should be from one-third micro- 



