422 



SUBMAEINE CABLE LAYING AND BEPAIEING. 



The ordinary bridge connections are used, as in Fig. 244, 

 and readings taken to false zero to eliminate earth current. 

 The galvanometer best suited for the test is the horizontal 

 astatic type with pivotted needles. It is hardly a method 

 suitable for ship work, as it takes too long to execute, but is 

 useful as a shore method which can be applied during the 

 interval between the interruption of the cable and the repairing 

 ship's arrival on the scene. 



The distant end should be free if there is no disconnection of 

 the conductor at the fault. The fault must iirst be thoroughly 

 cleaned by a strong zinc current, the effect of which is to break 

 down its resistance. The time this should be kept on depends 



Cable 



Fig. 244.— Lumsden's Test. 



upon the behaviour of the fault. If it is noticed that the re- 

 sistance no longer falls, but becomes steady, it shows the fault 

 to be clean, but it may take several hours before this is 

 observed. 



After the cleaning period the fault is just sealed over by 

 a few minutes' application of positive current, then the nega- 

 tive current is put on, causing a gradual fall in the resistance, 

 and the plugs in the bridge are adjusted quickly to keep 

 pace with the fall. This diminution of resistance arises from 

 decomposition of the salts at the fault, and immediately a 

 passage is opened through the coating, hydrogen is set free, 

 the resistance is increased and the needle moves rapidly away 



