FAULTS IN SUBMARINE CABLES . 
269 
may conceal it for a time. Rapidly reversed currents from a 
battery of 100 or more cells tend to enlarge the opening and 
render its resistance insignificant. A copper current passing 
through the fault to the sea seems to stop the opening better 
than a zinc current, because it causes a deposit of copper 
chloride and oxygen, whilst a zinc current gives rise to a dis¬ 
engagement of hydrogen and a deposit of salt. The bubbles 
Fig. 189. 
of gas formed under great pressure burst after a time, and the 
disengagement of gas temporarily enlarges the fault. When 
this takes place under the influence of the negative current, 
only a small enlargement of the opening is produced; but by 
the positive current the copper conductor is gradually eaten 
into, and this frequently occasions a solution of its continuity. 
There is no sign of this destructive action, for so long as the 
smallest trace of metal remains the resistance of the line is not 
sensibly affected. The operators in this case prefer to use the 
