364 SUBMARINE CABLE LAYING AND REPAIRING. 



kinds of faults make their presence felt on the signals at once, 

 and at times there are total breaks or dead-earth faults, which 

 immediately put a stop to communication. But, however 

 amall a fault is when first detected, it will gradually open up 

 under the action of signalling currents, even should the 

 original cause have ceased, and on this account it is usual to 

 proceed at once to remove it. Before the ship leaves, the 

 position of a fault is determined as closely as possible by tests 

 on shore from both ends, and after she arrives at the position 

 and cuts in, similar tests are made between the end on board 

 and the shore to obtain a nearer result. 



It will be seen that these tests are of great importance, as, 

 when carried out skilfully, the ship's time at sea is lessened, 

 and the business of the company quickly restored to its accus- 

 tomed channel. 



Cable Currents. — In a submarine cable there are various 

 currents prevalent, arising from different causes, which inter- 

 fere generally with testing. To eliminate their effect is the 

 general aim in the best methods. It is a first essential to have 

 the various currents clearly defined in the mind, and form a 

 separate idea of the action of each, although they chiefly act 

 together. These currents are considered separately as fol- 

 lows : — 



(1) Earth Currents. — These currents are due to natural 

 causes, and arise chiefly from climatic conditions and magnetic 

 disturbances. The varied rate of evaporation and condensation 

 in different localities produces different states of atmospheric 

 potential which act inductively on the earth. In two distant 

 localities the earth may be at very different potentials, and a 

 current then flows from the higher to the lower potential. 



The direction of such currents may follow or cross the line 

 of a cable, but the direct flow of earth current is prevented 

 so far as signalling is concerned by condensers placed between 

 the instruments and line. In testing, of course, the line must 

 come direct on to the bridge and galvanometer, through 

 which there is a direct path for earth currents to earth. 

 Earth currents are quite independent of any testing current, 

 and may be present in cables whether earthed or free, and 

 whether perfect or faulty. When a cable is earthed at both 



