498 



SUBMARINE CABLE LAYING AND REPAIRING. 



in the bridge, and then the current put on, and the galvano- 

 meter key opened too soon, the deflection will show " more re- 

 sistance wanted." This deflection will, however, creep back to 

 zero in a second or two. 



The effect is just the opposite to that experienced with laid 

 cables. There the electrostatic effect predominates, and the 

 current on first being applied rushes into the cable with great 

 strength, making the resistance appear less than it actually is. 



Another disturbing effect on board ship is the current set up 

 in the cable in tank due to the rolling of the ship. This makes 

 good copper resistance tests difficult to obtain, and it is next to 



Fig. 277.— Testing Cable in Tank. 



Impossible to obtain good results with discharge tests on cable 

 in tank. In tanks on shore there is only the inductance effect to 

 take into account, which for CR tests is got over by allowing a 

 few seconds before opening the galvanometer as described 

 above ; but when the tank and cable are rolling about at sea 

 there are disturbing currents produced in the cable due to its 

 moving relatively to the earth's magnetic field. In fact, it is 

 well known that cable tests much steadier in tanks ashore than 

 in tanks afloat. 



When testing for capacity and insulation resistance the induc- 

 tance effect may be got over by the simple expedient of connect- 

 ing both ends of the cable to the bridge, as in Fig. 277. The 

 current then enters the cable in opposite directions, and no mag- 

 netic field being produced, there is consequently no Inductance. 



