154 SUBMARINE CABLE LAYING AND EEPAIEING. 



and the permanent deflection on this instrument is proportional 

 to the potential at the end. The shore knows that all is well 

 if this deflection remains steady, duly changed in direction 

 every hour by the ship reversing, and the man on watch is 

 instantly made aware of the appearance of any fault or serious 

 drop in insulation by a fall in this dpflection. 



In the testing-room on board, the cable forms one arm of the 

 bridge, of which a fixed resistance forms the second arm, and 

 the slides the two ratios, ' For these connections plugs are put 

 in at A and B, the testing galvanometer being put in circuit 

 by plugging in the galvanometer two-way commutator on that 

 side. A plug is also put in connecting the two levers of the 

 speaking-key to ensure a safe earth connection without depend- 

 ing on the bridge contacts of the key. The current is kept 

 permanently on by one or other of the testing-key levers 

 reversing them every hour, and the slides are adjusted so that 

 the galvanometer is balanced. 



The galvanometer in circuit with the slides shows whether 

 the testing battery remains constant — an important point, as if 

 the battery fails shore's deflection will fall without any altera- 

 tion of the ship's balance. For convenience, the resistance of 

 this galvanometer is generally 1,000 ohms, ind the last coil 

 of the slides (of similar resistance) is cut out, so that the total 

 resistance of the slides is unchanged. 



The reading on the slides gives continuously the insulation 

 of the cable being payed out, and any fault is immediately made 

 apparent by the spot of light moving away from zero, showing 

 want of balance. " 



So far both ship and shore are continuously informed as to 

 the state of insulation during the progress of laying, and any 

 serious fall in the insulation is immediately apparent to both. 

 It remains to show how the continuity of the copper conductor 

 is proved. This is tested by the shore closing the discharge- 

 key K at pre-arranged intervals (generally every five minutes). 

 Ship's time is kept on shore for all observations ; the time at 

 noon is signalled from the ship every day, and the clock on 

 shore set accordingly. At the moment of closing the discharge- 

 key, the condenser takes a charge from the cable and causes 

 a throw on the ship's galvanometer, proving continuity. The 

 key is then released, and a discharge throw is obtained through 



