SUBMARINE TELEGRAPHY 199 



fcs it is called have in fact made enormous progress, and it is to 

 the discoveries and inventions in this branch of science that we 

 o^ve both those improvements in the quality of the materials 

 employed, and that certainty of detecting the smallest fault, 

 vrhich led so many practical engineers and electricians to a con- 

 viction of the feasibility of the great undertaking now so hap- 

 pily completed. It is on these electrical tests that a reasonable 

 belief may be based of the probable permanence of the two 

 Atlantic Cables, and it is to these improvements that attention 

 will now be directed. 



The electrical tests employed for the first cables made were 

 simple enough. It was necessary to ascertain that the copper 

 conductor in the cable was unbroken, and fit to transmit an 

 electric current. This was tested by placing a galvanometer in 

 a simple circuit formed by the battery, the copper conductor of 

 the cable, and the wire of the galvanometer. If the conductor 

 was unbroken, a current passed from one battery pole to the 

 other through the cable, and in its passage through the instru- 

 ment deflected a needle. The stronger the current, the more 

 the magnetised needle was deflected. If the conductor failed at 

 any point, no current passed. It was also desirable to know 

 that the conductor was insulated, so that no considerable portion 

 of the current entering one end of the cable would be lost before 

 arriving at the other end, where it would be required to produce 

 a signal ; to ascertain this the metallic circuit was broken one 

 pole of the batteiy remained connected with the conductor of 

 the cable through the galvanometer wire ; the other pole was 

 connected with a plate buried in damp earth, the cable was put 

 under water, and its far distant end was insulated. Thus the 

 battery was ready to send a current into the cable, and would 

 do so if the cable were at any point connected with the earth. 

 When the cable was well insulated, no current passed ; if there 

 was a fault, that is to say, a connection between the copper inside 

 the cable and the earth or water outside, a current passed and 

 deflected the galvanometer needle. The test consisted simply in 

 trying whether a current would pass through the conductor, and 

 would be stopped by the insulator ; the galvanometer being 

 an instrument which showed the presence or absence of a current 

 by its effect on a magnetised needle. Staunch conservatives 



