326 ATLANTIC TELEGRAPH. SECT. XXXII. 



diameter, and 2100 miles long. The use of the gutta percha is 

 to insulate the wires ; the other coatings are merely for protection. 



The Voltaic battery which generates the electricity consists 

 of 40 cells, the plates of which are alternately of zinc and 

 platinized silver, each about nine inches square, the exciting fluid 

 being dilute sulphuric acid. Although the force developed by 

 this battery is so great that a piece of iron three inches long and 

 three eighths of an inch in diameter placed in contact with the 

 poles may be consumed in a few minutes, it is absolutely inca- 

 pable of sending a current of electricity through wires 2500 miles 

 long, on account of their resistance, without the aid of Dr. Fara- 

 day's inductive action. It is only the primary agent for inducing 

 a current of sufficient strength. 



To accomplish that, many thousand yards of fine copper wire 

 coated with silk are wound round a hollow soft iron cylinder ; the 

 whole is then coated by gutta percha, and the end of the wire is 

 joined to the wires in the cable so as to form a continuous line 

 from Valentia to Newfoundland. A second copper wire, shorter 

 but thicker than the preceding, and also insulated by a coating of 

 silk, is wound round the cylinder above the gutta percha : when 

 the ends of this thick wire are brought into contact with the 

 poles of the battery, currents of electricity flow through it, 

 between pole and pole, and in their passage temporarily convert 

 the hollow iron cylinder into a powerful electro-magnet, which 

 by its reaction induces a current of electricity in the fine wire of 

 sufficient power to cross the Atlantic. The efficiency of the 

 electric telegraph depends upon the power we possess of breaking 

 and renewing the current at pleasure, since by that means dis- 

 tinct and successive signals are made from station to station. 

 In the Atlantic cable positive and negative electricity are trans- 

 mitted alternately ; the electricity is sent to America from alter- 

 nate poles, and the current returns again through the water, 

 which completes the circuit. 



The passage of electricity through a cable or telegraphic wire 

 in air is sensibly instantaneous ; that through a cable, whether 

 extended in water or under ground, requires time on account 

 of lateral induction through the gutta percha ; for the electricity, 

 in passing through the wires, induces the opposite electricity on 

 the surface of the water or moist earth in contact with the cable, 

 and in that respect it is precisely like a Leyden jar, the gutta 



