ELECTRO-TELEGRAPHY 



243 



814 



the earth ; but the battery power required to overcome the resistance of very great 

 lengths of wire is equally able to overcome the resistances presented by inferior insu- 

 lators, and to escape in considerable quantities at every pole ; so that the force which 

 reaches the far station would not bo equal to its work. It is for these long lines that 

 the greatest ingenuity has been expended in constructing insulators. Fine porcelain 

 is most in favour from its presenting a very smooth surface, and being less hygro- 

 metric than glass ; and it is distorted into most mysterious -looking shapes in order to 



present as great a distance, and one as much sheltered 

 as possible, between the part with which the line-wire 

 is in contact, and the part that is in contact with the 

 pole. 



Telegraph signals pass with far less rapidity through 

 buried and through submarine wires than along the 

 ancient aerial wires. The slow travellings mentioned 

 above were through wires of this kind. "We must 

 refer to treatises on Electricity for full details of the 

 conditions presented by a telegraph cable. In practice 

 it is found that on first sending a signal into a sub- 

 merged wire, the electricity is delayed on its road, 

 in order to produce a certain electrical 

 condition upon the surface of the gutta- 815 



percha that is in immediate contact 

 with the conducting wire. Nor is this 

 all ; before a second distinctive signal 

 can be sent, it is necessary that the 

 condition produced by the first signal 

 shall be destroyed; and this is an 

 operation requiring even more time 

 than was consumed in the mere act of 

 producing it. These two classes of 

 retardation, especially the latter, were 

 largely manifested in the Atlantic 

 ca,ble; and have called forth all the 

 ingenuity of electricians in order to 

 mitigate or to modify them. C.V.W. 



Fig. 814 represents the cable that 

 has been lying in the British Channel 

 between Dover and Calais, since Sep- 

 tember 1851. It contains four No. 16 

 copper-wires, each wire being doubly 

 covered with gutta-percha. The four 

 wires are then twisted into a rope ; and 

 the rope is thickly covered, first with 

 hempen yarn, tarred, and finally with 

 a jacket of ten No. 1 iron-wires. The 

 cable is shown in perspective and in 

 section. Fig. 815 shows the perspec- 

 tive and section of the Irish, a single- 

 wire cable. It consists of a single 

 central conductor, of one No. 16 copper- 

 wire, ckmbly covered with gutta-percha, 

 then with hempen yarn as before ; and 

 finally with a protecting jacket often 

 No. 8 iron-wires. The Calais cable 

 weighs 7 tons per mile ; the Irish 2 

 tons per mile. 



The first practical experiment with i|V 

 deep-sea telegraphs appears to have 

 been made by Mr. Brett in 1850. This 

 was a gutta-percha covered copper-wire sunk between Dover and Calais, which failed 

 on the first day. In 1851, a cable consisting of four covered copper- wires was sunk 

 across the Channel, which is still working. In 1853, Messrs. Newall made a line 

 seventy miles long between Dover and Ostend. In this cable the conducting wires 

 were increased to six. ' 



The Dover and Ostend cable was laid on May 6, 1853. This cable (fig. 816) is 70 

 miles long ; it is composed of six copper-wires, insulated by a coating of gutta-percha, 

 which are essentially the electrical cable, and these are secured by an armour of twelve 



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