UNDERGROUND WIRES. 



a metal plate, or other surface, attached to the line wire at the 

 place where it enters the station. It is found that these points 

 attract the atmospheric electricity, which passes to the ground by 

 the conductor connected with them, but do not attract the 

 electricity of the battery current. 



63. The wires extended from post to post are continued in passing 

 the successive stations of the line. The expedients by which the 

 current is turned aside from the main wire, and made to pass through 

 the telegraphic office of the station, differ more or less in their 

 details on different lines and in different countries, but are founded 

 on the same general principles. It will therefore be sufficient here 

 to describe one of those commonly used on the British lines. 



The conducting wire of the main line in passing the station is 

 cut and the ends jointed by a shackle, as represented in fig. 12, 

 in the case of a winding post. This shackle breaking the 

 metallic continuity would stop the course of the current. A wire 

 is attached to the line wire below the shackle so as to receive the 

 current which the latter would stop, and is carried on insulating 

 supports into the telegraphic office and put in connection with the 

 telegraphic instrument. Another wire connected with the other 

 side of the instrument receives the current on leaving it, and 

 being carried back on insulating supports to the line wire, is 

 attached to the latter above the shackle, and so brings back the 

 current which continues its progress along the line wire. 



64. Although the mode of carrying the conducting wires at a 

 certain elevation on supports above the ground has been the most 

 general mode of construction adopted on telegraphic lines, it has 

 been found in certain localities subject to difficulties and incon- 

 venience, and some projectors have considered that in all cases it 

 would be more advisable to carry the conducting wires underground. 



This underground system has been adopted in the streets of 

 London, and of some other large towns. The English and Irish 

 Magnetic Telegraph Company have adopted it on a great extent 

 of their lines, which overspread the country. The European 

 Submarine Telegraph Company has also adopted it on the line 

 between London and Dover, which follows the course of the old 

 Dover mail-coach road by Gravesend, Rochester and Canterbury. 



65. The methods adopted for the preservation and insulation or 

 these underground wires are various. 



The wires proceeding from the central telegraph station in 

 London are wrapped with cotton thread, and coated with a 

 mixture of tar, resin, and grease. This coating forms a perfect 

 insulator. Nine of these wires are then packed in a half-inch 

 leaden pipe, and four or five such pipes are packed in an iron pipe 

 about three inches in diameter. These iron pipes are then laid 



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