CHAP v.] TELEGRAPHIC LINES. 607 



CHAPTER V. 



TELEGRAPHIC LINES. 



I. Am LINES. SUBTERRANEAN LINES. 



WE have hitherto spoken of the apparatus which serve to produce or 

 receive the signals. It now remains for us to describe the lines which 

 transmit them, that is give passage to the electric currents, which are 

 the bases of telegraphy. 



An air line of the electric telegraph is formed of metallic wires 

 generally supported by wooden poles planted at equal distances along 

 the course of the line. At first these wires were of copper of 2 

 mm. in diameter. The metal chosen had the advantage of being a 

 very good conductor of electricity, but besides its high price, it had 

 the disadvantage of losing its elasticity under the influence of changes 

 'of temperature and of becoming brittle. Copper having been generally 

 abandoned, annealed iron has taken its place, which though more 

 resisting, is less costly, and to which a diameter of 3 or 4 millim. 

 is given. On lines of over 200 miles in length, where it is required to 

 have as little resistance as possible to the passage of the currents 

 iron wires of 6 or 6|- mm. diameter are employed chiefly in England. 



The iron wires of telegraphic lines are galvanized, that is to say 

 after being cleaned in acidulated water, are covered with a thin coating 

 of zinc ; the latter is oxidized in the air, and preserves the iron from 

 rust, and further prevents, by an electrical action, the oxidation of 

 those parts which are accidentally uncovered. 



The supporting poles, made of pine, injected with sulphate of 

 copper, are insulating when dry ; but to prevent the loss of electricity 

 in damp and rainy seasons, the wire is never directly attached to the 

 poles, but is insulated by glass, earthenware, or porcelain insulators. 



