HISTORY AND PROGRESS. 195 



Another method of stretching with the insulator itself is 

 by means of a wrought-iron winch, w, attached to the stalk, 

 as in Fig. 107. The ends of the line wire on each side are 

 passed through holes in the two drums d d, and wound up 

 tightly, the drums being prevented from running back by 

 clicks and ratchet-wheels on their axles. 



The Wire. The line wires are of iron ; very rarely of 

 copper. The superior conducting power and durability of 

 copper recommends it for employment, but the danger to 

 which such a line is constantly exposed of being cut and the 

 wire stolen is an argument against it. Were it not for this, 

 a copper line would be much cheaper in the end than an iron 

 one ; iron having a comparatively small conducting-power, 

 and, to give the same resistance for the same length, must 

 have a section at least seven times as great as would be 

 required for copper. This increased section increases, of 

 course, the weight of the line, which, as a consequence, neces- 

 sitates stronger posts and stronger insulators, in order to 

 allow each wire to be strained tightly between the supports 

 to keep it from touching the others. 



Various plans have been proposed for coating iron wires 

 in order to protect them from oxidation. The plan which 

 has met with most favour in reward of its merits, is that of 

 painting the wires with tar, or, as proposed by Romershausen, 

 of varnishing them from time to time with a good coat of 

 boiled linseed oil. By this method a thick crust is gradually 

 formed, which protects the wire completely. The only 

 objection to the process is its repetition, which renders it 

 difficult in climates like that of England, for the painting 

 can only be done successfully in fine weather. 



The other methods consist of covering the wire with a coat 

 of some metal which oxidises slowly. Zinc is frequently used 

 for the purpose ; it is applied by a process called " galvan- 

 ising," by which the surface of the iron is covered with a 

 thin film while the zinc is in a state of fusion. Dr. Siemens 

 says that the " galvanising " of iron wire is only useful when 

 the zinc is really melted together with the iron while the 

 two metals are in contact under heat, which is the onlv 



