io8 
THE TELEGRAPH. 
it can now be produced at much less cost than formerly with a 
great tensile strength and a very high conducting power. So 
superior in this last respect is copper to iron, that a copper 
wire having the same electric resistance as a given iron one is 
only one-sixth the weight of the latter. A copper wire will 
therefore cause much less strain on the posts and supports than 
an iron wire of the same conductivity. 
Joining the ivires .—The junction of the wires used to he 
effected by twisting them together for a distance of about 
7 inches. They were also soldered together at the middle of 
the twisted part to prevent oxidation, and ensure the continuity 
of the conductor. This process had the advantage of closely 
joining the wires, but it also lessened their power of resisting 
strains. For experience showed that when ruptures occurred 
from a fall of temperature, they generally occurred at the 
places where the twist of the wire began. 
For this reason, the process was abandoned and the following 
