opinion as to the relative advantages of iron and copper wires for transmitting the electric current, but for over-head wires the London and North Western Company prefer galvanised iron wires, as having greater tensile strength and superior powers of elongation and contraction, so as to allow for a considerable variation in temperature. The gauges of wire used differ according to circumstances, but the wire chiefly employed has a standard diameter of ·171 inch with a minimum tensile strength of 1,200 lbs., and is subjected to a torsion test of 20 twists in 6 inches. The average life of the wire varies very much according to locality. For instance, in the vicinity of chemical works, as at Widnes and St Helen's, the wire corrodes and requires renewal in about three years, while, in the pure atmosphere of the island of Anglesey, there are wires that have been in use for five-and-thirty years, and which recent tests show to have been very little impaired, whether as regards conductivity or tensile strength; under ordinary conditions, however, the average life of the standard wire is about 10 years.
As regards battery power, the Company use, for speaking telegraph circuits, three wire block telegraph circuits, and electric locks, the Daniell Sulphate Battery; for bell wires the Leclanché Battery; and for signal repeaters, Fuller's bichromate battery.
