Bare copper wire is the best, and is generally used for aerials. Wherever the stretch is 100 feet or over, however, so that the wires are subjected to considerable strain from their own weight, phosphor bronze is used because of its greater tensile strength. Commercial and navy stations employ stranded wire. High frequency currents have the peculiar property of traveling near the surface of wires and conductors. They do not permeate to the center of the wire, as do normal currents. The surface of a stranded wire is greater in comparison to its cross-section than a solid conductor of the same diameter, and therefore is often employed because it offers less resistance to currents of this sort.
FIG. 24.—Showing how wires are arranged and insulated.
Aluminum wire is very light, and causes very little strain on the pole or cross-arms. It offers more resistance than copper, but some of the larger sizes may be used with equally good results.
Iron wire must never be used, even if galvanized or tinned. It possesses a certain reactance tending to choke off the high frequency currents.
FIG. 25.—Aerial insulator.
The aerial is always very carefully insulated from its supports and surrounding objects by special insulators, capable of withstanding severe strains, made of a moulded material having an iron ring imbedded in each end.
FIG. 26.—Leading-in insulator.