Electric Power Transmission.
In the insulated cable that transmits electric power, metal wire is used, since metal is an excellent conductor of electricity. Copper alloy is frequently used. Weight is an important factor, because the large amounts of power generated and transmitted by modern power plants necessitate the use of thick cables. Maximum overhead line voltages have been increased from 345,000 volt in the 1950s to 765,000 volts at present. Even higher voltages can be attained, although it is contended that ultrahigh-voltage (UHV) lines are already emitting dangerous radiation.
It is possible that for high-capacity, long distance lines, direct-current (DC), rather than alternating-current (AC), power will be used in the future. Direct-current lines emit less radiation over long distances, are less expensive, and are more easily brought up to high voltages; receiving ends, since both generating plants and receiving devices are designed for alternating current. (See Power, generation and transmission of).
Communications cable operates at lower voltage and higher frequencies than electric-power cable. An ordinary telephone wire can easily carry signal of limited frequency, and trunk lines are capable of handling thousands of simultaneous conversations. While modern electronics permits multiple conversations to be carried on a single wire, great numbers of wires are often needed because of the high volume of calls. Interference (cross talk) between the circuits then becomes a problem. Other types of information network are now also carried over the communications network, including television signals that require higher frequencies than the ordinary telephone wire can carry.
The modern coaxial cable can carry both multiple signals and extremely high frequencies. A single cable consists of a thin copper wire accurately centered by non conducting plastic spacers inside a copper tube about 7 mm (0.28 in) in diameter. The signal travels in the space former between the two copper conductors-an arrangement that reduces the resistance of the cable to the passage of high frequency signals.
A typical telephone coaxial cable consists of 8 to 22 coaxial tubes sheathed in metal and plastic. In order to obtain many different voice channels for separate telephone conversations within a tube, a carrier wave (a separated sections; each of the conversations is funneled into a different section of the carrier wave by a device called a multiplexer.
The development of fiber optics has to the use of fiber cables that carry digital signals instead the analog signals of copper cables. The hair-thin glass fibers transmit encoded laser light and can handle huge number of messages simultaneously. Telephone systems in particular have made broad use of fiber-optic cable.