Cable

Cables came into widespread use in the second half of the 1800's, when telegraphs and telephones became common. Since the mid-1900's, communications systems that use radio signals broadcast from transmitters on earth or in artificial satellites have begun to replace some cables. For example, telephone companies now use radio waves to carry most long-distance telephone calls. Communications satellites send television programs and other electronic messages throughout the world. In many cases, however, cables still provide the most practical means of communication.

This article discusses communications cables. For more information on cables used to carry electric power, see Electric Power.

Various types of cables play an important role in everyday life. Communications cables transmit telephone calls and TV programs. Electric power cables distribute electricity from power companies to customers. Connectors are used in computers, radios, and other electronic devices. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by William Graham.

Simple cables are made up of a single pair of insulated wires twisted together. Multiconductor cables, such as telephone lines, contain hundreds--or even thousands--of conductors bound together. In many cases, multiconductor cables are enclosed in a heavy sheath made up of several layers of aluminum or plastic. Some thick cables also contain steel wire to provide strength.

Communications cables are laid underground and along the ocean floor, or they are mounted on poles. Burying cable protects it from harsh weather and keeps the land aboveground uncluttered. As a result, few new communications cables are strung aboveground today. Underwater cables serve as a communications link between continents. These underwater cables need tough outer coverings so that they can withstand strong ocean currents.

Most cables are designed to carry more than one message at a time. For example, a process called carrier transmission enables two pairs of wires to transmit as many as 96 telephone conversations simultaneously. Each conversation is carried by an electric current vibrating at a different frequency (number of cycles per second) so that the individual signals do not interfere with one another. Electronic filters at each end of the cable sort out the various conversations.

Engineers have developed two kinds of cables that can carry especially large quantities of messages at once. They are (1) coaxial cables and (2) fiber-optic cables.

A coaxial cable consists of one or more conductors called coaxials. A fiber-optic cable has a number of transparent fibers of glass and a central wire that strengthens the cable. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book illustrations by William Graham.

Coaxial cables are made up of two conductors. The outer conductor is a rigid or flexible metal tube, and the inner conductor is a wire running through its center. Insulation holds the wire in place. The tube and the wire have the same axis (center) and are therefore called coaxial. A typical coaxial cable has about the same diameter as a pencil, and as many as 22 cables may be bundled together to make a larger cable.

A coaxial cable's outer conductor shields the electric signals it carries from outside interference and helps prevent the signals from escaping. In addition, special amplifiers called repeaters are installed at various points along many coaxial cable systems to strengthen the signals. These amplifiers help prevent cable loss. Cable loss is the gradual weakening of signals as they travel along a cable. Repeaters consist of such electronic devices as transistors.

Many telephone calls, especially long-distance calls, travel over coaxial cables. When used for telephone conversations, coaxials work in pairs. One coaxial carries signals in one direction, while the other handles signals from the other direction. A pair of coaxials may handle as many as 13,200 telephone conversations at once.

Cable television systems use coaxial cables to transmit TV programs. A single cable can carry as many as 100 television signals at once. As a result, cable TV systems can transmit regular network programs as well as a variety of special features. See Television (Cable television systems).

Fiber-optic cables carry messages in the form of light. Such cables consist of a bundle of glass optical fibers, which look somewhat like transparent threads. Coded light signals travel through the core of the fibers. A thin layer of glass called cladding surrounds the core and helps prevent the light from escaping.

Special lasers are used to transmit the coded light signals through fiber-optic cables. The lasers flash on and off at extremely high speeds. Electronic devices at the receiving end of fiber-optic cables decode the light signals.

The largest fiber-optic cables can carry hundreds of thousands of telephone conversations or hundreds of television channels. Many communications companies have begun using fiber-optic cables instead of coaxial cables. No electrical interference occurs in fiber-optic cables, and there is less cable loss than there is in coaxial cables. Fiber-optic cables measure only 1/25 to 1/2 inch (1 to 13 millimeters) in diameter and thus take up much less space than coaxial cables.

During the late 1840's, numerous attempts were made to lay telegraph cables along the bottom of various bodies of water. Many of these attempts failed because the insulation on the cables was not able to shield the conductors from the water. In 1851, two English brothers, Jacob and John Brett, succeeded in laying a telegraph cable across the English Channel. In making their cable, they used a tough insulation consisting of a fiber called hemp and a rubberlike substance called gutta-percha. They also used iron to strengthen their cable.

The Atlantic telegraph cable. In 1854, Cyrus W. Field, an American businessman, organized the New York, Newfoundland, and London Telegraph Company and began plans for laying a transatlantic cable. In 1856, he re-formed the company and renamed it the Atlantic Telegraph Company. Field decided to lay his cable between Newfoundland and Ireland, because the ocean floor along this route is fairly level.

Field took over 12 years to complete his project. His first two cables, laid in 1857 and 1858, broke. In August 1858, two ships successfully laid a third cable for Field. This cable carried the first transatlantic telegraph message. But it failed after only four weeks of service.

In 1865, Field made his fourth attempt to lay a cable across the Atlantic. This cable also broke when the project was almost completed. Success finally came in 1866, when the British steamship the Great Eastern laid a cable that extended from Valentia, Ireland, to Heart's Content, Nfld. Also in 1866, crewmen retrieved and repaired the cable that had broken in 1865.

Much of the success of the submarine cable laid by the Great Eastern was due to the work of the British physicist William Thomson (later Lord Kelvin). He had developed a theory on how cables should work. He had also invented a device called a mirror galvanometer that enabled the transatlantic cable to transmit signals rapidly and continuously. The Atlantic cables marked the beginning of quick communication across the sea. By 1900, 15 transatlantic telegraph cables had been laid.

The spread of telephone cables. The American scientist Alexander Graham Bell patented the telephone in 1876. By the late 1800's, thousands of telephones had come into use. Early telephone and telegraph lines consisted of single insulated wires. As a result, many cities in the United States and Canada became cluttered with cables strung on wooden poles. In the late 1880's, engineers found that by twisting wires together they could produce a stronger cable and help limit the clutter. In 1902, the first underground telephone cable was installed, between New York City and Newark, N.J.

In 1907, the American inventor Lee De Forest patented a new kind of vacuum tube. His tube, also called an audion, could strengthen weak electric signals. In 1912, this tube was adapted as an amplifier for long-distance telephone calls. In 1931, two American engineers, Lloyd Espenschied and Herman A. Affel, patented the coaxial cable. First used commercially in 1941, coaxial cables could carry many more telephone conversations than the twisted cables then in use. In the 1950's, telephone companies began using transistors in repeaters.

The first transatlantic telephone cable was laid in 1956. This cable, which is still in operation, extends from Clarenville, Nfld., to Oban, Scotland. Since the 1950's, many cables have been laid along the ocean floor. Most carry telegraph signals as well as telephone calls.

Recent developments. In the late 1970's, telephone companies began replacing coaxial cables with fiber-optic cables. The first long-distance fiber-optic cable was completed in 1983. This cable carries telephone conversations between New York City and Washington, D.C. Fiber-optic cables began carrying messages between the United States and Europe in 1988 and between the United States and Japan in 1989.

Cable television, which began in the 1950's, became popular during the early 1980's. Many people began subscribing to cable television because it offers a wide range of special programs.

Contributor: Richard W. Moss, M.S.E.E., Principal Research Engineer, Georgia Tech Research Institute.

Related articles include:

Fiber optics; Telephone.

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