Telephone

In its most basic form, a telephone enables people to talk with one another at distances beyond the range of the human voice. More sophisticated telephones can send and receive not only voice messages, but also written words, drawings, photographs, and even video images. In addition, telephones can send information from one computer to another.

Telephones in people's homes are connected through a vast, complex telephone network. The network includes large computers, tremendous lengths of copper wire and hair-thin strands of glass, cables buried in the ground and laid along the bottom of the oceans, radio transmitters and receivers, and artificial satellites orbiting far above the earth.

Most telephones connect with the telephone network by means of wires that run through the walls of houses and other buildings. A small clip connects each telephone to the wiring. Other phones are installed in cars or carried in a bag or pocket. Such phones connect with the network by radio.

Alexander Graham Bell patented the telephone in 1876. Today, hundreds of millions of telephones serve people all over the world.

A telephone handset includes a transmitter or microphone, which produces an electric "copy" of the user's voice, and a receiver, which duplicates the voice of the person on the other end of the line. In a carbon transmitter, the sound wave of the user's voice makes a diaphragm vibrate. The vibrations cause a metal dome to exert pressure on carbon grains. Electric current flows through the grains with a strength related to this pressure, "copying" the voice. In a foil-electret microphone, the sound waves cause a sheet of metal-coated plastic to vibrate next to a metal disk. This leads to variations in the strength of a current in the microphone, "copying" the voice. In a receiver, the electric "copy" of the caller's voice flows through a wire, producing variations in the magnetic field of an electromagnet. As a result, a permanent magnet vibrates a diaphragm, generating sounds like those of the voice. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagrams by J. Harlan Hunt, Koralik Associates.

A telephone has three main parts: (1) a dialing mechanism, (2) a transmitter, and (3) a receiver.

The dialing mechanism enables a caller to enter telephone numbers. The dialing mechanism may be built into the handset, between the earpiece and the mouthpiece. Or it may be part of a separate base unit that connects by cord to the handset.

In most telephones manufactured since the 1960's, the dialing mechanism consists of a set of buttons or keys called a keypad. A standard keypad has 12 keys--the digits 0 through 9, a "*" key, and a "#" key. When pressed, each key generates either a certain number of electric pulses or a pair of accurately controlled tones. Computers in the telephone network use the sequence of pulses or tones to direct the call.

The dialing mechanism on some telephones consists of a disk called a rotary dial. The dial has finger holes that correspond to the digits 0 through 9. A caller enters a telephone number by rotating and releasing the dial. This action generates electric pulses.

The transmitter, also called the microphone, converts the sound waves of a person's voice into an electric current and sends this current farther into the telephone network. The transmitter is built into the handset, behind the mouthpiece. There are two main kinds of telephone transmitters--(1) the carbon transmitter and (2) the foil-electret condenser microphone.

The carbon transmitter has two main parts--a diaphragm and a carbon chamber. The diaphragm is a thin, round piece of aluminum. The carbon chamber lies behind the diaphragm, between two electric terminals. It contains many tiny grains of carbon. Low-voltage electric current travels through the carbon grains. A small, gold-plated brass dome located on the underside of the diaphragm also rests in the carbon chamber.

When a person speaks into the mouthpiece, the sound waves of the person's voice cause the diaphragm to vibrate. A loud sound creates a strong vibration, and a soft sound creates a weak vibration. The motions of the diaphragm cause the dome to vibrate within the carbon chamber. Each vibration of the dome compresses the carbon grains. A small vibration places light pressure on the grains, while a large vibration squeezes them tightly together. The greater the compression of the grains, the more easily electric current passes through them. So the louder the sound, the greater the flow of electric current. The variations in electric current are therefore an electric "copy" of the speaker's voice.

The foil-electret condenser microphone has a circular diaphragm consisting of a thin sheet of electrically charged plastic with a metal coating on one side. This diaphragm, called a foil electret, is stretched over a hollow metal disk called a backplate. The metal coating on the diaphragm faces away from the backplate.

The diaphragm touches the backplate only in certain areas. In other areas, the diaphragm and the backplate are separated by air pockets. The diaphragm can vibrate in these pockets when hit by sound waves.

The electric charge carried by the diaphragm sets up an electric field between the diaphragm and the backplate. The strength of this electric field depends in part on the distance between the diaphragm and the backplate.

When a person speaks into the mouthpiece, sound waves hit the diaphragm. The resulting vibrations change the distance between the diaphragm and the backplate--and thus the strength of the electric field. These variations in field strength trigger corresponding variations in an electric current. The current variations are an electric "copy" of the speaker's voice.

The receiver converts the electric current coming through the telephone line into a duplicate of the speaker's voice. The receiver is built into the handset, behind the earpiece.

In a receiver, an iron diaphragm in a flexible frame is surrounded by a ring-shaped permanent magnet. The permanent magnet exerts a constant pull on the diaphragm. Another magnet, called an electromagnet, is attached to the other side of the diaphragm. This magnet is made of metal with a coil of wire around it.

When the electric current flows through the coil, the electromagnet becomes magnetized. The current moves in two directions. When it moves in one direction, the resulting magnetism adds to the pull of the permanent magnet and causes it to attract the diaphragm more strongly. When the current moves in the other direction, the resulting magnetism opposes that of the permanent magnet and reduces its pull on the diaphragm. The variation in magnetic pull causes the diaphragm to vibrate at the same rate. It pulls and pushes the air in front of it, producing pressure changes that create sound waves almost the same as those of the speaker's voice. The sound waves strike the listener's ear, and the listener hears a duplicate of the speaker's voice.

An intercontinental call from the United States to the United Kingdom travels through the sky or under the ocean. Telephone signals pass from the caller's home phone to one or more local telephone offices called local exchange carriers, which pass the signals to a long-distance carrier. This office directs the call in one of two ways. In one method, the signals go to a ground station which beams them to a communications satellite. The satellite relays the signals to a British ground station. In the other transmission method, the long-distance carrier routes the signals to a fiber-optic cable that runs beneath the Atlantic Ocean. With both methods, the signals reach a British long-distance carrier. The call then passes through one or more local exchange carriers and finally reaches its intended destination. From The World Book™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved. World Book diagram by Rolin Graphics.

Routing the call. At one time, telephone operators switched (routed) telephone calls manually by plugging electric cords into a switchboard. Today, electronic switches--actually computers--route almost all calls.

After a telephone number has been dialed, pairs of copper wires carry the resulting electric signals to a central office, a facility of the local telephone company, or local exchange carrier. A switch sends the call on its way, using the telephone number as routing instructions.

If the call is a local call, the switch routes the signals to another switch owned by the same company. If the call is a long-distance call, the signals travel into the network of a long-distance company, or interexchange carrier. They may pass through several more switches in this network, but eventually they reach the central office of the local exchange carrier that serves the telephone being called. A switch in that office then routes the signals to this telephone. All the routing and switching occurs in seconds.

Transmitting the telephone signals. Once a connection has been established, the signals that make up the message must be carried between the two telephones. Most telephone calls today are sent through a system of digital switches. A digital switch works like a computer. It takes the continuous electric current that corresponds to a telephone message and transforms it into a series of electrical impulses. These impulses represent the digits "0" and "1"--the same digits used by computers. The sequence of "0's" and "1's," which now corresponds to the telephone message, moves through the telephone network. A digital switching system duplicates the sequence of digits as they move through each switch. This duplication maintains the strength and accuracy of the impulses over long distances.

An older type of switching system uses analog switches. An analog switch directs the current signal through the network without translating it into "0's" and "1's." Such a signal tends to fade over distance, so it requires continuous amplification (strengthening). This process frequently generates other sounds that interfere with the signal. Analog switches are rapidly being replaced by digital switches.

Switches, whether digital or analog, may be connected by cables consisting of many twisted pairs of copper wires or bundles of optical fibers. In fiber-optic cables, the hair-thin fibers carry telephone signals on a laser beam. A laser at one end of a fiber transforms electric signals corresponding to a telephone message into a sequence of flashes of light representing "0's" and "1's." At the other end of the fiber, a special device transforms the emerging flashes back into electric signals.

Fiber-optic cables can carry tremendous amounts of information. A strand of optical fiber can carry thousands of times as much information as a pair of copper wires. Both wire cables and fiber-optic cables may be buried in the ground or laid on the ocean floor.

Telephone messages are also transmitted through the air on short radio waves called microwaves. The telephone network uses a series of relay stations to direct microwave signals over vast stretches of land. A dish antenna at a relay station receives a signal, and another dish antenna retransmits the signal to the next station. To direct microwave signals across oceans, the telephone network uses artificial satellites--relay stations in the sky.

Cordless telephones have no cord connecting the handset to the base. Instead, radio waves carry sound messages between the handset and the base. A person can take the handset of a cordless telephone from room to room or even outdoors--within a limited range--while talking on the telephone. The base is connected to the telephone network like a regular phone.

In a cordless telephone, both the handset and the base can transmit and receive radio signals. The handset creates radio signals corresponding to spoken messages and broadcasts the signals through the air. The base picks up the radio signals, translates them into electric signals, and transmits the electric signals through wires to the network. Similarly, the base translates incoming electric signals into radio signals and transmits them to the handset.

Wireless telephone equipment communicates with the regular telephone network via radio waves rather than wires. Such wireless equipment includes cellular telephones, telephones in certain remote areas, and pagers.

Cellular telephones are mobile devices that transmit and receive radio signals. A cellular telephone communicates with an antenna transmitter that serves a small geographical area called a cell. The antenna transmitter in each cell communicates with the regular telephone network through a mobile telephone switching office.

Some cellular phones are mounted in cars. Others are carried in a bag or even in a pocket. When a person in a moving car uses a cellular phone, the antenna transmitter in the nearest cell handles the radio signals. As the car moves, the mobile telephone network passes the signals to the next cell without interrupting the call.

Telephones in remote areas. In some sparsely populated regions, people live so far apart that it would be too expensive to use wires or optical fibers to connect their homes with the telephone network. In these areas, Basic Exchange Telephone Radio Service (BETRS) bridges the gap. Each house has a transmitter that sends radio signals to the regular telephone network, in many cases by way of other transmitting stations.

Pagers. A pager, or beeper, is a small radio receiver that responds with a tone or vibration when activated by a signal. In most cases, the pager has its own telephone number. When this number is dialed from a telephone, the resulting signals travel to a special network. A transmitter in this network sends a radio signal to the pager. Many pagers display the phone number of the caller. The person being paged can then return the call using a nearby telephone.

Computer modems. Vast amounts of computer data travel through the telephone network. Many people use personal computers or computer terminals to contact other computer users or to obtain information. Using a computer to communicate over telephone lines requires a device called a modem and special communications software. A modem translates outgoing computer signals into signals the telephone network can use, and it translates incoming telephone signals into computer signals. Special modems can connect portable computers with the cellular network.

Once equipped, a computer can use the telephone network to contact computerized information services called on-line services. Some on-line services offer updated news bulletins, shopping services, encyclopedias, or games. Others feature back issues of newspapers, magazines, or other research materials. A subscriber to a bulletin board system (BBS) electronically "posts" messages and announcements, making them available to all other subscribers. Another on-line service is electronic mail (e-mail), through which individual users communicate with each other.

Facsimile machines, or fax machines, send written words and pictures over telephone lines. A fax machine resembles a copying machine but is equipped with a telephone or connected to one. To send a document, the user inserts it into the machine and dials the telephone number of the fax machine that is to produce a copy of the document. After the connection has been made, an electronic scanner on the transmitting machine generates electric signals that correspond to the image on the page. The telephone network uses these signals to instruct the receiving machine to print a copy of the document. Portable fax machines connect with cellular telephone networks.

Videophones. Ordinary telephone lines can transmit slow-motion video signals to a videophone, a telephone combined with a television camera and screen. Users can see pictures of each other during a call.

Information superhighway. The increased use of the telephone network for nonaudio applications has led to the idea of developing a vast, high-speed communications network called the information superhighway. This network would be formed by combining telephone systems with cable television systems and computer networks. The "highway" would be interactive, enabling users in homes, businesses, and institutions to transmit and receive video, voice communications, and computer data. See Internet.

Operator services use human operators or computers that imitate the human voice to provide various kinds of assistance. For example, a person who needs help to complete a call can dial "0" to reach an operator. A service called directory assistance provides callers with the telephone numbers of parties they wish to call.

Business services. Most businesses have more than one telephone. A business may set up its telephone system in either of two ways. In one arrangement, which is provided by the local telephone company, each telephone has its own telephone number. One name for this service is Centrex service. In the other arrangement, several lines owned by the telephone company are connected to a switching system owned by the business. In this system, known as a private branch exchange (PBX), the business usually has one telephone number, and each phone has a separate extension number.

Private line service, or dedicated service, is designed for businesses that have branch offices with which they communicate frequently. The phone company sets up a line or group of lines that run only between those offices. Private line service eliminates the need to send each of these calls over the public telephone network.

Private lines may be equipped to handle large amounts of computer data or transmissions by fax machines. Television stations can use private lines to transmit their coverage of an event, such as a football game, to a phone company switch.

In many businesses, a system called voice mail answers incoming calls. A voice recording stored on a computer chip gives callers instructions to help them reach the desired party. If that party is not available, the caller can leave a recorded message.

Another common business service is 800 service. In this service, the first three digits of a subscriber's telephone number--which would normally be the area code--are "800." The subscriber pays the charges for all calls received by that number, which is also known as a toll-free number. Many companies that sell goods or services by mail or by telephone use 800 service.

Residential services. Many home telephone users have access to a wide variety of services. For example, the telephone company can provide voice-mail service similar to that used by businesses. Some long-distance companies offer 800 numbers to home users.

Another service provides for different-sounding rings, depending on who is calling. Call waiting enables a person to put one telephone call on hold in order to speak with another caller. Call forwarding automatically sends all calls to a designated telephone number. Call blocking automatically rejects calls from telephones whose numbers are designated by the subscriber. A service called automatic caller identification, or caller I.D., enables a person with a special device attached to the telephone to see the caller's telephone number or name before answering.

Other services. Many hotels, restaurants, and other public places have pay telephones. These phones require callers to pay for calls with coins, credit cards, or special cards issued by local or long-distance telephone companies. Owners of pay telephones include local telephone companies and firms that provide long-distance service or operator services. Other telephone services supply information or advice, or enable callers to enter contests. The most popular of these is called 900 service because the customer dials "900" instead of an area code. Calling a 900 number can be very expensive. Providers of 900 services are required by law to give callers the price before providing a service.

Seven firms, called regional holding companies (RHC's), own telephone companies in different parts of the country. These companies provide about 75 percent of all local telephone service. The RHC's and the places where they offer local service are: (1) Ameritech--Illinois, Indiana, Michigan, Ohio, and Wisconsin; (2) Bell Atlantic--Delaware, the District of Columbia, Maryland, New Jersey, Pennsylvania, Virginia, and West Virginia; (3) BellSouth--Alabama, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee; (4) Nynex--New York and New England; (5) Pacific Telesis Group--California and Nevada; (6) SBC Communications Inc.--Arkansas, Kansas, Missouri, Oklahoma, and Texas; and (7) U S West--the remaining states, except Alaska and Hawaii. In 1996, Pacific Telesis and SBC announced plans to merge. That year, Bell Atlantic and Nynex also announced merger plans.

Another major firm that provides local service is GTE Corporation. GTE, which is not an RHC, offers local service in more than half of the states. Some of its largest operations are in California, Florida, and Hawaii.

There are about 500 long-distance companies in the United States. Some of these companies own all of their telephone lines and switches. Others--mostly the smaller companies--buy service from large firms and resell it to their customers. AT&T Corp. is the largest U.S. long-distance carrier. Other large long-distance companies include MCI and Sprint.

A third, smaller type of U.S. telephone company is the competitive access carrier. It links large customers to long-distance carriers. Competitive access carriers compete with local telephone companies for this business.

State government agencies set prices for local telephone service and other services within states. A U.S. government agency called the Federal Communications Commission (FCC) has authority over phone services that cross state lines, such as long-distance service.

In other countries. For many years, the government provided telephone service in most countries. But since the 1980's, many nations have begun selling part ownership--or, in some cases, the whole telephone company--to private citizens.

On June 2, 1875, one of the metal reeds of the harmonic telegraph stuck. Bell's assistant, Thomas A. Watson, plucked the reed to loosen it. Bell, who was in another room, heard the sound in his receiver. He realized that the vibrations of the reed had caused variations of electric current. In turn, the electric current had reproduced the same variations in the receiver he was using.

On March 10, 1876, Bell finally succeeded in speaking words over a telephone. He was about to test a new transmitter. In another room, Watson waited for the test message. Suddenly, Bell spilled some acid from a battery on his clothes. He cried out: "Mr. Watson, come here. I want you!" Watson heard every word clearly and rushed into the room.

In June 1876, Bell exhibited his telephone at the Centennial Exposition in Philadelphia. Scientists praised his work. But the public showed little interest until early in 1877, when Bell gave many demonstrations.

Early telephones. In August 1876, Bell received the first one-way long-distance call. This call came over an 8-mile (13-kilometer) telegraph line between Brantford, Ontario, and Paris, Ontario. In October 1876, Bell and Watson held the first two-way long-distance telephone conversation. They spoke between Boston and Cambridgeport, a part of Cambridge, Massachusetts, a distance of 2 miles (3 kilometers). In 1877, Charles Williams, an electrical workshop owner, installed the first line intended exclusively for telephone use. It extended 3 miles (5 kilometers) between Williams's home in Somerville, Massachusetts, and his shop in Boston.

The first telephones used no switchboards. A pair of iron wires connected each pair of phones. As more telephones came into use, each was connected to all the other phones. More than 1,000 connections were required to link only 50 telephones. Switchboards solved this problem by bringing together the wires from all telephones in an area. The first switchboard began operating in 1877 in Boston.

Improvements. Almon B. Strowger, an American inventor, patented an automatic switching system in 1891. The first commercial switchboard based on his patent opened in LaPorte, Indiana, in 1892. The caller pressed buttons to get the number, then turned a crank to ring the phone. Also in 1892, phone service began between New York City and Chicago.

In 1896, the first dial telephones went into operation in Milwaukee. Transcontinental phone service began between New York City and San Francisco in 1915. Transatlantic radiotelephone service between New York City and London began in 1927. The first long-distance coaxial cable linked New York City and Philadelphia in 1936.

Undersea telephone cables between North America and Europe began operating in 1956. A cable between the U.S. mainland and Hawaii began operating in 1957. A cable from Japan was joined to this cable in 1964, connecting Japan to the U.S. mainland. The first commercial communications satellite, Early Bird, was launched in 1965.

In 1970, International Direct Distance Dialing (IDDD) began operating between New York City and London. IDDD, which now serves many of the world's cities, enables people to dial overseas directly. In 1980, a fiber-optic system for transmitting local calls was installed in Atlanta, Georgia. A fiber-optic system between New York City and Washington, D.C., began in 1983. Fiber-optic cables began carrying messages across the Atlantic Ocean in 1988 and across the Pacific in 1989. The FCC issued the first license for commercial cellular systems in 1983. By the late 1980's, cellular service was available in most of the United States.

The Bell System. Bell, Watson, Gardiner G. Hubbard, and Thomas Sanders formed the Bell Telephone Company in 1877. Hubbard was Bell's father-in-law, and Sanders was the father of one of Bell's pupils. They had helped pay for Bell's experiments.

In 1878, the first telephone exchange opened in New Haven, Connecticut. It had 21 customers. Exchanges soon opened throughout the United States and Canada.

In 1877, the Western Union Telegraph Company entered the telephone business. Western Union used transmitters developed by Thomas A. Edison, the great American inventor. Its receivers had been developed by Elisha Gray, another American inventor. The Bell company met the competition by using the improved transmitters of Emile Berliner, a German immigrant, and Francis J. Blake, an American.

In September 1878, the Bell company sued Western Union to protect Bell's telephone patents. Western Union claimed that Gray, not Bell, had invented the telephone. Gray had notified the United States Patent Office that he was working on a device to transmit speech. However, he made this notification a few hours after Bell had applied for his first patent on Feb. 14, 1876. The United States Patent Office had issued Bell his patent on March 7, 1876. In 1879, Western Union acknowledged Bell's patents and agreed to stay out of the telephone business. The Western Union case was the first of about 600 lawsuits over Bell's patents. In 1888, the Supreme Court of the United States upheld Bell's patents.

In 1878, two companies were formed as successors to the Bell Telephone Company: the New England Telephone Company and a new Bell Telephone Company. The New England Telephone Company licensed telephone service in New England, and the Bell Telephone Company licensed phone service in the rest of the United States. The following year, the two companies were combined to form the National Bell Telephone Company, a nationwide licensing company.

In 1880, the American Bell Telephone Company was formed as a successor to the National Bell Telephone Company. In 1881, it bought Western Electric Manufacturing Company, which had made phones for Western Union, and renamed it Western Electric Company.

In 1885, the American Telephone and Telegraph Company (AT&T) was established to operate the long-distance network. AT&T took over American Bell in 1899 and became the parent company of the Bell System.

Competition in the telephone industry became an important issue in the early 1900's, when AT&T began buying small telephone companies. In 1913, the U.S. attorney general warned AT&T that some of its planned purchases of phone companies could violate antitrust laws, which are designed to protect competition. AT&T promised not to buy any more competing telephone companies and to allow all phone companies to connect to its long-distance network.

The FCC ruled in 1968 that people could buy their own telephones and other equipment. Previously, all phone equipment was leased to users by telephone companies. In 1969, the FCC cleared the way for MCI Telecommunications to offer long-distance service.

A lawsuit filed by the U.S. government in 1974 brought dramatic changes to the telephone industry. The government charged AT&T with anticompetitive practices. The case was long and complicated. In January 1982, the government and AT&T announced a settlement. In April 1982, U.S. District Judge Harold H. Greene approved the settlement with certain changes.

The approved settlement required that AT&T give up its local telephone companies on Jan. 1, 1984. On that day, the local companies were grouped into the seven RHC's. In addition, the settlement allowed AT&T to keep its equipment-manufacturing factories, its long-distance business, and its research facilities. AT&T was also allowed to enter the computer services and information-processing businesses. The RHC's, however, were forbidden to manufacture equipment, provide long-distance service, or furnish computerized information services. In 1991, a court lifted the ban against information services. In 1994, the American Telephone and Telegraph Company changed its name to AT&T Corp.

In the early 1990's, telephone companies and firms that provided other communications services moved to compete with one another. The Telecommunications Act of 1996 loosened many restrictions that had limited competition in the telephone industry. For example, it allowed local telephone companies, long-distance companies, and cable-television companies to enter each other's businesses. The act also allowed the RHC's to manufacture telecommunications equipment.

Contributor: Arthur R. Brodsky, M.S.J., Senior Editor, Communications Daily.

Related articles include:

Biographies
Bell, Alexander Graham; Berliner, Emile; Edison, Thomas Alva.

Other related articles
Cable; Electromagnet; Fiber Optics; Radio.

How do fiber-optic cables carry telephone signals?

How does a digital switch work?

What happens when a person uses a cellular telephone?

What is caller identification?

When a person speaks into the mouthpiece of a telephone, what happens to a foil-electret condenser microphone?

What were the first words spoken over a telephone?

What is voice mail?

When was the first local fiber-optic system installed?

How does a cordless telephone work?

Skurzynski, Gloria. Get the Message: Telecommunications in Your High-Tech World. Bradbury, 1993. Younger readers.

Webb, Marcus. Telephones. Lucent Bks., 1992.

Williams, Gene B. All Thumbs Guide to Telephones and Answering Machines. TAB, 1994.

Master Index