A telephone is a device used for communication over long distances. It converts sound, especially the human voice, into electronic signals that are transmitted over long distances via cables or other transmission media. Since its invention, the telephone has been greatly improved and many variants of the original prototype have been developed to serve the various needs of people. Today, telephones are indispensable to business, government, offices, and households.

The telephone was first patented in 1876 by Alexander Graham Bell, though its invention was the culmination of work done by many individuals, simultaneously. Bell wanted to improve the telegraph, a wire-based electrical system that had been an established means of communication. The telegraph was basically limited to receiving and sending one message at a time. Bell designed a “harmonic telegraph”, which was based on the principle that several notes could be sent simultaneously along the same wire if the notes or signals differed in pitch. All that he needed was a working transmitter with a membrane capable of varying electronic currents and a receiver that would reproduce these variations in audible frequencies. While experimenting with his harmonic telegraph, Bell discovered he could hear the sound over a wire. The sound was that of a twanging clock spring. Thus the idea of the telephone was realized. Bell’s first design has a single-port that required the user to alternately speak into and then listen through the same hole.

The handset of a telephone consists of a microphone (transmitter) to speak into and an earphone (receiver)which reproduces the voice of the distant person. The dial may be located either on the handset or on a base unit to which the handset is connected by a cord containing wires. The base unit also contains an alerting device, called the ringer, and a switchhook, which is a spring-loaded button that gets pressed, when the handset is put on the base unit. The switchhook actually breaks the electrical circuit thus terminating the call. There is also a duplex coil that blocks the caller’s voice from reaching his ear.

A telephone’s mouthpiece that contains a transmitter has a thin, round metal disk called a diaphragm. When a person talks into the telephone, the sound waves strike the diaphragm and make it vibrate. The diaphragm vibrates at various speeds, depending on the variations in air pressure caused by the varying tones of the speaker’s voice. Behind the diaphragm lies a small coup filled with tiny grains of carbon, which contain a low voltage electric current. Louder sounds create stronger vibrations that squeeze the carbon grains very tightly, as against weaker vibrations of quieter sounds that squeeze the carbon grains more loosely. As the vibrating diaphragm presses against these carbon grains, the current copies the pattern of the sound waves and travels over a telephone wire to the receiver of another telephone. The receiver also has a diaphragm with two magnets – a permanent magnet that constantly holds the diaphragm close to it and an electromagnet. The electric current passing through the electromagnet becomes stronger or weaker according to the speaker’s speech pattern. As the diaphragm moves in and out, it pulls and pushes the air in front of it. The pressure on the air sets up sound waves that are the same as the ones sent into the transmitter. The sound waves strike the ear of the listener and he hears the words of the speaker.

Telephones are made in a variety of forms. A landline telephone is connected by a pair of wires to the telephone network, while a mobile phone communicates through radio transmissions. A cordless telephone has a portable handset that communicates by radio transmission with the handset base station which is connected to orbiting satellites, instead of terrestrial cell sites.

The telephone network consists of a worldwide net of telephone lines, fiber optic cables, microwave transmission, cellular networks, communications satellites, and undersea telephone cables connected by switching centers.