In order to overcome weakening of the signal by distance and the curvature of the earth, these microwave relay antennas are placed at line-of-sight intervals about 48 km (about 30 mi) apart. This microwave transmission service was established in the U.S. by the Western Union Telegraph Company. For intercontinental communication, artificial geosynchronous satellites are used as relay antennas for voice, data, graphic, and video signals between ground-based stations (see Communications Satellite; Space Exploration).


I. Introduction

Telephone, instrument that sends and receives voice messages and data. Telephones convert speech and data to electrical energy, which is sent great distances. All telephones are linked by complex switching systems called central offices or exchanges, which establish the pathway for information to travel. Telephones are used for casual conversations, to conduct business, and to summon help in an emergency (as in the 911 service in the United States). The telephone has other uses that do not involve one person talking to another, including paying bills (the caller uses the telephone to communicate with a bank's distant computer) and retrieving messages from an answering machine. In 1998 there were 661 main telephone lines per 1,000 people in the United States and 634 main telephone lines per 1,000 people in Canada.

About half of the information passing through telephone lines occurs entirely between special-purpose telephones, such as computers with modems. A modem converts the digital bits of a computer's output to an audio tone, which is then converted to an electrical signal and passed over telephone lines to be decoded by a modem attached to a computer at the receiving end. Another special-purpose telephone is a facsimile machine, or fax machine, which produces a duplicate of a document at a distant point.

II. Parts of a Telephone

A basic telephone set contains a transmitter that transfers the caller's voice; a receiver that amplifies sound from an incoming call; a rotary or push-button dial; a ringer or alerter; and a small assembly of electrical parts, called the antisidetone network, that keeps the caller's voice from sounding too loud through the receiver. If it is a two-piece telephone set, the transmitter and receiver are mounted in the handset, the ringer is typically in the base, and the dial may be in either the base or handset. The handset cord connects the base to the handset, and the line cord connects the telephone to the telephone line.

More sophisticated telephones may vary from this pattern.