Telephone

Telephone, apparatus for transmission and reproduction of sound by means of frequency electric waves. The telephone was invented in 1876 by Alexander Graham Bell. Bell's transmitter worked by the voltage induced in a coil by a piece of iron attached to a vibrating diaphragm; the same apparatus, working in reverse, was used as a receiver. The carbon microphone (invented by Thomas Edison, 1878) provided a more sensitive transmitter. In 1878 the first commercial exchange was opened in New Haven, Conn.; local telephone networks spread rapidly in the United States and elsewhere. Repeaters, or amplifiers, made long-distance telephone calls possible. Today, microwave radio links, communications satellites, and optical fibers are used. Telephone subscribers are connected to a local exchange, these in turn being linked by trunk lines connecting a hierarchy of switching centers so that alternative routes may be used. When a call is dialed, each digit is coded as pulses or pairs of tones that work electromechanical or electronic switches.

See also: Bell, Alexander Graham.