telephone

In the telephone, sound vibrations are converted to an electric signal and back again. The mouthpiece contains a carbon microphone that produces an electrical signal which varies in step with the spoken sounds. The signal is routed to the receiver via local or national exchanges. The earpiece contains an electromagnetic loudspeaker which reproduces the sounds by vibrating a diaphragm.

Telephone operators connecting calls, 1940s. Before automatic exchanges were invented, all calls went through a manual exchange system. A light on a switchboard alerted an operator that someone wished to place a call, and the operator plugged an electrical cable into a jack corresponding to the caller requesting service, which allowed the operator and caller to converse. The operator then plugged an adjacent cable into the called-party's jack, and operated an electrical switch to connect the caller to the called-party's telephone.

Instrument for communicating at a distance by voice, developed by Scottish-US inventor Alexander Graham Bell in 1876, consisting of an earpiece that receives electrical signals and a mouthpiece that sends electrical signals. The transmitter (mouthpiece) consists of a carbon microphone, with a diaphragm that is vibrated by sound waves when a person speaks into it. The diaphragm vibrations compress grains of carbon to a greater or lesser extent, altering their resistance to an electric current passing through them. This sets up variable electrical signals, which travel along the telephone lines to the receiver of the person being called. The earpiece contains an electromagnet attached to a diaphragm. As the incoming electrical signal varies, the strength of the electromagnet also varies, resulting in a variable movement of an armature of the electromagnet. These movements cause the diaphragm to vibrate, producing the pattern of sound waves that originally entered the mouthpiece.

The standard instrument has a handset, which houses the transmitter (mouthpiece), and receiver (earpiece), resting on a base, which has a push-button mechanism for dialling a telephone number. Cordless telephones combine a push-button mechanism, a mouthpiece, and an earpiece in one unit, and are connected to the base unit not by wires but by radio. They can be used at distances of up to 100 m/330 ft from the base unit.

In the 1990s, digital networks provided wireless mobility with the mobile phone, and enabled the adaptation of personal computers for telephone use with the Internet phone.