Telephone

A telephone is an apparatus for transmitting to and receiving sounds or speech from a distant point over wires usually by means of electricity. Telephones are now widely used; this system of communication was invented by Alexander Graham Bell, who patented it in 1876 and 1877. Modern telephone transmitters, which are made up essentially of a carbon microphone, contain loosely packed carbon grains to be compressed and released. When someone speaks into the telephone, the diaphragm vibrates and causes a compression of the carbon grains and their release. This motion varies the current flow in the associated electric circuit. The current, when transmitted to a distant identical instrument, causes the diaphragm in it to vibrate in response to the fluctuations induced by the nearby magnetic field. Telephone lines used include ordinary open-wire lines; lead-sheathed cables consisting of many lines; coaxial cables; and, most recently, fibre optic cables. Coaxial cables and fibre-optic lines are placed underground, but other cables may be either overhead or underground. Long-distance transmission of telephone messages is often accomplished by means of radio and microwave transmissions. In some cases microwaves are sent to an orbiting communications satellite, from which they are relayed back to a distant point on the earth. Sophisticated services, including automatic switching systems, automatic dialing, call forwarding, and conference calling, have been developed in recent years. Telephone systems are increasingly important to the transmission of messages and data between digital devices, such as computers. To improve accuracy and speed, analog telephone systems (in which the shape of the electrical signal is analogous to the shape of the sound wave) are being replaced by digital systems. A digital system rapidly samples the sound waves and encodes them as binary pulses that can be decoded by the receiver. Such a system can carry more messages over the same link at the same time. In addition, computers can communicate over telephone circuits without modems, greatly enhancing the efficiency of networks.  Another device that works almost on the same principle is the cellular phone and the portable phone.

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