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In 1876 the telephone was invented by Scottish-born speech therapist and inventor Alexander Graham Bell (1847-1922). His device used a thin diaphragm to convert the vibrations of the human voice into electrical signals, then reconverted them into sound waves. Bell experimented on this system while developing his father's work on a method of teaching deaf people to speak, in which symbols represented the position of the lips and tongue. The first intelligible words transmitted by telephone were: 'Mr Watson, come here, I want to see you!' spoken by Bell to his assistant, Thomas Watson.

The electrical signals in a modern phone travel to the earpiece of the person being called via copper cables, optical cables, communications satellites or Cellphone radio links. In the earpiece, the signals pass through the coils of a small electromagnet - causing its magnetism to fluctuate. This makes a diaphragm vibrate and sets up sound waves that are replicas of those that entered the microphone in the mouthpiece. The mouthpiece and earpiece have reverse functions and in modern telephones both are incorporated in a single handset.

The first public telephone exchange opened in Pittsburgh, USA, in 1877, and all calls were connected by someone at a switchboard. However, few private houses had a telephone until the 1920s. The modern telephone network also carries fax messages, computer data and electronic mail, and, on videophones, images.

Bell's other developments included the photophone, which transmitted sounds on a beam of light, and a hydrofoil which in 1919 attained a speed of 70 mph.

The decibel, used for measuring the loudness and intensity of sound, is named after Alexander Graham Bell.

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