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Edison's Transmitter
Early transmitters, such as the one developed by Alexander Graham
Bell, used magnetic induction to create an electrical current of
varying strength when voice vibrations struck a thin metal disk. The
variations in current produced a variation in the strength of an
electromagnet in the receiver. The varying magnetic pull on a
flexible metal disk in the receiver produced sound vibrations.
 A better signal was generated by adapting the mechanism invented
in 1878 by Thomas Edison in the carbon-button microphone. When the
caller spoke into the transmitter, the sound waves created pressure
on a brass button centred over a block of carbon. The carbon was
gathered from kerosene lamp black or scraped from chimneys and
pressed into compacts in the shape of a button. The button carried
electrical current which passed through the carbon. Carbon is a
semiconductor and carries more current when it is under pressure.
The changing pressure of the button caused changes in the current
passing though the carbon. A variable current corresponding to the
pressure of the sound waves was produced. The pattern of variable
current reproduced the sound in the telephone receiver at the
listener’s end.
 This principle was copied in other transmitters, such as the one
developed by the American scientist and photographer Francis Blake
in the late 1870s. Later versions used carbon granules to create the
variation in electrical current.
Copyright © 2004
Heritage Community Foundation and
Telephone Historical Centre All Rights Reserved
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 on display at the Telephone Historical Centre. Edison developed the carbon button transmitter which was more effective than Bell's liquid transmitter. Edison also developed the loudspeaking tlephone receiver and patented over 10,000 inventions during his lifetime.',3))
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