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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.

Inventor: Thomas A. EdisonA 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 listenerís end.

Blake transmitterThis 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.


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