The telephone essentially converted the human voice into an electrical signal; in turn-of-the-century phones this was done by allowing sound waves produced by a voice to vibrate a taut diaphragm—usually a disc made of thin aluminum—that was backed by another thin metal disc. A mild electric current ran between the two discs, which were separated by a chamber filled with the tiny carbon granules Edison had invented. As sound waves from a voice vibrated the top diaphragm, waves of varying pressure moved through the granules below it. The varying pressure would in turn vary the resistance to the
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