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Thomas Edison: Telephone, Electricity and Phonograph, 1915

Telephone Transmitter

The telephone, invented by Alexander Graham Bell in 1876, converted the sound waves from the human voice to electric impulses, conducted the impulses through a wire, and converted them back to the human sound at the other end of the wire. The originating transmitter contained a parchment membrane that vibrated in response to sound. A metal button attached to the membrane sent the varied movements to an electromagnet and electric current corresponding to the vibrations was induced. This induced current traveled to the receiving device and where the process was reversed, the electricity caused movement of a magnet which then caused a membrane to vibrate and emit the corresponding sounds.

Thomas Edison worked to improve a drawback in Bell's invention: the weakness of the electric signal limited the quality and distance of the message. His approach was to improve the sensitivity of sound detection at the transmitter by replacing the parchment membrane with a disc of compressed carbon set between metal plates. The electrical resistance of carbon is extremely sensitive to the minute pressure changes caused by sound waves. Edison's solution—improved later by substitution of granulated carbon and then roasting of the granules—became a basic component of telephones for almost a hundred years.