Acoustic Telegraph
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The Acoustic Telegraph was a method for multiplexing (transmitting more than one) signal on a single telegraph wire. It used signals at different acoustic frequencies. A telegrapher used a conventional Morse key to tap out the message, the key pulses being sent as pulses of a specific frequency. At the receiving end a device tuned to one frequency resonated to the pulses but not to others on the same wire.
Inventors who worked on the acoustic telegraph included Thomas Edison, Elisha Gray and Alexander Graham Bell.
