A tikker was a device that allowed early crystal radio circuits to detect continuous-wave telegraphy. Ordinary crystal detectors cannot demodulate continuous waves; the direct current (DC) signal is inaudible in headphones. One form of the tikker consisted of a fine wire resting on a rotating drum, which interrupted the DC voltage to produce an audible tone.
The tikker was invented in 1908 by Valdemar Poulsen and Peder Oluf Pedersen. Poulsen also experimented with using the tikker to send speech with spark gap transmitters such as the arc converter.  Poulsen is perhaps best known for developments in magnetic recording.
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