A tikker was a device that allowed early crystal radio circuits to detect continuous-wave telegraphy

Telegraphy

Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...

. Ordinary crystal detectors cannot demodulate continuous waves; the direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 (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

Valdemar Poulsen

Valdemar Poulsen was a Danish engineer who developed a magnetic wire recorder in 1899.-Biography:He was born on 23 November 1869 in Copenhagen...

 and Peder Oluf Pedersen

Peder Oluf Pedersen

Peder Oluf Pedersen was a Danish engineer and physicist. He is notable for his work on electrotechnology and his cooperation with Valdemar Poulsen on the developmental work on the telegraphone and the Poulsen Arc Transmitter....

. Poulsen also experimented with using the tikker to send speech with spark gap transmitters such as the arc converter

Arc converter

The arc converter, sometimes called the arc transmitter or Poulsen arc after its inventor Danish engineer Valdemar Poulsen, is a device that used an electric arc to convert direct current electricity into radio frequency alternating current...

. Poulsen is perhaps best known for developments in magnetic recording.