Schematic diagram of a radio receiver from the first decade of the 1900s using a "tikker", to receive radiotelegraph transmissions. Around 1905, continuous wave (CW) radio transmitters began replacing the previous spark-gap transmitters which generated damped waves. The "tikker" or "ticker" invented by Valdamar Poulsen in 1902 was one of the first devices that could make CW Morse code transmissions audible. It consisted of an interrupter such as a motor driven contact wheel which had metal contacts spaced around its rim, which periodically interrupted the carrier wave from the tuned circuit, producing a tone in the earphones. The tikker was only used for a short time, until Fessenden's heterodyne receiver replaced it around 1913.