More important for the future, the coherer could not detect AM (radio) transmissions. As a simple switch that registered the presence or absence of radio waves, the coherer could detect the on-off keying of wireless telegraphy transmitters, but it could not rectify nor demodulate the waveforms of AM radiotelephone signals, which began to be experimented with in the first years of the 20th century. This problem was solved by the rectification capability of the hot wire barretter and electrolytic detector, developed by Reginald Fessenden around 1902. These were replaced by the crystal detector around 1907, and then around 1912-1918 by vacuum tube technologies such as John Ambrose Fleming's thermionic diode and Lee De Forest's Audion (triode) tube.