An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency alternating current for use as a radio transmitter. It was one of the first devices capable of generating the continuous radio waves needed for transmission of amplitude modulation (sound) by radio. It was used from about 1910 in a few "superpower" longwave radiotelegraphy stations to transmit transoceanic message traffic by Morse code to similar stations all over the world. Although obsolete by the early 1920s due to the development of vacuum-tube transmitters, the Alexanderson alternator continued to be used until World War 2. It is on the list of IEEE Milestones as a key achievement in electrical engineering.