English: Alexanderson 200-kw alternator radio transmitter installed at the US Navy's New Brunswick, NJ station, 1920. The Alexanderson alternator was an early type of radio transmitter used between 1906 and the mid 1920s, mostly at powerful governmental and Naval radio stations. It consisted of a specialized electric generator spun at extremely high speed to generate radio waves, and was one of the first continuous wave radio technologies to transmit sound (AM). In this device the rotor had 600 poles, rotated at 2170 RPM and the output frequency was 22.1 kHz. The stator was water-cooled. Alexanderson alternators were very expensive, inefficient, and finicky to adjust, and were replaced by vacuum tubes in the 1920s.