Alexanderson alternator
electromechanical generator of high-frequency, high-power current for radio transmitters
An Alexanderson alternator is a rotating machine, developed by Ernst Alexanderson beginning 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 modulated (AM) signals 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Although superseded in the early 1920s by the development of vacuum-tube transmitters, the Alexanderson alternator continued to be used until World War II. It is on the list of IEEE Milestones as a key achievement in electrical engineering.
History
Prior developments
After radio waves were discovered in 1887, the first generation of radio transmitters, the spark gap transmitters, produced strings of damped waves, pulses of radio waves which died out to zero quickly. By the 1890s it was realized that damped waves had disadvantages: their energy was spread over a broad frequency band so transmitters on different frequencies interfered with each other, and they could not be modulated with an audio signal to transmit sound. Efforts were made to invent transmitters that would produce continuous waves – a sinusoidal alternating current at a single frequency.
In an 1891 lecture, Frederick Thomas Trouton pointed out that if an electrical alternator were run at a great enough cycle speed (that is, if it turned fast enough and was built with a large enough number of magnetic poles on its armature), it would generate continuous waves at radio frequency. Starting with Elihu Thomson in 1889, a series of researchers built high-frequency alternators: Nikola Tesla (1891, 15 kHz), Salomons and Pyke (1891, 9 kHz), Parsons and Ewing (1892, 14 kHz), Siemens (5 kHz), B. G. Lamme (1902, 10 kHz), but none reached the frequencies required for radio transmission, above 20 kHz.
Construction
In 1904, Reginald Fessenden contracted with General Electric for an alternator that generated a frequency of 100,000 hertz for continuous wave radio.
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