English: Closeup of the rotor of an Alexanderson alternator a rotating machine used from about 1906 to 1930 as a longwave radio transmitter in high power transoceanic wireless telegraphy radio stations which transmitted telegrams by Morse code between nations. The Alexanderson machine was an induction generator, The rotor was a solid piece of high tensile steel with narrow slots cut in the periphery. The narrow "teeth" between the slots functioned as the rotor's magnetic "poles". The stator was a circular steel piece with a U shaped cross section which embraced the rim of the rotor, creating a magnetic field passing axially through the rim. The stator had a number of "teeth" or poles equal to the rotor. As the rotor turned, the rotor teeth would alternately line up with the stator teeth and fall between them. When a rotor tooth was between the stator teeth, it would increase the magnetic flux between the poles. When the tooth passed between them, the flux would decrease. This alternating magnetic flux would induce an alternating current in windings around the poles. This rotor from a powerful 200 kW machine had 300 pole pairs and could turn at up to 20,000 revolutions per minute, generating frequencies up to 100 kHz. The Alexanderson alternator was designed in 1906 by Reginald Fessenden and Ernst Alexanderson