English: The rotor of a General Electric 200 kW Alexanderson alternator, a type of radio transmitter used from 1906 to 1930. The rotor consists of a disk made of high tensile magnetic steel with narrow slots cut in its periphery, creating a series of blades which form the magnetic "poles" of the machine. The slots are filled with bronze to give the rotor a smooth surface. The rotor has no conductive windings and the machine operates by induction. A stator with an equal number of poles around the periphery of the rotor creates a magnetic field axially through the side of the rotor. When the stator and rotor poles line up the magnetic flux through the pole increases. When the rotor poles are between two stator poles the magnetic flux decreases. This induces a radio frequency alternating current in the stator wondings. The rotor weighs 5500 lb. The air gap between the rotor and stator on each side is only 1 mm, maintained by a complicated thrust bearing system. It turned at 2170 RPM and the alternator transmitted at 22. 1 kHz. It was installed in September 1918 at RCA's New Brunswick, N.J. station