TEPATJP Beginning in 1941, seven of the twenty original 200 KW alternators were put into service by the U.S. By the end of the war the Alexanderson alternator was operating to reliably provide transoceanic radio service. Higher radio frequencies thus require more poles, a higher rotational speed, or both. The machine operates by variable reluctance (similar to an electric guitar pickup), changing the magnetic flux linking two coils. The rotor is turned at a high speed by an electric motor through a speed-increaser gearbox. The output of the LC circuit is rectified, and the resulting voltage is compared with a constant reference voltage to produce a feedback signal to control the motor speed. The space between the teeth is filled with nonmagnetic material, to give the rotor a smooth surface to decrease aerodynamic drag. David E. Fisher and Marshall J. Fisher, Tube, the Invention of Television Counterpoint, Washington D.C.|Jepang: プレイステーション, Hepburn: Pureisutēshon?