English: A rotor from a small Alexanderson alternator radio transmitter, made by en:Reginald Fessenden, an obsolete type of radio transmitter used between 1906 and the mid 1920s to transmit some of the first AM (sound) broadcasts. The alternator functioned as an AC electric generator which was spun at very high speed, so it produced radio frequency alternating current. The rotor is 6 inches in diameter, made from high strength magnetic cobalt steel and had 300 narrow notches cut in its rim (visible) to make 300 "teeth" or poles. To reduce air resistance the space between the teeth is filled with nonmagnetic bronze. The armature, which embraces the rim of the wheel with a gap of 1/8 in. has a field winding with 300 poles. The generator works by variable reluctance. When a tooth is in line with the pole of the field winding, the magnetic field through the pole is strong. When the rotor turns so that the tooth is between the poles of the field winding, the lower reluctance reduces the magnetic field. As the wheel turns the resulting varying magnetic field induces a radio frequency current in the field coils. The rotor turns at 30,000 revolutions per minute (500 rev. per second) and there are 300 poles, so the output frequency is 150,000 Hz (150 kHz), at 150 to 200 volts. This small alternator was designed for shipboard use, and was mounted on gimbals so that its axis could stay constant while the ship rocks, to reduce stress on the bearings. Caption: "Field magnet disc of H.F. alternator"
Alterations to image: Removed aliasing artifacts (background crosshatching) introduced when the halftone photo was scanned, using Gimp FFT filter.