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"it's not very hard to see how a stereo phonograph works... 16:24 (tech_notes#110)

A playback needle is attached to a tiny magnet, which is placed near a coil of wire that creates a voltage when the magnet moves. The coil is then attached to a transistor that controls the power from the power supply, which is connected to a speaker."

from:

http://www.ethanwiner.com/HWTutor.html