906. How do record players and their needles work? - JW
As a phonograph record turns, the needle of its playing arm slides through a narrow spiral groove on the record's surface. This groove is cut with a 90° angle at its bottom and both of its sides have undulations in them. As the needle slides through the groove, it rides up and down on these undulations. The needle's movement causes currents to flow in two separate pick-ups that are attached to the needle. One pick-up responds to needle motions caused by the right edge of groove and the other pick-up responds to needle motions caused by the left edge of the groove. The physical mechanism for converting needle motion into electric current depends on the needle cartridge—it can involve moving magnets, moving coils of wire, or squeezed piezoelectric crystals. Since the groove undulations represent air pressure fluctuations at the right and left microphones during recording, the currents from the two pick-ups represent those pressure fluctuations during playback. With the help of amplifiers and speakers, these currents are used to reproduce the sounds that were recorded at the two microphones.