As the record spins around with the platter, the stylus glides through the grooves. The stylus is held in place by the cartridge, which is attached to the tone arm. A counter weight attached to the opposite end keeps the pressure on the needle very low. As the record spins the contact between the stylus and groove walls causes vibrations in the stylus. These vibrations move up the needle to the top, where a magnet, surrounded by four wire coils, is attached. The vibrations create a mechanical force, which is proportional to the groove modulation on a phonograph record, which is used to change the position of the magnetic fields of the magnet. This induces a voltage in the coil which is equal to the number of turns in the coil multiplied by the time rate of change of the magnetic flux linkage through the coil.

Image from The Way Things Work

This voltage (signal) is then sent to the phono input of an amplifier. From there the amplifier boosts the voltage to a level that is powerful enough to be heard by our ears.

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