A recording head (a small electromagnet) magnetizes the passing tape in such a way that the magnetic particles on it are realigned in a pattern that corresponds to the loudness and frequency (rate of vibration) of incoming sounds. The resulting pattern remains on the tape until erased or changed.
The tape of an audiocassette has a weak magnetic field (an area where a magnetic force is present) around it that varies from point to point depending on the pattern of its magnetic particles. The playback head contains a coil of wire. When the magnetized tape moves past the coil, an electrical current is created. The current will alternate in direction depending on the alignment of the magnetic particles as they pass by the playback head. The magnetic pattern originally recorded on the tape is transformed into a precisely corresponding electrical signal.Extracted from http://www.scienceclarified.com/
