Originally, the recording head is actually a hoof electromagnet, the poles are very close, leaving only a slit in the middle. The entire head is sealed inside the metal shell. The base of the recording tape is coated with a layer of magnetic powder, which is actually a lot of ferromagnetic small particles.
Tape close to the recording head through, audio current makes the recording head gap between the strength and direction of the magnetic field constantly changing, the magnetic powder on the tape is magnetized into a magnetic pole direction and magnetic strength of the different "small magnet", the sound signal is recorded on the tape. The structure of the playback head is similar to that of the recording head. As the tape passes through the slit of the playback head, the magnetic field generated by the "small magnet" on the tape crosses the coil of the sound head. Because the polarity and magnetism of the "small magnet" are different, the magnetic flux produced in its online circle is constantly changing, so that the inductive current is generated in the coil, and the sound can be made in the speaker when zoomed in.