How does magnetic recording work?
During the recording process, an electromagnet in the recording head magnetizes the surface of a specially coated tape. This tape is coated with a thin layer of plastic that’s impregnated with tiny cigar-shaped magnetic particles. As the tape moves past the recording head, the head magnetizes these particles back and forth to a certain depth, according to the audio signal reaching the recorder from the microphone. The higher the pitch of the sound, the more frequently the direction of magnetization reverses. The louder the volume of the sound, the deeper the magnetization extends into the layer. During playback, this magnetized layer moves past the playback head and induces electric currents in it. These currents are then amplified and used to reproduce the sound. A much more detailed discussion of this process appears in my book.