Magnetic metal oxide development led to a revolution in the sound-recording industry. Metal oxides in powder form are bonded to flexible plastic tape (1), usually PVC, forming a moving surface on which magnetic patterns can be imposed corresponding to sound, visual, or other signals. These tapes are used in machines that consist of an erase head (2) using high-frequency input (3) to demagnetize the tape as it is driven past, and a record (4) and replay head (5). These either magnetize the tape according to the signal input (6) or reconvert the previously imposed magnetic patterns into the signals that formed them (i.e. plays back) (7). Stereo or twin-track recording combines two record/replay heads in one. The erase head is taken out of circuit when playing back and, to prevent accidental erasure, most tape recorders have a built fail-safe arrangement.
A tape recorder is an instrument for sound recording on magnetic tape, and subsequent playback. The tape, consisting of small magnetic particles of iron oxides on a thin plastic film base, is wound from the supply reel to the take-up reel to the take-up reel by a rotating capstan which controls the speed. The tape passes in turn the erase head, which by applying an alternating field reduces the overall magnetization to zero; the recording head, and the playback head. Cassettes contain thin tape handily packaged, running at 17/8 inches per sec. The somewhat larger cartridges contain an endless loop of tape on a single reel. Most recorders use two, four, or more tracks side by side on the tape.