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The National MagLab is funded by the National Science Foundation and the State of Florida.

Tape Recorder

Two heads — or even three — are better than one when it comes to understanding how tape recorders harness electromagnetic induction.

A device for recording audio signals via magnetized tape was first invented in 1935. By the late 1960s, magnetic tape cassettes were common, largely supplanting phonograph records.

Take a look at the tape recorder demonstrated here.

Current passing through tiny electromagnets generates a magnetic field above a magnetized tape. This is at the heart of how a tape recorder works.


  1. Note the three heads in the recorder. The Erase Head, the Record Head, and the Playback Head. Each head is a small electromagnet consisting of a coil of wire wrapped around a ring of ferromagnetic material.
  2. See how the base of each ring has a small gap positioned directly over the tape.
  3. The tape is a plastic film coated with minuscule magnetic particles.
  4. When sound is picked up by the tape recorder, it’s converted to an electrical signal, which is sent to the record head.
  5. The amplitude and frequency of the signal vary, resulting in a corresponding change in the electrical signal and the magnetic field they induce. Consequently, the magnetic particles on the tape are affected differently. Their magnetic domains shift in alignment with whatever field they pass beneath, thus imprinting a kind of magnetic picture of the audio signal on the tape.
  6. Immediately after a signal is recorded onto tape, it can be played back using the playback head. When recorded tape scrolls under the playback head, the moving magnetic fields induce a varying current in the head. This voltage produces an electrical representation of the magnetic signal on the tape. The electrical signal is sent to an amplification/speaker system, converting the electrical energy back to mechanical energy, so that the recorded music is audible.
  7. To erase what’s on the tape, a high amplitude, high frequency signal is sent via the wire to the erase head. The resulting magnetic field induced in the head jumbles up the magnetic particles in the tape, erasing any sound preserved in the way those particles were arranged on the tape.