Operation Of The Playback Head
How can the information, which was stored in a pattern of magnetically aligned particles on tape, be converted
back into sound waves? The magnetic pattern must be transformed into an electric current, which then can be amplified and cause a speaker to vibrate.
The operation of the playback is based on a discovery made in the 1830s by an English physicist, Michael Faraday. He knew about Oersted's earlier observation that magnetism is created by an electric current. Faraday wondered if the opposite process might occur, where an electric current could be created from magnetism. By experimenting with magnets and coils of wire, he was able to show that a moving magnet did create a small current in a coil. His discovery was called electromagnetic induction because current was "induced" in the coil by the moving magnet. The induction principle, combining magnetism and motion, is the basis for the operation of the generators that produce electricity at power plants.
The tape of an audiocassette has a weak magnetic field around it that varies from point to point depending on the orientation of its magnetic particles. The playback head contains a coil of wire. When the magnetized tape moves past the coil, Faraday's condition for inducing a current in the coil is fulfilled. The induced current will alternate in direction depending on the orientation 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.
The electric current from the playback head is amplified and sent to an audio speaker, which vibrates in synchronism with the varying current. The back-and-forth motion of the speaker creates pressure waves in the air. This causes the listener's ear drums to vibrate, producing the sensation of sound.
When someone wants to listen to a previously recorded tape, only the playback head is activated. However, to record new information on a tape requires two operations: the erase head must be activated, followed by the recording head. It is possible to activate all three heads, so that the first one erases, the second one records and the third one plays back what has just been recorded.
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