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Over time, the passage of magnetically charged cassette tape over the metallic parts of the tape deck will impart a magnetic polarity that can impair the ability of subsequent tapes to record accurately. Cassette demagnetizers (sometimes known as degaussers) were developed to remove this polarity.
Cassette demagnetizers of the "cassette" type physically resemble a cassette tape shell but contain circuitry to demagnetize the tape heads of the tape deck. Another type of demagnetizer, the "wand" type can demagnetize anything that it is brought into contact with, including the tape heads and capstans. Nakamichi manufactured a "wand" type demagnetizer in the 1980s. This design had the advantage that it could be used to demagnetize other metal parts of the tape path (not just the actual record/playback heads) however it required careful use (to avoid increasing the magnetization of the heads) and if used too close to actual cassette tapes could inadvertently act as a "bulk eraser".
Demagnetizers contain electronic circuitry and require a power source—either a battery or a power cord.
A third design consisted of a cassette shell with a head cleaning tape wound on the spools and a disc shaped magnet mounted above the head cleaner tape such that when the play button was activated the head cleaner physically cleaned the head surface and simultaneously made the magnet rotate, creating the alternating magnetic field required for demagnetizing.
Some cassette deck manufacturers even produced decks with a self demagnetise button. These worked by feeding the record head with a strong high frequency signal which was gradually reduced in amplitude to zero over a few seconds.