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|When the part is
magnetized, these domains are caused to align in the direction of the coercing
field. Some of the domains are more difficult to align than others, and
so some small degree of magnetization may be reached at a low field, and
more at a higher one, until all the domains have turned (or reformed) in
the direction of the external field. In magnetically "soft" materials,
such as low carbon steel, as soon as the external field is removed, some
or all of the domains go back to a field-canceling jumble of directions.
In magnetically "hard" permanent materials, however, most or all
of the domains remain aligned, resulting in a net external magnetic field.
Many modern magnet materials are said to be ANISOTROPIC, meaning that they have a preferred axis of magnetic field, built-in during manufacture. The part may be magnetized in either direction along this axis, north-south or south-north. The part is extremely resistant to magnetizing in any other direction, however. Many of these materials are easier to magnetize the first time, from the virgin state, than they are to remagnetize again in the opposite direction. For this reason, if a part made of material with these characteristics must be remagnetized, it must be aligned in the fixture in the same way that it was originally magnetized, not north pole to south pole, and so on.
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