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are far more powerful than those of only a few years ago, which has greatly
increased their usefulness in a wide range of products. Magnets are usually
shipped from the factory to a manufacturer in the unmagnetized state. There
are a number of reasons for this. There are legal restrictions on the shipping
of active magnets. They may affect instruments, destroy nearby delicate
equipment, and erase magnetic tape and disk memory information. Larger masses
of magnets can be dangerous to personnel, causing objects to fly at high
speed, and pinching or crushing body parts. In addition, magnetized magnets
attract some kinds of dirt particles, and they are extremely difficult to
clean once they have become contaminated. The new types of magnets can be
made and used in extremely thin sections, and materials are often brittle.
Once they are magnetized, additional forces are caused, which could greatly
increase breakage in shipment. For these reasons, it is usually preferable
to magnetize magnets in place in the final product, or just before assembly.
Older magnet materials could often be magnetized by such means as exposing them to a fixed magnetic field caused by other magnets, focused through a steel pole structure, or by using a short time duration current (perhaps a second) of rectified AC current directly from the power lines into a fixture of many turns (and high inductance), among other techniques. The newer high-coercivity magnets are much more resistive to demagnetizing effects, and are correspondingly much harder to magnetize.
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