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04/11/10(Sun)01:31:57 No. 215629XXX A
magnet (from Greek μαγνήτις λίθος magnḗtis líthos, Magnesian stone) is a
material or object that produces a magnetic field. This magnetic field
is invisible but is responsible for the most notable property of a
magnet: a force that pulls on other ferromagnetic materials like iron
and attracts or repels other magnets. A permanent magnet is an
object made from a material that is magnetized and creates its own
persistent magnetic field. An everyday example is a refrigerator magnet
used to hold notes on a refrigerator door. Materials that can be
magnetized, which are also the ones that are strongly attracted to a
magnet, are called ferromagnetic (or ferrimagnetic). These include iron,
nickel, cobalt, some rare earth metals and some of their alloys (e.g.,
Alnico), and some naturally occurring minerals such as lodestone.
Although ferromagnetic (and ferrimagnetic) materials are the only ones
attracted to a magnet strongly enough to be commonly considered
magnetic, all other substances respond weakly to a magnetic field, by
one of several other types of magnetism. Some ferromagnetic
materials can be magnetised by a magnetic field but do not tend to
remain magnetised when the field is removed; these are termed soft.
Permanent magnets are made from magnetically hard ferromagnetic
materials that stay magnetized. An electromagnet is made from a
coil of wire which acts as a magnet when an electric current passes
through it, but stops being a magnet when the current stops. Often an
electromagnet is wrapped around a core of ferromagnetic material like
steel, which enhances the magnetic field produced by the coil. The
overall strength of a magnet is measured by its magnetic moment, while
the local strength of the magnetism in a material is measured by its
magnetization.