ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
magnetism once the magnetizing force is removed. Materials of
the type that easily lose most of their magnetic: strength are
called temporary magnets. The amount of magnetism which remains
in a temporary magnet is referred to as its residual magnetism.
The ability of a material to retain an amount of residual
magnetism is called the retentivity of the material.
The difference between a permanent and a temporary magnet has
been indicated in terms of reluctance, a permanent magnet having
a high reluctance and a temporary magnet having a low
reluctance. Magnets are also described in terms of the
permeability of their materials, or the ease with which magnetic
lines of force distribute themselves throughout the material. A
permanent. magnet, which is produced from a material with a
high reluctance, has a low permeability. A temporary magnet,
produced from a material with a low reluctance, would have a
high permeability.
b. Magnetic Polls. The magnetic force surrounding a magnet is
not uniform. There exists a great concentration of force at
each end of the magnet and a very weak force at the center.
Proof of this fact can be obtained by dipping a magnet into iron
filings. It is found that many filings will cling to the ends
of the magnet: while very few adhere to the center. The( two
ends, which are the regions of the concentrated lines of force,
are called the poles of the magnet. Magnets have two magnetic
poles, and both poles have equal magnetic strength.
(1) Law of Magnetic Poles. If a bar magnet is suspended
freely on a string, it will align itself in a north and south
direction. When this experiment is repeated, it is found that
the same pole of the magnet will always swing toward the north
geographical pole of the earth. Therefore, it is called the
northseeking pole or simply the north pole. The other pole of
the magnet is the southseeking pole or the south pole.
A practical use of the directional characteristic of the magnet
is the compass, a device in which a freely rotating magnetized
needle indicator points toward the north pole. The realization
that the poles of a suspended magnet always move to a definite
position gives an indication that the opposite poles of a magnet
have opposite magnetic polarity.
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