All magnets have two points where the magnetic force is greatest. These
two points are known as the poles. For a rectangular or cylindrical bar magnet,
these poles would be at opposite ends. One pole is called the north-seeking
pole, or north pole, and the other pole is called the south-seeking, or south
pole. This terminology reflects one of the earliest uses of magnetic materials
such as lodestone. When suspended from a string, the north pole of these first
crude compasses would always "seek" or point towards the north. This
aided sailors in judging the direction to steer to reach distant lands and
return home.
The atoms of ferromagnetic substances may be thought of
as tiny magnets with an N-pole and an S-pole. These atomic magnets, or dipoles,
interact with their nearest neighboring dipoles and a group of them line up
with their magnetic axes in the same direction to form a magnetic domain. In an
unmagnetized piece of iron there are millions of these domains, but they are
pointing in random directions so that the piece of iron, as a whole, is not
magnetized.
When such a
piece of iron is placed in a magnetic field (that is, near another magnet), the
dipoles act like small compasses and rotate until they are aligned with the
field. The piece of iron will then consist of a large number of dipoles
pointing north and one end will become an N-pole, while the other end will
become an S-pole. Induced Magnetism is the magnetism of
an object as a result of an external influence. Typically, the external
influence is a magnetic field due to another object. (One also hears the term
transient magnetism applied to the same phenomenon. To be considered transient,
the magnetism must disappear when the external influence disappears, but to be
"induced" it just has to appear.)
Though this is a technical term, it has a meaning that is the normal
meaning of "induced," as in "caused by." This is to be
contrasted with other modifiers, like "permanent magnetism" or
"remnant magnetism" or "spontaneous magnetism" which are
all still magnetism of an object but with a modifier further describing the
magnetic state.
