Another curious property of magnetism is that we cannot develop
north magnetism in a bar without developing south magnetism at the
same time. If it were otherwise, important consequences would
result. A separate north pole of a magnet would, if attached to a
floating object and thrown into the ocean, start on a journey
towards the north all by itself. A possible method of bringing
this result about may suggest itself. Let us take an ordinary bar
magnet, with a pole at each end, and break it in the middle; then
would not the north end be all ready to start on its voyage north,
and the south end to make its way south? But, alas! when this
experiment is tried it is found that a south pole instantly
develops itself on one side of the break, and a north pole on the
other side, so that the two pieces will simply form two magnets,
each with its north and south pole. There is no possibility of
making a magnet with only one pole.
It was formerly supposed that the central portions of the earth
consisted of an immense magnet directed north and south. Although
this view is found, for reasons which need not be set forth in
detail, to be untenable, it gives us a good general idea of the
nature of terrestrial magnetism.
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