A single needle is not relied upon to secure the direction of the
card, the latter being attached to a system of four or even more
magnets, all pointing in the same direction. The compass must have
no iron in its construction or support, because the attraction of
that substance on the needle would be fatal to its performance.
From this cause the use of iron as ship-building material
introduced a difficulty which it was feared would prove very
serious. The thousands of tons of iron in a ship must exert a
strong attraction on the magnetic needle. Another complication is
introduced by the fact that the iron of the ship will always
become more or less magnetic, and when the ship is built of steel,
as modern ones are, this magnetism will be more or less permanent.
We have already said that a magnet has the property of making
steel or iron in its neighborhood into another magnet, with its
poles pointing in the opposite direction. The consequence is that
the magnetism of the earth itself will make iron or steel more or
less magnetic. As a ship is built she thus becomes a great
repository of magnetism, the direction of the force of which will
depend upon the position in which she lay while building.
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