h which a current of electricity was
passing was held over and parallel with a magnetic needle which was free
to move, and therefore pointed to the north, if the current was moving
_towards_ the north, the north pole was deflected to the west; if the
current was moving towards the south, the south pole of the magnet was
deflected towards the west; and that in all cases the magnet tended to
set itself at right angles to the current; also that this angular
displacement depended upon the strength of the current. Thus originated
the _galvanometer_, an instrument that not only detects the existence of
an electric current, but enables us to determine its direction and its
strength. Our present knowledge of electrical laws is due, in a very
large measure, to observations made with this instrument. Of course it
has been very much modified, and made almost incredibly sensitive: yet,
in all galvanometers, the fundamental principle involved in their
structure is that of the action of a current of electricity upon a
magnet, which was first noticed by Oersted.
MAGNETS.
It is related by Nicander that among the shepherds who tended their
flocks upon the sides of Mount Ida was one named Magnes, who noticed,
that, while taking his herds to pasture, his shepherd's crook adhered to
some of the rocks. From this man's name some have supposed the name
_magnet_ to have been derived. It is, however, generally believed to
have received its name from the ancient city of Magnesia in Asia Minor,
near which the loadstone or magnetic substance was found. This rock,
which possesses the remarkable property of attracting and holding to
itself small pieces of iron or steel, is now known to be one of the ores
of iron, and is called magnetite by mineralogists. The iron is
chemically combined with oxygen, and forms 72.5 per cent of its weight.
There is another ore of iron, known as hematite, which contains seventy
per cent of iron; but the difference of two and a half per cent of iron
in the ore is enough to make the difference between a magnetically inert
substance, and one which may be able to lift a mass of iron equal to
many times its own weight.
Sir Isaac Newton is said to have worn in a finger-ring a small loadstone
weighing three grains, which would lift seven hundred and fifty grains,
which is equal to two hundred and fifty times its own weight. The most
powerful magnet now known is owned by M. Obelliane of Paris. It can lift
forty times it
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