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 its own weight. Large pieces, however, do not support
proportionally greater weights, seldom more than one or two times their
own weight.
There are in many places in the world immense beds of magnetic iron-ore.
Such are to be found in the Adirondack region in Northern New York, and
in Chester County, Pennsylvania. The celebrated iron-mines of Sweden
consist of it, and in Lapland there are several large mountains of it.
It must not be inferred, that, because the mineral is called magnetite,
all specimens possess the property called magnetism. The large masses
seldom manifest any such force, any more than ordinary pieces of iron or
steel manifest it: yet any of it will be attracted by a magnet in the
same way as iron will be. The most powerful native magnets are found in
Siberia, and in the Hartz, a range of mountains in Northern Germany.
When a piece of this magnetically endowed ore is placed in a mass of
iron-filings, it will be seen that the filings adhere to it in greatest
quantity upon two opposite ends or sides, and these are named the poles
of the magnet. If the piece be suspended by a string so as to turn
freely, it will invariably come to rest with the same pole turned
towards the north; and this pole is therefore called the north pole of
the magnet, and the action is called the directive action. This
directive action was known to the Chinese more than three thousand years
ago. In traversing those vast steppes of Tartary they employed magnetic
cars, in which was the figure of a man, whose movable, outstretched arm
always pointed to the south. Dr. Gilbert affirms that the compass was
brought from China to Italy in 1260, by a traveller named Paulus
Venetus.
When a piece of hardened steel is rubbed upon a natural magnet, it
acquires the same directive property; and, as the steel could be easily
shaped into a convenient form for use, a steel needle has generally been
used for the needle of a compass. The directive power of the magnet has
been and still is of incalculable value to all civilized nations. Ocean
navigation would be impossible without it, and territorial boundaries
are fixed by means of it; but there are other properties and relations
of a magnet, which have been discovered within the last fifty years,
which are destined to be as important to mankind as that of the compass
has been.
In 1825 William Sturgeon of Woolwich, Eng., discovered that if a copper
wire were wound around a piece of soft iron, and a current of
electricity sent through the wire, the soft iron would become a magnet,
but would retain its magnetism no longer than while the current of
electricity was passing through the coil. The magnetism developed in
this way was called electro-magnetism, and the iron so wound was called
an electro-magnet. The first electro-magnet was made by winding bare
wire upon the soft iron. This method will not produce very strong
magnets. In 1830 Prof. Henry insulated the wire by covering it with
silk, and was the first to produce powerful magnets.
On a soft iron bar of fifty-nine pounds weight he used twenty-six coils
of wire, thirteen on each leg, all joined to a common conductor by their
opposite ends, and having an aggregate length of seven hundred and
twenty-eight feet. This apparatus was found able to sustain a weight of
twenty-five hundred pounds. This electro-magnet is now owned by Yale
College.
The power of the electro-magnet is enormously greater than that of any
permanent magnet. A permanent magnet made by Jamin of Paris, which is
made up of many strips of thin steel bound together, and weighing four
pounds, is able to support a weight of one hundred pounds; but Dr. Joule
made an electro-magnet, by arranging the coils to advantage, that would
support thirty-five hundred times its own weight, or one hundred and
forty times the proportionate load of Sir Isaac Newton's ring magnet.