The form of a horse-shoe is generally given to magnetized bars, when
both poles are wanted to act together, which frequently happens in
various experiments, such as for lifting weights by the force of
magnetic attraction, and for magnetizing steel bars by the process of
double touch, for which they are exceedingly convenient. The following
is the method of making a powerful magnetic battery of the horse-shoe
form. Twelve bars or plates of steel are to be taken, and having
been previously bent to the required form, that is, the horse-shoe
shape, they are then bound together by means of rivets at their ends;
before being finally fastened they are each separately magnetized and
afterwards finally united.
Horse-shoe magnets should have a short bar of soft iron adapted to
connect the two poles, and should never be laid by without such a piece
of iron adhering to them. Bar magnets should be kept in pairs with
their poles turned in contrary directions, and they should be kept from
rust. Both kinds of magnets have their power not only preserved but
increased, by keeping them surrounded with a mass of dry filings of
soft iron, each particle of which will re-act by its induced magnetism
upon the point of the magnet to which it adheres, and maintain in that
point its primitive magnetic state.
EXPERIMENT TO SHOW THAT SOFT IRON POSSESSES MAGNETIC PROPERTIES WHILE
IT REMAINS IN THE VICINITY OF A MAGNET.
Let a magnet and a key be held horizontally near one of its poles, or
near its lower edge. Then if another piece of iron, such as a small
nail, be applied to the other end of the key, the nail will hang
from the key, and will continue to do so while the magnet is slowly
withdrawn; but when it has been removed beyond a certain distance, the
nail will drop from the key, because the magnetism induced in the key
becomes at that distance too weak to support the weight of the nail.
That this is the real cause of its falling off may be proved by taking
a still lighter fragment of iron, such as a piece of very slender wire,
and applying it to the key. The magnetism of the key will still be
sufficiently strong to support the wire, though it cannot the nail,
and it will continue to support it even when the magnet is yet further
removed; at length, however, it drops off.