ough a wire.

_II.--Theories of Magnetism_

Certain bodies--as, for instance, the iron ore called loadstone, the
earth itself, and pieces of steel which have been subjected to certain
treatment--are found to possess the following properties, and are called
magnets.

If a magnet be suspended so as to turn freely about a vertical axis, it
will in general tend to set itself in a certain azimuth, and, if
disturbed from this position, it will oscillate about it.

It is found that the force which acts on the body tends to cause a
certain line in the body--called the axis of the magnet--to become
parallel to a certain line in space, called the "direction of the
magnetic force."

The ends of a long thin magnet are commonly called its poles, and like
poles repel each other; while unlike poles attract each other. The
repulsion between the two magnetic poles is in the straight line joining
them, and is numerically equal to the products of the strength of the
poles divided by the square of the distance between them; that is, it
varies as the inverse square of the distance. Since the form of the law
of magnetic action is identical with that of electric action, the same
reasons which can be given for attributing electric phenomena to the
action of one "fluid," or two "fluids" can also be used in favour of the
existence of a magnetic matter, fluid or otherwise, provided new laws
are introduced to account for the actual facts.

At all parts of the earth's surface, except some parts of the polar
regions, one end of a magnet points in a northerly direction and the
other in a southerly one. Now a bar of iron held parallel to the
direction of the earth's magnetic force is found to become magnetic. Any
piece of soft iron placed in a magnetic field is found to exhibit
magnetic properties. These are phenomena of _induced_ magnetism. Poisson
supposes the magnetism of iron to consist in a separation of the
magnetic fluids within each magnetic molecule. Weber's theory differs
from this in assuming that the molecules of the iron are always magnets,
even before the application of the magnetising force, but that in
ordinary iron the magnetic axes of the molecules are turned
indifferently in every direction, so that the iron as a whole exhibits
no magnetic properties; and this theory agrees very well with what is
observed.

The theories establish the fact that magnetisation is a phenomenon, not
of large masses of iron, but of molecules; t