PARAMAGNETIC AND DIAMAGNETIC FORCES

[Footnote: Abstract of a discourse delivered in the Royal Institution,
February 1, 1856.]

THE notion of an attractive force, which draws bodies towards the
centre of the earth, was entertained by Anaxagoras and his pupils, by
Democritus, Pythagoras, and Epicurus; and the conjectures of these
ancients were renewed by Galileo, Huyghens, and others, who stated
that bodies attract each other as a magnet attracts iron. Kepler
applied the notion to bodies beyond the surface of the earth, and
affirmed the extension of this force to the most distant stars. Thus
it would appear, that in the attraction of iron by a magnet originated
the conception of the force of gravitation. Nevertheless, if we look
closely at the matter, it will be seen that the magnetic force
possesses characters strikingly distinct from those of the force which
holds the universe together. The theory of gravitation is, that every
particle of matter attracts every other particle; in magnetism also we
have attraction, but we have always, at the same time, repulsion, the
final effect being due to the difference of these two forces. A body
may be intensely acted on by a magnet, and still no motion of
translation will follow, if the repulsion be equal to the attraction.
Previous to magnetization, a dipping needle, when its centre of
gravity is supported, stands accurately level; but, after
magnetization, one end of it, in our latitude, is pulled towards the
north pole of the earth. The needle, however, being suspended from
the arm of a fine balance, its weight is found unaltered by its
magnetization. In like manner, when the needle is permitted to float
upon a liquid, and thus to follow the attraction of the north magnetic
pole of the earth, there is no motion of the mass towards that pole.
The reason is known to be, that although the marked end of the needle
is attracted by the north pole, the unmarked end is repelled by an
equal force, the two equal and opposite forces neutralizing each
other.

When the pole of an ordinary magnet is brought to act upon the
swimming needle, the latter is attracted,--the reason being that the
attracted end of the needle being nearer to the pole of the magnet
than the repelled end, the force of attraction is the more powerful of
the two. In the case of the earth, its pole is so distant that the
length of the needle is practically zero. In like manner, when a
piece of iron is presen