iron and steel retain previous magnetizations, and an apparent external
neutrality would in most cases be the superposition of one magnetism
upon another of equal external force in the opposite direction, as shown
at B, Fig. 1. Perfectly soft iron we can easily free, by vibrations,
from the slightest trace of previous magnetism, and study the neutrality
produced under varying conditions.

[Illustration: FIG. 1.]

If we take a flat bar of soft iron, of 30 or more centimeters in
length, and hold it vertically (giving while thus held a few torsions,
vibrations, or, better still, a few slight blows with a wooden mallet,
in order to allow its molecules to rotate with perfect freedom), we find
its lower end to be of strong north polarity, and its upper end south.
On reversing the rod and repeating the vibrations, we find that its
lower end has precisely a similar north polarity. Thus the iron is
homogeneous, and its polarity symmetrical. If we now magnetize this rod
to produce a strong south pole at its lower portion, we can gradually
reverse this polarity, by the influence of earth's magnetism, by
slightly tapping the upper extremity with a small wooden mallet. If
we observe this rod by means of a direction needle at all parts, and
successively during its gradual passage from one polarity to the other,
there will be no sudden break into a haphazard arrangement, but a
gradual and perfectly symmetrical rotation from one direction to that of
the opposite polarity.

If this rod is placed east and west, having first, say, a north polarity
to the right, we can gradually discharge or rotate the molecules to
zero, and as gradually reverse the polarity by simply inclining the rod
so as to be slightly influenced by earth's magnetism; and at no portion