During these researches I have remarked a peculiar property of
magnetism, viz., that not only can the molecules be rotated through any
degree of arc to its maximum, or saturation, but that, while it requires
a comparatively strong force to overcome its rigidity or resistance to
rotation, it has a small field of its own through which it can move with
excessive freedom, trembling, vibrating, or rotating through a small
degree with infinitely less force than would be required to rotate it
permanently on either side. This property is so marked and general that
we can observe it without any special iron or apparatus.
Let us take a flat rod of ordinary hoop iron, 30 or more centimeters
in length. If, while holding this vertically, we give freedom to its
molecules by torsions, vibrations, or, better still, by a few blows with
a wooden mallet upon its upper extremity, we find, as is well known,
that its lower portion is strongly north, and its upper south. If we
reverse this rod, we now find it neutral at both extremities. We might
here suppose that the earth's directing force had rotated the molecules
to zero, or transversely, which in reality it has done, but only to the
limit of their comparatively free motion; for if we reverse the rod to
its original position, its previous strong polarity reappears at both
extremities, thus the central point of its free motion is inclined to
the rod, giving by its free motion great symmetrical inclination and
polarity in one direction, but when reversed the inclination is reduced
to zero.
In Fig. 3, D shows the bar of iron when strongly polarized by earth's
magnetic influence, under vibrations, with a sufficient force to have
rotated its elastic center of action. C shows the same bar with its
molecules at zero, or transversal, the directing force of earth being
insufficient without the aid of mechanical vibration to allow them to
change. The dotted lines of D suppose the molecule to be in the center
of its free motion, while at C the molecules have rotated to zero, as
they are prevented from further rotation by being at the extreme end of
its free motion.
If, now, we hold the rod vertically, as at C, giving neutrality, and
give a few slight blows with a wooden mallet to its upper extremity, we
can give just the amount of freedom required for it to produce evident
polarity, and we then have equal polarity, no matter which end of the
bar is below, the center of its free rotation here being perfect, and
the rod perfectly neutral longitudinally when held east and west. If, on
the other hand, we have given too much freedom by repeated blows of the
mallet, its center of free motion becomes inclined with the molecules,
and we arrive at its first condition, except that it is now neutral at
D and polarized at C. From this it will be seen that we can adjust
this center of action, by vibrations or blows, to any point within the
external directing influence.
[Illustration: FIG. 3.]
We can perceive this effect of free rotation in a limited space in all
classes of iron and steel, being far greater in soft Swedish iron than
in hard iron or steel. A similar phenomenon takes place if we magnetize
a rod held vertically in the direction of earth's magnetism. It
then gives greater polarity than if magnetized east or west, and if
magnetized in a contrary sense to earth's magnetism, it is very feebly
magnetized, or, if the rod is perfectly soft, it becomes neutral after
strong magnetization. This property of comparative freedom, and the
rotation of its center of action, can be demonstrated in a variety of
ways. One remarkable example of it consists in the telephone. All those
who are thoroughly acquainted with electro-magnetism, and know that
it requires measurable time to charge an electro-magnet to saturation
(about one-fifteenth of a second for those employed in telegraphy),
were surprised that the telephone could follow the slightest change of
timbre, requiring almost innumerable changes of force per second. I
believe the free rotation I have spoken of through a limited range
explains its remarkable sensitiveness and rapidity of action, and,
according to this view, it would also explain why loud sounding
telephones can never repeat all the delicacy of timbre that is easily
done with those only requiring a force comprised in the critical limits
of its free rotation. This property, I have found, has a distinct
critical value for each class of iron, and I propose soon to publish
researches upon the molecular construction of steel and iron, in which
I have made use of this very property as a guide to the quality of the
iron itself.
The elastic rotation (in a limited space) of a molecule differs entirely
from that known as mechanical elasticity. In perfectly soft iron we have
feeble _mechanical_ elasticity, while in tempered steel we have
that elasticity at its maximum. The contrary takes place as regards
_molecular_ elasticity. In tempered steel the molecules are extremely
rigid, and in soft iron its molecular elasticity is at its maximum. Its
free motion differs entirely from that given it by torsion or stress. We
may assume that a molecule is surrounded by continuous ether, more of
the nature of a jelly than of that of a gas; in such a medium a molecule
might freely vibrate through small arcs, but a rotation extending beyond
its critical limit would involve a much greater expenditure of force.
The discovery of this comparatively free rotation of molecules, by means
of which, as I have shown, we can (without in any degree disturbing the
external mechanical elasticity of the mass) change the axes of their
free motion in any direction desired, has led me into a series of
researches which have only indirectly any relation with the theory of
magnetism. I was extremely desirous, however, of finding an experimental
evidence which in itself should demonstrate all portions of the theory,
and the following experiment, I believe, answers this purpose.
Let us take a square soft iron rod, five millimeters in diameter by
thirty or more centimeters in length, and force the molecules, by aid
of blows from a wooden mallet, as previously described, to have their
centers of free motion in one direction; the rod will (as already shown)
have polarity at both ends, when held vertically; but if reversed, both
ends become completely neutral.
If now we turn the rod to its first position, in which it shows strong
polarity, and magnetize it while held vertically, by drawing the north
pole of a sufficiently powerful permanent magnet from its upper to its
lower extremity, we find that this rod, instead of having south polarity
at its lower portion, as we should expect from the direction of the
magnetization, is completely neutral at both extremities, but if we
reverse the rod its fullest free powers of magnetization now appear in
the position where it was previously neutral. Thus, by magnetization, we
have completely rotated its free path of action, and find that we can
rotate this path as desired in any direction by the application of a
sufficient directing power.
If we take a rod as described, with its polarities evident when held
vertically, and its neutrality also evident when its ends are reversed
in the same magnetic field, we find that its polarity is equal at both
ends, and that it is in every way symmetrical with a perfect magnet. If
we _gradually_ reverse the ends and take observations of its condition
through each degree of arc passed over, we find an equal symmetrical
diminution of evident external polarity, until we arrive at neutrality,
when it has no external trace of inherent polarity; but its inherent
polarity at once becomes evident by a simple return to its former
position. Thus the rod has passed through all the changes from polarity
to neutrality, and from neutrality to polarity, and these changes have
taken place with complete symmetry.
The limits of this paper do not allow me to speak of the numerous
theoretical evidences as shown by the use of my induction balance. I
believe, however, that I have cited already experimental evidences to
show that what has been attributed to coercive force is really due to
molecular freedom or rigidity; that in inherent molecular polarity we
have a fact admitted by Coulomb, Poisson, Ampere, De la Rive, Weber,
Du Moncel, Wiedermann, and Maxwell; and that we have also experimental
evidence of molecular rotation and of the symmetrical character of
polarity and neutrality.
The experiments which I have brought forward in this paper, in addition
to those mentioned in my paper read before the Royal Society, will,
I hope, justify me in having advanced a theory of magnetism which I
believe in every portion allows at least experimental evidences of its
probable truth.
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