NEUTRALITY.

The apparatus needed for researches upon evident external polarity

requires no very great skill or thought, but simply an apparatus to

measure correctly the force of the evident repulsion or attraction; in

the case of neutrality, however, the external polarity disappears, and

we consequently require special apparatus, together with the utmost care

and reflection in its use.

From numerous researches previously made by means of the induction

balance, the results of which I have already published, I felt convinced

that in investigating the cause of magnetism and neutrality I should

have in it the aid of the most powerful instrument of research ever

brought to bear upon the molecular construction of iron, as indeed of

all metals. It neglects all forces which do not produce a change in the

molecular structure, and enables us to penetrate at once to the interior

of a magnet or piece of iron, observing only its peculiar structure

and the change which takes place during magnetization or apparent

neutrality.

The induction balance is affected by three distinct arrangements of

molecular structure in iron and steel, by means of which we have

apparent external neutrality.

Fig 1 shows several polar directions of the molecules as indicated

by the arrows. Poisson assumed as a necessity of his theory, that

a molecule is spherical; but Dr. Joule's experimental proof of the

elongation of iron by one seven-hundred and-twenty-thousandth of its

length when magnetized, proves at least that its form is not spherical;

and, as I am unable at present to demonstrate my own views as to its

exact form, I have simply indicated its polar direction by arrows--the

dotted oval lines merely indicating its limits of free elastic rotation.

In Fig. 1, at A, we have neutrality by the mutual attraction of each

pair of molecules, being the shortest path in which they could satisfy

their mutual attractions. At B we have the case of superposed magnetism

of equal external value, rendering the wire or rod apparently neutral,

although a lower series of molecules are rotated in the opposite

direction to the upper series, giving to the rod opposite and equal

polarities. At C we have the molecules arranged in a circular chain

around the axis of a wire or rod through which an electric current

has passed. At D we have the evident polarity induced by the earth's

directive influence when a soft iron rod is held in the magnetic

meridian. At E we have a longitudinal neutrality produced in the same

rod when placed magnetic west, the polarity in the latter case being

transversal.

In all these cases we have a perfectly symmetrical arrangement, and I

have not yet found a single case in well-annealed soft iron in which I

could detect a heterogeneous arrangement, as supposed by Ampere, De la

Rive, Weber, Wiedermann, and Maxwell.

We can only study neutrality with perfectly soft Swedish iron. Hard

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

of this passage from one polarity to neutrality, and to that of the

opposite name, will there be found a break of continuity of rotation or

haphazard arrangement. If we rotate this rod slowly, horizontally or

vertically, taking observations at each few degrees of rotation of an

entire revolution, we find still the same gradual symmetrical change

of polarity, and that its symmetry is as complete at neutrality as in

evident polarity.

In all these cases there is no complete neutrality, the longitudinal

polarity simply becoming transversal when the rod is east and west.

F, G, H, I, J, Fig. 1, show this gradual change, H being neutral

longitudinally, but polarized transversely. If, in place of the rod,

we take a small square soft iron plate and allow its molecules freedom

under the sole influence of the earth's magnetism, then we invariably

find the polarity in the direction of the magnetic dip, no matter in

what position it be held, and a sphere of soft iron could only be

polarized in a similar direction Thus we can never obtain complete

external neutrality while the molecules have freedom and do not form an

internal closed circle of mutual attractions; and whatever theory we may

adopt as to the cause of polarity in the molecule, such as Coulomb's,

Poisson's, Ampere's, or Weber's, there can exist no haphazard

arrangement in perfectly soft iron, as long as it is free from all

external causes except the influence of the earth; consequently these

theories are wrong in one of their most essential parts.

We can, however, produce a closed circle of mutual attraction in iron

and steel, producing complete neutrality as long as the structure is not

destroyed by some stronger external directing influence.

Oersted discovered that an external magnetic needle places itself

perpendicular to an electric current; and we should expect that, if the

molecules of an iron wire possessed inherent polarity and could rotate,

a similar effect would take place in the interior of the wire to that

observed by Oersted. Wiedermann first remarked this effect, and it has

been known as circular magnetism. This circle, however, consists really

in each molecule having placed itself perpendicular to the current,

simply obeying Oersted's law, and thus forming a complete circle in

which the mutual attractions of the molecules forming that circle are

satisfied, as shown as C, Fig. 1. This wire becomes completely neutral,

any previous symmetrical arrangement of polarity rotating to form its

complete circle of attractions; and we can thus form in hard iron and

steel a neutrality extremely difficult to break up or destroy. We have

evident proof that this neutrality consists of a closed chain, or

circle, as by torsion we can partially deflect them on either side; thus

from a perfect externally neutral wire, producing either polarity, by

simple mechanical angular displacement of the molecules, as by right or

left handed torsion.

If we magnetize a wire placed east and west, it will retain this

polarity until freed by vibrations, as already remarked. If we pass an

electric current through this magnetized wire, we can notice the gradual

rotation of the molecules, and the formation of the circular neutrality.

If we commence with a weak current, gradually increasing its strength,

we can rotate them as slowly as may be desired. There is no sudden break

or haphazard moment of neutrality: the movements to perfect zero are

accomplished with perfect symmetry throughout.

We can produce a more perfect and shorter circle of attractions by the

superposition of magnetism, as at B, Fig. 1. If we magnetize a piece

of steel or iron in a given direction with a strong magnetic directing

power, the magnetism penetrates to a certain depth. If we slightly

diminish the magnetizing power, and magnetize the rod in a contrary

direction, we may reduce it to zero, by the superposition of an exterior

magnetism upon one of a contrary name existing at a greater depth; and

if we continue this operation, gradually diminishing the force at each

reversal, we can easily superpose ten or more distinct symmetrical

arrangements, and, as their mutual attractions are satisfied in a

shorter circle than in that produced by electricity, it is extremely

difficult to destroy this formation when once produced.

The induction balance affords also some reasons for believing that the

molecules not only form a closed circle of attractions, as at B, but

that they can mutually react upon each other, so as to close a circle

of attractions as a double molecule, as shown at A. The experimental

evidence, however, is not sufficient to dwell on this point, as the

neutrality obtained by superposition is somewhat similar in its external

effects.

We can produce a perfectly symmetrical closed circle of attractions of

the nature of the neutrality of C, Fig. 3, by forming a steel wire into

a closed circle, 10 centimeters in diameter, if this wire is well joined

at its extremities by twisting and soldering. We can then magnetize this

ring by slowly revolving it at the extremity of one pole of a strong

permanent magnet; and, to avoid consequent poles at the part last

touching the magnet, we should have a graduating wedge of wood, so that

while revolving it may be gradually removed to greater distance. This

wire will then contain no consequent points or external magnetism: it

will be found perfectly neutral in all parts of its closed circle. Its

neutrality is similar to C, Fig. 3; for if we cut this wire at any point

we find extremely strong magnetic polarity, being magnetized by this

method to saturation, and having retained (which it will indefinitely)

its circle of attractions complete.

I have already shown that soft iron, when its molecules are allowed

perfect freedom by vibration, invariably takes the polarity of the

external directing influence, such as that of the earth, and it does so

even with greater freedom under the influence of heat. Manufacturers of

electro-magnets for telegraphic instruments are very careful to choose

the softest iron and thoroughly anneal it; but very few recognize the

importance as regards the position of the iron while annealing it under

the earth's directing influence. The fact, however, has long since been

observed.

Dr. Hooke, 1684, remarked that steel or iron was magnetized when heated

to redness and placed in the magnetic meridian. I have slightly varied

this experiment by heating to redness three similar steel bars, two

of which had been previously magnetized to saturation, and placed

separately with contrary polarity as regards each other, the third being

neutral. Upon cooling, these three bars were found to have identical and

similar polarity. Thus the molecules of this most rigid material, cast

steel, had become free at red heat, and rotated under the earth's

magnetic influence, giving exactly the same force on each; consequently

the previous magnetization of two of these bars had neither augmented

nor weakened the inherent polarity of their molecules. Soft iron gave

under these conditions by far the greatest force, its inherent polarity

being greater than that of steel.

I have made numerous other experiments bearing upon the question of

neutrality, but they all confirm those I have cited, which I consider

afford ample evidence of the symmetrical arrangement of neutrality.