pleasure or cause to disappear. We magnetise our strip, of steel by
drawing it along the pole of a magnet; we can demagnetise it, or
reverse its magnetism, by properly drawing it along the same pole in
the opposite direction. What, then, is the real nature of this
wondrous change? What is it that takes place among the atoms of the
steel when the substance is magnetised? The question leads us beyond
the region of sense, and into that of imagination. This faculty,
indeed, is the divining-rod of the man of science. Not, however, an
imagination which catches its creations from the air, but one informed
and inspired by facts; capable of seizing firmly on a physical image
as a principle, of discerning its consequences, and of devising means
whereby these forecasts of thought may be brought to an experimental
test. If such a principle be adequate to account for all the
phenomena--if from an assumed cause the observed acts necessarily
follow, we call the assumption a theory, and, once possessing it, we
can not only revive at pleasure facts already known, but we can
predict others which we have never seen. Thus, then, in the
prosecution of physical science, our powers of observation, memory,
imagination, and inference, are all drawn upon. We observe facts and
store them up; the constructive imagination broods upon these
memories, tries to discern their interdependence and weave them to an
organic whole. The theoretic principle flashes or slowly dawns upon
the mind; and then the deductive faculty interposes to carry out the
principle to its logical consequences. A perfect theory gives
dominion over natural facts; and even an assumption which can only
partially stand the test of a comparison with facts, may be of eminent
use in enabling us to connect and classify groups of phenomena. The
theory of magnetic fluids is of this latter character, and with it we
must now make ourselves familiar.
With the view of stamping the thing more firmly on your minds, I will
make use of a strong and vivid image. In optics, red and green are
called complementary colours; their mixture produces white. Now I ask
you to imagine each of these colours to possess a self-repulsive
power; that red repels red, that green repels green; but that red
attracts green and green attracts red, the attraction of the
dissimilar colours being equal to the repulsion of the similar ones.
Imagine the two colours mixed so as to produce white, and suppose two
strips of wood painted with this white-; what will be their action
upon each other? Suspend one of them freely as we suspended our
darning-needle, and bring the other near it; what will occur? The red
component of the strip you hold in your hand will repel the red
component of your suspended strip; but then it will attract the green,
and, the forces being equal, they neutralise each other. In fact, the
least reflection shows you that the strips will be as indifferent to
each other as two unmagnetised darning-needles would be under the same
circumstances.
But suppose, instead of mixing the colours, we painted one half of
each strip from centre to end red, and the other half green, it is
perfectly manifest that the two strips would now behave towards each
other exactly as our two magnetised darning-needles--the red end would
repel the red and attract the green, the green would repel the green
and attract the red; so that, assuming two colours thus related to
each other, we could by their mixture produce the neutrality of an
unmagnetised body, while by their separation we could produce the
duality of action of magnetised bodies.
But you have already anticipated a defect in my conception; for if we
break one of our strips of wood in the middle we have one half
entirely red, and the other entirely green, and with these it would be
impossible to imitate the action of our broken magnet. How, then,
must we modify our conception? We must evidently suppose _each
molecule of the wood_ painted green on one face and red on the opposite
one. The resultant action of all the atoms would then exactly
resemble the action of a magnet. Here also, if the two opposite
colours of each atom could be caused to mix so as to produce white, we
should have, as before, perfect neutrality.
For these two self-repellent and mutually attractive colours,
substitute in your minds two invisible self-repellent and mutually
attractive fluids, which in ordinary steel are mixed to form a neutral
compound, but which the act of magnetisation separates from each
other, placing the opposite fluids on the opposite face of each
molecule. You have then a perfectly distinct conception of the
celebrated theory of magnetic fluids. The strength of the magnetism
excited is supposed to be proportional to the quantity of neutral
fluid decomposed. According to this theory nothing is actually
transferred from the exciting magnet to the excited steel. The act of
magnetisation consists in the forcible separation of two fluids which
existed in the steel before it was magnetised, but which then
neutralised each other by their coalescence. And if you test your
magnet, after it has excited a hundred pieces of steel, you will find
that it has lost no force--no more, indeed, than I should lose, had my
words such a magnetic influence on your minds as to excite in them a
strong resolve to study natural philosophy. I should rather be the
gainer by my own utterance, and by the reaction of your fervour. The
magnet also is the gainer by the reaction of the body which it
magnetises.
Look now to your excited piece of steel; figure each molecule with its
opposed fluids spread over its opposite faces. How can this state of
things be permanent? The fluids, by hypothesis, attract each other;
what, then, keeps them apart? Why do they not instantly rush together
across the equator of the atom, and thus neutralise each other? To
meet this question philosophers have been obliged to infer the
existence of a special force, which holds the fluids asunder. They
call it _coercive force_; and it is found that those kinds of steel
which offer most resistance to being magnetised--which require the
greatest amount of 'coercion' to tear their fluids asunder--are the
very ones which offer the greatest resistance to the reunion of the
fluids, after they have been once separated. Such kinds of steel are
most suited to the formation of _permanent_ magnets. It is manifest,
indeed, that without coercive force a permanent magnet would not be at
all possible.
Probably long before this you will have dipped the end of your magnet
among iron filings, and observed how they cling to it; or into a
nail-box, and found how it drags the nails after it. I know very well
that if you are not the slaves of routine, you will have by this time
done many things that I have not told you to do, and thus multiplied
your experience beyond what I have indicated. You are almost sure to
have caused a bit of iron to hang from the end of your magnet, and you
have probably succeeded in causing a second bit to attach itself to
the first, a third to the second; until finally the force has become
too feeble to bear the weight of more. If you have operated with
nails, you may have observed that the points and edges hold together
with the greatest tenacity; and that a bit of iron clings more firmly
to the corner of your magnet than to one of its flat surfaces. In
short, you will in all likelihood have enriched your experience in
many ways without any special direction from me.
Well, the magnet attracts the nail, and the nail attracts a second
one. This proves that the nail in contact with the magnet has had the
magnetic quality developed in it by that contact. If it be withdrawn
from the magnet its power to attract its fellow nail ceases. Contact,
however, is not necessary. A sheet of glass or paper, or a space of
air, may exist between the magnet and the nail; the latter is still
magnetised, though not so forcibly as when in actual contact. The
nail thus presented to the magnet is itself a temporary magnet. That
end which is turned towards the magnetic pole has the opposite
magnetism of the pole which excites it; the end most remote from the
pole has the same magnetism as the pole itself, and between the two
poles the nail, like the magnet, possesses a magnetic equator.
Conversant as you now are with the theory of magnetic fluids, you have
already, I doubt not, anticipated me in imagining the exact condition
of an iron nail under the influence of the magnet. You picture the
iron as possessing the neutral fluid in abundance; you picture the
magnetic pole, when brought near, decomposing the fluid; repelling the
fluid of a like kind with itself, and attracting the unlike fluid;
thus exciting in the parts of the iron nearest to itself the opposite
polarity. But the iron is incapable of becoming a permanent magnet.
It only shows its virtue as long as the magnet acts upon it. What,
then, does the iron lack which the steel possesses? It lacks coercive
force. Its fluids are separated with ease; but, once the separating
cause is removed, they flow together again, and neutrality is
restored. Imagination must be quite nimble in picturing these
changes--able to see the fluids dividing and reuniting, according as
the magnet is brought near or withdrawn. Fixing a definite pole in
your mind, you must picture the precise arrangement of the two fluids
with reference to this pole, and be able to arouse similar pictures in
the minds of your pupils. You will cause them to place magnets and
iron in various positions, and describe the exact magnetic state of
the iron in each particular case. The mere facts of magnetism will
have their interest immensely augmented by an acquaintance with the
principles whereon the facts depend. Still, while you use this theory
of magnetic fluids to track out the phenomena and link them together,
you will not forget to tell your pupils that it is to be regarded as a
symbol merely,--a symbol, moreover, which is incompetent to cover all
the facts, but which does good practical service whilst we are waiting
for the actual truth. [Footnote: This theory breaks down when applied
to diamagnetic bodies which are repelled by magnets. Like soft iron,
such bodies are thrown into a state of temporary excitement, in virtue
of which they are repelled; but any attempt to explain such a
repulsion by the decomposition of a fluid will demonstrate its own
futility.]
The state of excitement into which iron is thrown by the influence, of
a magnet, is sometimes called 'magnetisation by influence.' More
commonly, however, the magnetism is said to be 'induced' in the iron,
and hence this mode of magnetising is called 'magnetic induction.'
Now, there is nothing theoretically perfect in Nature: there is no
iron so soft as not to possess a certain amount of coercive force, and
no steel so hard as not to be capable, in some degree, of magnetic
induction. The quality of steel is in some measure possessed by iron,
and the quality of iron is shared in some degree by steel. It is in
virtue of this latter fact that the unmagnetised darning-needle was
attracted in your first experiment; and from this you may at once
deduce the consequence that, after the steel has been magnetised, the
repulsive action of a magnet must be always less than its attractive
action. For the repulsion is opposed by the inductive action of the
magnet on the steel, while the attraction is assisted by the same
inductive action. Make this clear to your minds, and verify it by
your experiments. In some cases you can actually make the attraction
due to the temporary magnetism overbalance the repulsion due to the
permanent magnetism, and thus cause two poles of the same kind
apparently to attract each other. When, however, good hard magnets
act on each other from a sufficient distance, the inductive action
practically vanishes, and the repulsion of like poles is sensibly
equal to the attraction of unlike ones.
I dwell thus long on elementary principles, because they are of the
first importance, and it is the temptation of this age of unhealthy
cramming to neglect them. Now follow me a little farther. In
examining the distribution of magnetism in your strip of steel you
raised the needle slowly from bottom to top, and found what we called
a neutral point at the centre.
Now does the magnet really exert no influence on the pole presented to
its centre? Let us see.
Let SN, fig. 13, be our magnet, and let n represent a particle of
north magnetism placed exactly opposite the middle of the magnet. Of
course this is an imaginary case, as you can never in reality thus
detach your north magnetism from its neighbour. But supposing us to
have done so, what would be the action of the two poles of the magnet
on n? Your reply will of course be that the pole S attracts n while
the pole N repels it. Let the magnitude and direction of the
attraction be expressed by the line n m, and the magnitude and
direction of the repulsion by the line n o. Now, the particle n being
equally distant from s and N, the line n o, expressing the repulsion,
will be equal to m n, which expresses the attraction. Acted upon by
two such forces, the particle n must evidently move in the direction n
p, exactly midway between m n and n o. Hence you see that, although
there is no tendency of the particle n to move towards the magnetic
equator, there is a tendency on its part to move parallel to the
magnet. If, instead of a particle of north magnetism, we placed a
particle of south magnetism opposite to the magnetic equator, it would
evidently be urged along the line n q; and if, instead of two separate
particles of magnetism, we place a little magnetic needle, containing
both north and south magnetism, opposite the magnetic equator, its
south pole being urged along n q, and its north along n p, the little
needle will be compelled to set itself parallel to the magnet s N.
Make the experiment, and satisfy yourselves that this is a true
deduction.
Substitute for your magnetic needle a bit of iron wire, devoid of
permanent magnetism, and it will set itself exactly as the needle
does. Acted upon by the magnet, the wire, as you know, becomes a
magnet and behaves as such; it will turn its north pole towards p, and
south pole towards q, just like the needle.
But supposing you shift the position of your particle of north
magnetism, and bring it nearer to one end of your magnet than to the
other; the forces acting on the particle are no longer equal; the
nearest pole of the magnet will act more powerfully on the particle
than the more distant one. Let SN, fig. 14, be the magnet, and n the
particle of north magnetism, in its new position. It is repelled by
N, and attracted by S. Let the repulsion be represented in magnitude
and direction by the line n o, and the attraction by the shorter line
n M. The resultant of these two forces will be found by completing the
parallelogram m n o p, and drawing its diagonal n p. Along n p, then,
a particle of north magnetism would be urged by the simultaneous
action of S and N. Substituting a particle of south magnetism for n,
the same reasoning would lead to the conclusion that the particle
would be urged along it q. If we place at n a short magnetic needle,
its north pole will be urged along n p, its south pole along n q, the
only position possible to the needle, thus acted on, being along the
line p q, which is no longer parallel to the magnet. Verify this
deduction by actual experiment.
In this way we might go round the entire magnet; and, considering its
two poles as two centres from which the force emanates, we could, in
accordance with ordinary mechanical principles, assign a definite
direction to the magnetic needle at every particular place. And
substituting, as before, a bit of iron wire for the magnetic needle,
the positions of both will be the same.
Now, I think, without further preface, you will be able' to comprehend
for yourselves, and explain to others, one of the most interesting
effects in the whole domain of magnetism. Iron filings you know are
particles of iron, irregular in shape, being longer in some directions
than in others. For the present experiment, moreover, instead of the
iron filings, very small scraps of thin iron wire might be employed. I
place a sheet of paper over the magnet; it is all the better if the
paper be stretched on a wooden frame as this enables us to keep it
quite level. I scatter the filings, or the scraps of wire, from a
sieve upon the paper, and tap the latter gently, so as to liberate the
particles for a moment from its friction. The magnet acts on the
filings through the paper, and see how it arranges them! They
embrace the magnet in a series of beautiful curves, which are
technically called 'magnetic curves,' or 'lines of magnetic force.'
Does the meaning of these lines yet flash upon you? Set your magnetic
needle, or your suspended bit of wire, at any point of one of the
curves, and you will find the direction of the needle, or of the wire,
to be exactly that of the particle of iron, or of the magnetic curve,
at that point. Go round and round the magnet; the direction of your
needle always coincides with the direction of the curve on which it is
placed. These, then, are the lines along which a particle of south
magnetism, if you could detach it, would move to the north pole, and a
bit of north magnetism to the south pole. They are the lines along
which the decomposition of the neutral fluid takes place. In the case
of the magnetic needle, one of its poles being urged in one direction,
and the other pole in the opposite direction, the needle must
necessarily set itself as a _tangent_ to the curve. I will not seek to
simplify this subject further. If there be anything obscure or
confused or incomplete in my statement, you ought now, by patient
thought, to be able to clear away the obscurity, to reduce the
confusion to order, and to supply what is needed to render the
explanation complete. Do not quit the subject until you thoroughly
understand it; and if you are then able to look with your mind's eye
at the play of forces around a magnet, and see distinctly the
operation of those forces in the production of the magnetic curves,
the time which we have spent together will not have been spent in
vain.
FIG. 15.
In this thorough manner we must master our materials, reason upon
them, and, by determined study, attain to clearness of conception.
Facts thus dealt with exercise an expansive force upon the intellect;
they widen the mind to generalisation. We soon recognise a
brotherhood between the larger phenomena of Nature and the minute
effects which we have observed in our private chambers. Why, we
enquire, does the magnetic needle set north and south? Evidently it
is compelled to do so by the earth; the great globe which we inherit
is itself a magnet. Let us learn a little more about it. By means of
a bit of wax, or otherwise, attach the end of your silk fibre to the
middle point of your magnetic needle; the needle will thus be
uninterfered with by the paper loop, and will enjoy to some extent a
power of dipping' its point, or its eye, below the horizon. Lay your
bar magnet on a table, and hold the needle over the equator of the
magnet. The needle sets horizontal. Move it towards the north end of
the magnet; the south end of the needle dips, the dip augmenting as
you approach the north pole, over which the needle, if free to move,
will set itself exactly vertical. Move it back to the centre, it
resumes its horizontality; pass it on towards the south pole, its
north end now dips, and directly over the south pole the needle
becomes vertical, its north end being now turned downwards. Thus we
learn that on the one side of the magnetic equator the north end of
the needle dips; on the other side the south end dips, the dip varying
from nothing to 90°. If we go to the equatorial regions of the earth
with a suitably suspended needle we shall find there the position of
the needle horizontal. If we sail north one end of the needle dips;
if we sail south the opposite end dips; and over the north or south
terrestrial magnetic pole the needle sets vertical. The south
magnetic pole has not yet been found, but Sir James Ross discovered
the north magnetic pole on June 1, 1831. In this manner we establish
a complete parallelism between the action of the earth and that of an
ordinary magnet.
The terrestrial magnetic poles do not coincide with the geographical
ones; nor does the earth's magnetic equator quite coincide with the
geographical equator. The direction of the magnetic needle in London,
which is called the magnetic meridian, encloses an angle of 24° with
the astronomical meridian, this angle being called the Declination of
the needle for London. The north, pole of the needle now lies to the
west of the true meridian; the declination is westerly. In the year
1660, however, the declination was nothing, while before that time it
was easterly. All this proves that the earth's magnetic constituents
are gradually changing their distribution. This change is very slow:
it is therefore called the secular change, and the observation of it
has not yet extended over a sufficient period to enable us to guess,
even approximately, at its laws.
Having thus discovered, to some extent, the secret of the earth's
magnetic power, we can turn it to account. In the line of 'dip' I
hold a poker formed of good soft iron. The earth, acting as a magnet,
is at this moment constraining the two fluids of the poker to
separate, making the lower end of the poker a north pole, and the
upper end a south pole. Mark the experiment: When the knob is
uppermost, it attracts the north end of a magnetic needle; when
undermost it attracts the south end of a magnetic needle. With such a
poker repeat this experiment and satisfy yourselves that the fluids
shift their position according to the manner in which the poker is
presented to the earth. It has already been stated that the softest
iron possesses a certain amount of coercive force. The earth, at this
moment, finds in this force an antagonist which opposes the
decomposition of the neutral fluid, The component fluids may be
figured as meeting an amount of friction, or possessing an amount of
adhesion, which prevents them from gliding over the molecules of the
poker. Can we assist the earth in this case? If we wish to remove
the residue of a powder from the interior surface of a glass to which
the powder clings, we invert the glass, tap it, loosen the hold of the
powder, and thus enable the force of gravity to pull it down. So also
by tapping the end of the poker we 'loosen the adhesion of the
magnetic fluids to the molecules and enable the earth to pull them
apart. But, what is the consequence? The portion of fluid which has
been thus forcibly dragged over the molecules refuses to return when
the poker has been removed from the line of dip; the iron, as you see,
has become a permanent magnet. By reversing its position and tapping
it again we reverse its magnetism. A thoughtful and competent teacher
will know how to place these remarkable facts before his pupils in a
manner which will excite their interest. By the use of sensible
images, more or less gross, he will first give those whom he teaches
definite conceptions, purifying these conceptions afterwards, as the
minds of his pupils become more capable of abstraction. By thus
giving them a distinct substratum for their reasonings, he will confer
upon his pupils a profit and a joy which the mere exhibition of facts
without principles, or the appeal to the bodily senses and the power
of memory alone, could never inspire.
*****
As an expansion of the note on magnetic fluids, the following extract
may find a place here: 'It is well known that a voltaic current
exerts an attractive force upon a second current, flowing in the same
direction; and that when the directions are opposed to each other the
force exerted is a repulsive one. By coiling wires into spirals,
Ampère was enabled to make them produce all the phenomena of
attraction and repulsion exhibited by magnets, and from this it was
but a step to his celebrated theory of molecular currents. He
supposed the molecules of a magnetic body to be surrounded by such
currents, which, however, in the natural state of the body mutually
neutralised each other, on account of their confused grouping. The
act of magnetisation he supposed to consist in setting these molecular
currents parallel to each other; and, starting from this principle, he
reduced all the phenomena of magnetism to the mutual action of
electric currents.