THEORY AND NATURE OF MAGNETISM.
Iron and steel have a peculiar property called magnetism. It is an
attraction in many ways unlike the attraction of cohesion or the
attraction of gravitation. It is very certain that magnetism is an
inherent property of the molecules of iron and steel, and, to a small
degree, other forms of matter. That is to say, the molecules are little
natural magnets of themselves. It is as unnecessary to inquire why they
are magnets as it is to inquire why the molecules of all ordinary
substances possess the attraction of cohesion. The one is as easy to
explain as the other. People of all ages have insisted upon making a
greater mystery of all electrical and magnetic phenomena than they do of
other natural forces. Ampere's theory is that electric currents are
flowing around the molecules which render them magnetic; but it is just
as easy to suppose that magnetism is an inherent quality of the
molecule. (The word molecule is here used as referring to the smallest
particle of iron.)
These little molecular magnets, so small that 100,000 million million
million of them can be put into a cubic inch of space, have their
attractions satisfied by forming into little molecular rings, with their
unlike poles together, so that when the iron is in a natural or
unmagnetized condition it does not attract other iron. If I should take
a ring of hardened steel and cut it into two or more pieces and
magnetize them, each one of the pieces would be an independent magnet.
If now I put them together in the form of a ring they will cling
together by their mutual attraction for each other. Before I put them
together into a ring each piece would attract and adhere to other pieces
of iron or steel. But as soon as they are put together in the ring they
are satisfied with their own mutual attraction, and the ring as a whole
will not attract other pieces of iron.
Suppose the pieces forming the ring--it may be only two, if you
choose--are as small as the molecules we have described, the same thing
would be true of them. Each molecular ring would have its magnetic
attractions satisfied and would not attract other molecules outside of
its own little circle. When the iron is in the neutral state it will not
as a mass attract another piece of iron, because the millions of little
natural magnets of which it is made up have their attractive force all
turned in upon themselves.
Now, if we make a helix, or coil, of insulated wire and put a piece of
iron into it, and pass a current of electricity through the helix, the
iron becomes a magnet. Why? Because the electric current has the power
to break up these molecular magnetic rings and turn all their like poles
in one direction, so that their attractions are no longer satisfied
among themselves, and with a combined effort they reach outside and
attract any piece of iron that is within reach. In this state we say it
is magnetized. Most people think that we have put something into the
iron, but we have not; we have only developed and made active its
inherent power. It must be kept in mind that it takes power to develop
this magnetic power from its state of neutrality and that something is
never made from nothing. When this power is developed it will do work in
falling back to its natural state. The power is natural to the molecules
of the metal. It is only being exerted in a new direction. The millions
of little natural magnets have been forced to combine their attractions
into one whole and exert it on something outside of themselves. They are
under a strain in this condition, like a bent bow, and there is a
tendency to fly back to the natural position, and if it is soft iron and
not steel, they will fly back as soon as the power that wrenched them
apart and is holding them apart is taken away. This power is the
electric current. Now break the current, and the little natural magnets,
that have been so ruthlessly torn from their home circle attachments,
fly back to them again with the speed of lightning, and the iron rod as
a whole is no longer a magnet. The power to become so under the
electrical strain is in it still--only latent.
The kind of magnet that we have been describing is called an
electromagnet. It is a magnet only so long as the electric current is
passing around it. There is another kind of magnet called a permanent
magnet that will remain a magnet after the current is taken away. The
permanent magnet is made of steel and hardened; then its poles are
placed, to the poles of a powerful magnet, either electro or permanent,
when its molecular rings are wrenched apart and arranged in a polarized
position as heretofore described. Now take it away from the magnet and
it will be found to retain its magnetism. The molecules tend to fly back
the same as those of the soft iron, but they cannot because hardened
steel is so much finer grained than soft iron, and the molecules are so
close together that they are held in position by a friction that is
called its coercive force. The soft iron is comparatively free from this
coercive force, because its molecules are free to move on each other, so
that when they are wrenched out of their natural position they fly back
by their own attractions as soon as the force holding them apart is
taken away. The molecules of hardened steel are unable to fly back,
although they tend to do it just as much as in the iron, and so it is
called a permanent magnet. Its molecules also are under a strain, like a
bent bow. (The form of such a magnet is usually that of a horse-shoe, or
U.)
Let us use a homely illustration that may help us to understand. Let ten
boys represent the molecules in a piece of iron. Let them pair off into
five pairs and each one clasp his mate in his arms; each one, say, is
exerting a force of ten pounds, and it would require a force of twenty
pounds to pull any one of the pairs apart. The five pairs are exerting a
force of one hundred pounds, but this force is not felt outside of
themselves. Now let them unclasp themselves and take hold of a rope that
is tied to a post, and all pull with the same force that they were
using, to wit, ten pounds each, and all pull in the same direction, and
they would put a strain of one hundred pounds upon the post, the same
power that they were exerting upon themselves before they combined their
efforts on something outside of themselves. So with the magnet. So long
as the force of each molecule is wholly spent upon its neighbor there is
nothing left for exterior use. But as soon as they all line up and pull
conjointly in the same direction their combined force is felt outside.
The analogy may not be perfect, but it will help you to get a mental
picture of what takes place in iron when it is magnetized.
We have now described the magnet and the inherent power residing in the
molecular structure of iron. It is this magic power slumbering in its
molecules and the ability of the electric current to arouse them to
action at will and to hold them in action and at will let them fly back
to their normal position, that gives to electricity and magnetism--twin
sisters in nature's household--their great value as the servants of man.
There would be no virtue in winding up a weight if it could not run down
and do work in its fall. Simply bending a bow would never send the arrow
flying over its course; it must be released as well. The magnet could
not accomplish the great work it does if we could only charge it and not
have the ability to discharge it. Without this ability the electric
motor would not revolve, the electric light would not burn, the click of
the telegraph would not be heard, the telephone would not talk, nor
would the telautograph write.
I have said that the permanent magnet would hold its charge after once
having been magnetized. This is true only in a sense and under favorable
conditions. If made of the best of steel for the purpose and hardened
and tempered in just the right way, it will hold its charge if it is
given something to do. If a piece of iron is placed across its poles it
also becomes a magnet and its molecules turn and work in harmony with
those of the mother magnet. These magnetic lines of force reach around
in a circuit. Even before the iron, or "keeper," as it is called, is put
across its poles there are lines of force reaching around through the
air or ether from one pole to another. (For a description of Ether see
Chap. V.) This is called the "field" of the magnet, and when the iron is
placed in this field the lines of force pass through it in a closed
circuit, and if the "keeper" is large enough to take care of all the
lines of force in the field the magnet will not attract other bodies,
because its attraction is satisfied, like its prototype in the molecular
ring described above.
We speak of lines of force, not that force is necessarily exerted in a
bundle of lines but as a convenient way of telling the strength of a
magnetic field. The practical limit of the magnetization of soft iron
(called saturation) is 18,000 lines to the square centimeter. As long as
we give our magnet something to do, up to the measure of its capacity,
it will keep up its power. We may make other magnets with it, thousands,
yea, millions of them, and it not only does not lose its power but may
be even stronger for having done this work. If, however, we hang it up
without its "keeper," and give it nothing to do, it gradually returns to
its natural condition in the home circle of molecular rings. Little by
little the coercive force is overcome by the constant tendency of the
molecule to go back to its natural position among its fellows.
The magnet furnishes many beautiful lessons, as indeed do all the
natural phenomena. Every man has within him a latent power that needs
only to be aroused and directed in the right way to make his influence
felt upon his fellows. Like the magnet, the man who uses his power to
help his fellows up to the measure of his limitations not only has been
a benefactor to his race, but is himself a stronger and better man for
having done so. But, again, like the magnet, if he allows these
God-given powers to lie still and rust for want of legitimate use he
gradually loses the power he had and becomes simply a moving thing
without influence or use in a world in which he vegetates. But let us
leave philosophy and go back to science.
One of the striking exhibitions of magnetism is found in the earth. The
earth itself is a great magnet; and there is good reason for believing
that it is an electromagnet of great power. The magnetic poles of the
earth are not exactly coincident with the geographical poles, and they
are not constant. There is a gradual deviation going on, but as it
follows a certain law mariners are able to tell just what the deviation
should be at a certain time. The magnetic pole revolves around the polar
axis of the earth once in about 320 years. A thermal current (one
produced by heat) of electricity seems to flow around the earth caused
by the irregularities of temperature at the earth's surface, as the sun
makes his daily round. These earth currents vary at times, and other
phenomena are the occasion. This will be discussed when we come to
electric storms.
The value of the earth's magnetism is seen most in the science of
navigation. A magnetic needle is only a slender permanent magnet
suspended very delicately, and when not under local influence it points
north and south on the magnetic axis. The law of its action may be
explained as follows: Take a straight bar magnet of fairly good power
and suspend a magnetic needle over it. The needle will arrange itself
parallel to the bar magnet. The north pole of the needle will point
toward the south pole of the bar magnet. In the presence of the magnet
the needle is not affected by the earth, but yields to a superior force.
If, however, the bar magnet is taken out of the way of the needle it
will immediately arrange itself north and south. Of course if the
earth's magnetic axis changes the needle will vary with it. This
variation is uniform and in navigation is reduced to a science, so that
the mariner knows how much to allow for the variation. Columbus, as
heretofore mentioned, was supposed to have first noticed this variation
and it made him trouble. He did not know how to account for it, and as
his crew thought the laws of nature were changing because they were so
far from home he saw the necessity for some sort of explanation. So,
like the brave man that he was, he hatched up a theory that satisfied
the crew, and although in the light of the closing years of the
nineteenth century it was a questionable one, it worked well enough in
practice to serve his purpose.
We have already stated that the earth was a great magnet, and that
probably it was an electromagnet, caused by earth currents circulating
around the globe. You want to know how the earth can be a magnet unless
it has an iron core like an electromagnet. Magnetism or magnetic lines
of force may be developed without the presence of iron. When we pass a
current of electricity through a wire, magnetic lines of force are
thrown out at right angles with the direction of the current. This will
be fully explained further on. If we wind the wire into a coil, or
helix, these magnetic lines are concentrated. If now we suspend this
helix, or, better, float it on water so that it can move freely, and
pass a current of electricity through it, the helix will arrange itself
north and south the same as a magnetic needle. Its attractive properties
are feeble in comparison with that of the iron, but it obeys the laws of
a magnet. The earth is probably a magnet of this kind, consisting mostly
of lines of force.
However, the iron in the earth is affected magnetically, as we have
evidence in the loadstone. The earth has the power also to magnetize
iron through the medium of its magnetic field, that reaches out in lines
of force from pole to pole like those of the artificial magnet. If we
hold a bar of iron in line with the magnetic axis of the earth and dip
it in line with the dipping needle and then strike it a few blows on the
end, it will be found to be feebly magnetic. The blows have partly
loosened the molecules and during the moment that they unclasped
themselves the earth's magnetism has through its lines of force caught
them for a time and held them a little out of their natural position--as
they are in a state of rest. The peculiar changing light that we
sometimes see in the northern sky, that is called the Aurora Borealis
(Northern Light), is indirectly due to intense magnetic lines of force
that radiate from the north magnetic pole of the earth. Those lines of
force are able to cause the rarified air molecules to become feebly
incandescent, giving them the appearance that we see in a tube that is a
partial vacuum when electricity is passed through it. While these
auroral displays may be seen almost any night in the far north, they
vary greatly in their intensity, so it is only once in a while that they
are visible in the temperate latitudes.
What are called magnetic storms occur occasionally, and at such times
the telegraph service will sometimes be paralyzed on all the east and
west lines for many hours. Strong earth-currents will flow east and
west, and be so powerful and so erratic that it is sometimes impossible
to use the telegraph. It sometimes happens that the operators can throw
off their batteries and work on the earth-current alone. Sometimes it is
necessary to make a complete metallic circuit to get away from the
influence of the earth in order to use the telegraph. Currents equal to
the force of 2,000 cells of ordinary battery have been developed
sometimes in telegraph wires. This of course is a mere fraction of what
is passing through the earth under the wire through which the current
flowed. On the 17th and 18th of November, 1882, a magnetic storm
occurred that extended around the globe, as it was felt wherever there
were telegraph wires. These magnetic storms are attended by brilliant
displays of the aurora, and this fact strengthens the theory that the
earth is a great electromagnet; for the stronger the electrical current
the more powerful we should expect the magnetism to be, and this is
shown by the action of the magnetic needle at such times. The stronger
the magnet the more intense will be the lines of force, and naturally
the more intense the light, if indeed these lines of force are the cause
of the light. There is evidently some close relation between the two.
Another coincidence is that at the times of these storms there is an
unusual display of sun-spots. These sun-spots seem to be great holes
that have been blown through the photosphere of the sun. The photosphere
is a great luminous body of gaseous matter that is believed to envelop
the sun, so that we do not see the core of the sun unless it is when we
look into one of these spots. In some way, evidently, the sun affects
the earth by radiating magnetic lines of force which are cut by the
earth's revolution, and so creating currents of electricity. The sun is
the field-magnet, and the earth is the revolving armature of nature's
great dynamo-electric machine. It would seem that the radiant energy
that comes out through these spots or these holes in the sun's envelope,
are more potent to develop earth-currents than the ordinary rays; and
so, when for a brief while in the revolution of the earth about the sun,
these extra potent rays strike the earth, an unusual energy is
developed, and these unusual phenomena are the consequence. These
phenomena seem to occur periodically; some years (about eleven)
intervening.
All the forces and phenomena of nature are thus seen to be in a state of
unrest. And it is to this unrest, which does not stop with visible
things, but pervades even the atoms of matter throughout the universe,
that we are indebted for the ability to carry on all the activities of
life, and for life itself. For universal quiet would mean universal
death. The cyclone and tornado that devastate and strike terror to a
whole region are only eccentricities of nature when she is setting her
house to rights. The play of natural forces has disturbed her
equilibrium, and she is but making an effort to restore it.