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.
Ampère’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.