MAGNETS AND MAGNETISM
In many parts of the world there is to be found a kind of iron ore,
some specimens of which have the peculiar power of attracting iron,
and of turning to the north if suspended freely. This is called the
_lodestone_, and it has been known from very remote times. The name
Magnetism has been given to this strange property of the lodestone, but
the origin of the name is not definitely known. There is an old story
about a shepherd named Magnes, who lived in Phrygia in Asia Minor. One
day, while tending his sheep on Mount Ida, he happened to touch a dark
coloured rock with the iron end of his crook, and he was astonished and
alarmed to find that the rock was apparently alive, for it gripped his
crook so firmly that he could not pull it away. This rock is said to
have been a mass of lodestone, and some people believe that the name
magnet comes from the shepherd Magnes. Others think that the name is
derived from Magnesia, in Asia Minor, where the lodestone was found in
large quantities; while a third theory finds the origin in the Latin
word _magnus_, heavy, on account of the heavy nature of the lodestone.
The word lodestone itself comes from the Saxon _laeden_, meaning to
lead.
It is fairly certain that the Chinese knew of the lodestone long before
Greek and Roman times, and according to ancient Chinese records this
knowledge extends as far back as 2600 B.C. Humboldt, in his _Cosmos_,
states that a miniature figure of a man which always turned to the
south was used by the Chinese to guide their caravans across the plains
of Tartary as early as 1000 B.C. The ancient Greek and Roman writers
frequently refer to the lodestone. Thales, of whom we spoke in Chapter
I., believed that its mysterious power was due to the possession of a
soul, and the Roman poet Claudian imagined that iron was a food for
which the lodestone was hungry. Our limited space will not allow of an
account of the many curious speculations to which the lodestone has
given rise, but the following suggestion of one Famianus Strada, quoted
from Houston’s _Electricity in Every-Day Life_, is really too good to
be omitted.
“Let there be two needles provided of an equal Length and Bigness,
being both of them touched by the same lodestone; let the Letters of
the Alphabet be placed on the Circles on which they are moved, as the
Points of the Compass under the needle of the Mariner’s Chart. Let the
Friend that is to travel take one of these with him, first agreeing
upon the Days and Hours wherein they should confer together; at which
times, if one of them move the Needle, the other Needle, by Sympathy,
will move unto the same letter in the other instantly, though they are
never so far distant; and thus, by several Motions of the Needle to the
Letters, they may easily make up any Words or Sense which they have a
mind to express.” This is wireless telegraphy in good earnest!
The lodestone is a natural magnet. If we rub a piece of steel with a
lodestone we find that it acquires the same properties as the latter,
and in this way we are able to make any number of magnets, for the
lodestone does not lose any of its own magnetism in the process. Such
magnets are called artificial magnets. Iron is easier to magnetize than
steel, but it soon loses its magnetism, whereas steel retains it; and
the harder the steel the better it keeps its magnetism. Artificial
magnets, therefore, are made of specially hardened steel. In this
chapter we shall refer only to steel magnets, as they are much more
convenient to use than the lodestone, but it should be remembered that
both act in exactly the same way. We will suppose that we have a pair
of bar magnets, and a horse-shoe magnet, as shown in Fig. 13.
[Illustration: FIG. 13.--Horse-shoe and Bar Magnets, with Keepers.]
If we roll a bar magnet amongst iron filings we find that the filings
remain clinging to it in two tufts, one at each end, and that few or
none adhere to the middle. These two points towards which the filings
are attracted are called the poles of the magnet. Each pole attracts
filings or ordinary needles, and one or two experiments will show that
the attraction becomes evident while the magnet is still some little
distance away. If, however, we test our magnet with other substances,
such as wood, glass, paper, brass, etc., we see that there is no
attraction whatever.
If one of our bar magnets is suspended in a sort of stirrup of copper
wire attached to a thread, it comes to rest in a north and south
direction, and it will be noticed that the end which points to the
north is marked, either with a letter N or in some other way. This is
the north pole of the magnet, and of course the other is the south
pole. If now we take our other magnet and bring its north pole near
each pole of the suspended magnet in turn, we find that it repels the
other north pole, but attracts the south pole. Similarly, if we present
the south pole, it repels the other south pole, but attracts the north
pole. From these experiments we learn that both poles of a magnet
attract filings or needles, and that in the case of two magnets unlike
poles attract, but similar poles repel one another. It will be noticed
that this corresponds closely with the results of our experiments
in Chapter I., which showed that an electrified body attracts
unelectrified bodies, such as bits of paper or pith balls, and that
unlike charges attract, and similar charges repel each other. So far as
we have seen, however, a magnet attracts only iron or steel, whereas
an electrified body attracts any light substance. As a matter of fact,
certain other substances, such as nickel and cobalt, are attracted by a
magnet, but not so readily as iron and steel; while bismuth, antimony,
phosphorus, and a few other substances are feebly repelled.
The simplest method of magnetizing a piece of steel by means of one of
our bar magnets is the following: Lay the steel on the table, and draw
one pole of the magnet along it from end to end; lift the magnet clear
of the steel, and repeat the process several times, always starting at
the same end and treating each surface of the steel in turn. A thin,
flat bar of steel is the best for the purpose, but steel knitting
needles may be made in this way into useful experimental magnets.
We have seen that a magnet has two poles or points where the magnetism
is strongest. It might be thought that by breaking a bar magnet in the
middle we should get two small bars each with a single pole, but this
is not the case, for the two poles are inseparable. However many pieces
we break a magnet into, each piece is a perfect magnet having a north
and south pole. Thus while we can isolate a positive or a negative
charge of electricity, we cannot isolate north or south magnetism.
If we place the north pole of a bar magnet near to, but not touching, a
bar of soft iron, as in Plate II. _a_, we find that the latter becomes
a magnet, as shown by its ability to support filings; and that as
soon as the magnet is removed the filings drop off, showing that the
iron has lost its magnetism. If the iron is tested while the magnet
is in position it is found to have a south pole at the end nearer the
magnet, and a north pole at the end farther away; and if the magnet
is reversed, so as to bring its south pole nearer the iron, the poles
of the latter are found to reverse also. The iron has gained its new
properties by magnetic induction, and we cannot fail to notice the
similarity between this experiment and that in Fig. 2, Chapter II.,
which showed electro-static induction. A positively or a negatively
electrified body induces an opposite charge at the nearer end, and a
similar charge at the further end of a conductor, and a north or a
south pole of a magnet induces opposite polarity at the nearer end, and
a similar polarity at the further end of a bar of iron. In Chapter II.
we showed that the attraction of a pith ball by an electrified body
was due to induction, and from what we have just learnt about magnetic
induction the reader will have no difficulty in understanding why a
magnet attracts filings or needles.
[Illustration: PLATE II.
(_a_) EXPERIMENT TO SHOW MAGNETIC INDUCTION.]
[Illustration: (_b_) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY
AN ELECTRIC CURRENT.]
Any one who experiments with magnets must be struck with the distance
at which one magnet can influence filings or another magnet. If a layer
of iron filings is spread on a sheet of paper, and a magnet brought
gradually nearer from above, the filings soon begin to move about
restlessly, and when the magnet comes close enough they fly up to it
as if pulled by invisible strings. A still more striking experiment
consists in spreading filings thinly over a sheet of cardboard and
moving a magnet to and fro underneath the sheet. The result is most
amusing. The filings seem to stand up on their hind legs, and they
march about like regiments of soldiers. Here again invisible strings
are suggested, and we might wonder whether there really is anything of
the kind. Yes, there is. To put the matter in the simplest way, the
magnet acts by means of strings or lines of force, which emerge from it
in definite directions, and in a most interesting way we can see some
of these lines of force actually at work.
Place a magnet, or any arrangement of magnets, underneath a sheet of
glass, and sprinkle iron filings from a muslin bag thinly and evenly
all over the glass. Then tap the glass gently with a pencil, and the
filings at once arrange themselves in a most remarkable manner. All the
filings become magnetized by induction, and when the tap sets them free
for an instant from the friction of the glass they take up definite
positions under the influence of the force acting upon them. In this
way we get a map of the general direction of the magnetic lines of
force, which are our invisible strings.
Many different maps may be made in this way, but we have space for only
two. Plate III. _a_ shows the lines of two opposite poles. Notice how
they appear to stream across from one pole to the other. It is believed
that there is a tension along the lines of force not unlike that in
stretched elastic bands, and if this is so it is easy to see from the
figure why opposite poles attract each other.
Plate III. _b_ shows the lines of force of two similar poles. In
this case they do not stream from pole to pole, but turn aside as
if repelling one another, and from this figure we see why there is
repulsion between two similar poles. It can be shown, although in a
much less simple manner, that lines of electric force proceed from
electrified bodies, and in electric attraction and repulsion between
two charged bodies the lines of force take paths which closely resemble
those in our two figures. A space filled with lines of magnetic force
is called a _magnetic field_, and one filled with lines of electric
force is called an _electric field_.
A horse-shoe magnet, which is simply a bar of steel bent into the shape
of a horse-shoe before being magnetized, gradually loses its magnetism
if left with its poles unprotected, but this loss is prevented if the
poles are connected by a piece of soft iron. The same loss occurs with
a bar magnet, but as the two poles cannot be connected in this way it
is customary to keep two bar magnets side by side, separated by a strip
of wood; with opposite poles together and a piece of soft iron across
the ends. Such pieces of iron are called _keepers_, and Fig. 13 shows a
horse-shoe magnet and a pair of bar magnets with their keepers. It may
be remarked that a magnet never should be knocked or allowed to fall,
as rough usage of this kind causes it to lose a considerable amount of
its magnetism. A magnet is injured also by allowing the keeper to slam
on to it; but pulling the keeper off vigorously does good instead of
harm.
If a magnetized needle is suspended so that it is free to swing either
horizontally or vertically, it not only comes to rest in a north
and south direction, but also it tilts with its north-pointing end
downwards. If the needle were taken to a place south of the equator it
would still tilt, but the south-pointing end would be downwards. In
both cases the angle the needle makes with the horizontal is called the
_magnetic dip_.
[Illustration: PLATE III.
(_a_) LINES OF MAGNETIC FORCE OF TWO OPPOSITE POLES.
(_b_) LINES OF MAGNETIC FORCE OF TWO SIMILAR POLES.]
It is evident that a suspended magnetized needle would not invariably
come to rest pointing north and south unless it were compelled to do
so, and a little consideration shows that the needle acts as if it
were under the influence of a magnet. Dr. Gilbert of Colchester, of
whom we spoke in Chapter I., gave a great deal of time to the study
of magnetic phenomena, and in 1600 he announced what may be regarded
as his greatest discovery: _The terrestrial globe itself is a great
magnet_. Here, then, is the explanation of the behaviour of the
magnetized needle. The Earth itself is a great magnet, having its poles
near to the geographical north and south poles. But a question at once
suggests itself: “Since similar poles repel one another, how is it that
the north pole of a magnet turns towards the north magnetic pole of the
earth?” This apparent difficulty is caused by a confusion in terms. If
the Earth’s north magnetic pole really has north magnetism, then the
north-pointing end of a magnet must be a south pole; and on the other
hand, if the north-pointing end of a magnet has north magnetism, then
the Earth’s north magnetic pole must be really a south pole. It is a
troublesome matter to settle, but it is now customary to regard the
Earth’s north magnetic pole as possessing south magnetism, and the
south magnetic pole as possessing north magnetism. In this way the
north-pointing pole of a magnet may be looked upon as a true north
pole, and the south-pointing pole as a true south pole.
Magnetic dip also is seen to be a natural result of the Earth’s
magnetic influence. Here in England, for instance, the north magnetic
pole is much nearer than the south magnetic pole, and consequently its
influence is the stronger. Therefore a magnetized needle, if free to
do so, dips downwards towards the north. At any place where the south
magnetic pole is the nearer the direction of the dip of course is
reversed. If placed immediately over either magnetic pole the needle
would take up a vertical position, and at the magnetic equator it would
not dip at all, for the influence of the two magnetic poles would be
equal. A little study of Fig. 14, which represents a dipping needle
at different parts of the earth, will make this matter clearer. N and
S represent the Earth’s north and south magnetic poles, and the arrow
heads are the north poles of the needles.
[Illustration: FIG. 14.--Diagram to illustrate Magnetic Dip.]
Since the Earth is a magnet, we should expect it to be able to induce
magnetism in a bar of iron, just as our artificial magnets do, and
we can show that this is actually the case. If a steel poker is held
pointing to and dipping down towards the north, and struck sharply
with a piece of wood while in this position, it acquires magnetic
properties which can be tested by means of a small compass needle. It
is an interesting fact that iron pillars and railings which have been
standing for a long time in one position are found to be magnetized.
In the northern hemisphere the bases of upright iron pillars are north
poles, and their upper ends south poles, and in the southern hemisphere
the polarity is reversed.
The most valuable application of the magnetic needle is in the compass.
An ordinary pocket compass for inland use consists simply of a single
magnetized needle pivoted so as to swing freely over a card on which
are marked the thirty-two points of the compass. Ships’ compasses are
much more elaborate. As a rule a compound needle is used, consisting of
eight slender strips of steel, magnetized separately, and suspended
side by side. A compound needle of this kind is very much more reliable
than a single needle. The material of which the card is made depends
upon whether the illumination for night work is to come from above or
below. If the latter, the card must be transparent, and it is often
made of thin sheet mica; but if the light comes from above, the card is
made of some opaque material, such as very stout paper. The needle and
card are contained in a sort of bowl made of copper. In order to keep
this bowl in a horizontal position, however the ship may be pitching
and rolling, it is supported on gimbals, which are two concentric rings
attached to horizontal pivots, and moving in axes at right angles to
one another. Further stability may be obtained by weighting the bottom
of the bowl with lead. There are also liquid compasses, in which the
card is floated on the surface of dilute alcohol, and many modern
ships’ compasses have their movements regulated by a gyrostat.
The large amount of iron and steel used in the construction of modern
vessels has a considerable effect upon the compass needle, and unless
the compass is protected from this influence its readings are liable
to serious errors. The most satisfactory way of giving this protection
is by placing on each side of the compass a large globe of soft iron,
twelve or more inches in diameter.
On account of the fact that the magnetic poles of the Earth do not
coincide with the geographical north and south poles, a compass
needle seldom points exactly north and south, and the angle between
the magnetic meridian and the geographical meridian is called the
_declination_. The discovery that the declination varies in different
parts of the world was made by Columbus in 1492. For purposes of
navigation it is obviously very important that the declination at all
points of the Earth’s surface should be known, and special magnetic
maps are prepared in which all places having the same declination are
joined by a line.
It is an interesting fact that the Earth’s magnetism is subject to
variation. The declination and the dip slowly change through long
periods of years, and there are also slight annual and even daily
variations.
At one time magnets were credited with extraordinary effects upon
the human body. Small doses of lodestone, ground to powder and mixed
with water, were supposed to prolong life, and Paracelsus, a famous
alchemist and physician, born in Switzerland in 1493, believed in the
potency of lodestone ointment for wounds made with steel weapons. Baron
Reichenbach, 1788–1860, believed that he had discovered the existence
of a peculiar physical force closely connected with magnetism, and he
gave this force the name _Od_. It was supposed to exist everywhere,
and, like magnetism, to have two poles, positive and negative; the left
side of the body being od-positive, and the right side od-negative.
Certain individuals, known as “sensitives,” were said to be specially
open to its influence. These people stated that they saw strange
flickering lights at the poles of magnets, and that they experienced
peculiar sensations when a magnet was passed over them. Some of them
indeed were unable to sleep on the left side, because the north pole of
the Earth, being od-negative, had a bad effect on the od-negative left
side. The pretended revelations of these “sensitives” created a great
stir at the time, but now nobody believes in the existence of _Od_.
Professor Tyndall was once invited to a seance, with the object of
convincing him of the genuineness of spiritualism. He sat beside a
young lady who claimed to have spiritualistic powers, and his record of
his conversation with her is amusing. The Reichenbach craze was in full
swing at the time, and Tyndall asked if the lady could see any of the
weird lights supposed to be visible to “sensitives.”
“_Medium._--Oh yes; but I see the light around all bodies.
_I._--Even in perfect darkness?
_Medium._--Yes; I see luminous atmospheres round all people. The
atmosphere which surrounds Mr. R. C. would fill this room with
light.
_I._--You are aware of the effects ascribed by Baron Reichenbach to
magnets?
_Medium._--Yes; but a magnet makes me terribly ill.
_I._--Am I to understand that, if this room were perfectly dark, you
could tell whether it contained a magnet, without being informed
of the fact?
_Medium._--I should know of its presence on entering the room.
_I._--How?
_Medium._--I should be rendered instantly ill.
_I._--How do you feel to-day?
_Medium._--Particularly well; I have not been so well for months.
_I._--Then, may I ask you whether there is, at the present moment, a
magnet in my possession?
The young lady looked at me, blushed, and stammered, ‘No; I am not
_en rapport_ with you.’
_I sat at her right hand, and a left-hand pocket, within six inches
of her person, contained a magnet._”
Tyndall adds, “Our host here deprecated discussion as it ‘exhausted the
medium.’”