MAGNETISM.
THE LOADSTONE—MAGNETIC CURVES—THE MAGNETIC NEEDLE—THE MARINER’S
COMPASS—MAGNETO-ELECTRICITY.
We have already mentioned some of the properties of the loadstone
or magnet; but as we are now about to enter more fully into the
considerations of its attributes and of the compass, etc., we will add
some further interesting particulars.
Ancient writers (Pliny, Homer, and Aristotle) mentioned the existence
of the magnet, and Humboldt refers to the knowledge of it possessed by
the ancients. Pliny says “the magnet-stone is found in Cantabria,” and
we have heard of the loadstones that are supposed to support Mahomet’s
coffin at Medina. The origin of this fable was (probably) owing to the
order given by Ptolemy to his architect, Dinochares. Ptolemy wished
the roof of a temple at Alexandria to be roofed with the magnet-stone,
so that the own image of his sister, Arsinoe, should remain suspended
therein. But the death of the king and his architect prevented the
project from being carried out.
The name “magnet” is said to have been derived from a shepherd named
MAGNES, who, when tending his flock on Mount Ida, found that
his iron crook was attracted to a certain stone; and six hundred
years before the Christian era Thales wrote respecting amber and the
magnet; and because they attracted various substances, he supposed they
possessed life and power. They were the germs of the science now so
developed in their applications, and whose full powers we are scarcely
yet acquainted with.
We may remark that other bodies besides iron and steel are capable of
magnetization; nickel and cobalt have the like property. Magnetism,
properly so called, treats of certain bodies known as MAGNETS,
describes the properties they possess, and the influence of magnetic
force upon other substances. Electricity and magnetism are always
associated, but practically the force is the same, electricity being
the current or motive power, so to speak; and it is to Faraday that the
world is indebted for the discovery of magneto-electricity.
Epinus’ theory of magnetism was that all bodies possessed a substance
he termed magnetic fluid, the particles of which repelled each other.
But while supposed to repel each other, they attracted particles in
other bodies. Thus they attracted iron. Coulomb asserted that there
were two fluids—a north and south fluid. Ampère’s theory was that
magnetic bodies are made up of molecules, round which currents are
always circulating in all directions when non-magnetized, but when
magnetized the currents all flow in the same direction. The space
through which a magnet diffuses its influence was called by Faraday
the _Magnetic Field_. The lines of _magnetic force_ will be understood
from the accompanying illustration (fig. 263). If we cover a magnet
with a paper and scatter iron filings over it, we shall see the manner
in which the filings arrange themselves. They radiate in curves from
the poles of the magnet, and are dependent upon its form. If it be a
straight bar magnet, evenly magnetized, they will turn inward in oval
curves.
[Illustration: Fig. 263.—Lines of magnetic force.]
[Illustration: Fig. 264.—Magnetization.]
The manner of magnetization has already been mentioned, but here we
will give further illustrations of the method of _magnetization_. Four
magnets are used, two being placed with their opposite poles apart,
and upon them is placed the bar of which a magnet is to be made. Two
other magnets separated by a piece of wood are then brought near, and
subsequently drawn from the centre to the ends of the bar. This is the
_separated touch_ system; the _double touch_ of Mitchell is completed
by moving the upper two magnets from end to end backwards and forwards,
and finally lifting them away from the centre.
A magnet, then, is a bar of steel endowed with certain properties, such
as attracting iron, etc.; and electro-magnetism is the term applied to
the production of magnetism by means of electricity, the medium being
the electro-magnet. To understand the science it will be necessary to
mention Ampère’s theory of magnetism.
It was Œrsted who observed that when a magnet is placed within reach of
an electric current and free to move, it sets itself at right angles
to the direction of the current; and Ampère defined the law already
referred to when treating of the electric current,—viz., “that if a
person be imagined as placed in the wire so that the current shall pass
through him from feet to head, if he turn his face toward the magnetic
needle the north pole will always be deflected to his left-hand side.”
When the current is passed above the needle from south to north poles,
the deflection is to the west; when from north to south the deflection
is east. When the current is below the needle the contrary is the
case. Ampère decided that currents circulating in the same direction
attracted each other, and when running in opposite ways they repelled
each other. He supposed currents to circulate within all magnetic
substances, and then—that is, when the body is magnetized—these
currents flow in planes parallel to each other, and the material which
offers the least resistance to the circulation of these currents
becomes the most magnetic.
The earth being supposed to be an immense magnet has currents
circulating through it in a direction from east to west; and having the
property or power of turning a magnetized bar in a direction similar
to that in which the bar would be turned by a magnet, the earth is
considered a magnetic mass. This influence is due to what is called
“terrestrial magnetism.” If we suspend a bar by a thread it will point
in no particular direction, but may be turned towards any side we
please. But when once the needle is magnetized it will point north and
south; or, as we say (but not correctly), the north pole of the magnet
points to the north of the globe. It is really the south pole that
points to the north, and the north pole of the magnet points _south_,
as can be proved by suspending the bar over another magnetized bar. So
if the earth be considered a magnet our English terms are inconsistent
with our theories. Continental writers are more correct.
The line of the magnetic needle’s direction, which differs in different
places, is called the magnetic meridian, and the amount of its
divergence from the astronomical meridian is termed its declination or
variation. When the amount of this variation is known it is allowed
for, and the needle can be considered as pointing due north and south.
But the needle does not assume a position perfectly parallel to the
horizon. It dips down in different hemispheres. As we approach the
north pole the dip or inclination will become greater, and the same
effect is observable at the south pole. Again, there are certain places
on the earth where the attraction is so evenly balanced that the needle
is perfectly horizontal. The line uniting these places is the magnetic
equator. This does not coincide with the earth’s equator any more than
the magnetic poles coincide with the geographical poles of the earth.
The declination of the needle varies from the meridian of Greenwich
at different times. If we travel to the west the variation increases
westerly, and is greatest in the Atlantic. It then decreases; the
needle points due north in North America. Going still forward the
variation becomes easterly, increases, and decreases to nothing in
Eastern Russia. Thence the variations are westerly. Columbus discovered
the variation of the compass needle in September 1493. In places where
the needle is due north and south the lines drawn through them are
termed lines of no variation.
The variation, however, is not always the same in the same place. In
the year 1580, in London, for instance, the variation was 11° 11´ E.
A little more than one hundred years later London was on the line of
no variation, and now the tendency is westerly. On the other hand,
there are places where there is no deviation, and Sir John Herschell
says that West India property has been saved from litigation in
consequence of the invariability of magnetic declination there, for all
surveys were made by the compass. Lines of equal variation are called
isogonial; those of equal dip or inclination, isoclinical; and those of
equal intensity, isodynamical.
As we have said, the magnetic elements are not always the same, and the
variations of the compass are daily and annually observed with certain
instruments. What are termed secular variations take place at long
intervals, as the following table will show:—
In 1576 the angle of declination in England was 11° 15´ East.
1622 ” ” 6° 12´ ”
1660 ” ” 0° ” ”
1730 ” ” 13° West.
1760 ” ” 19° 30´ ”
1800 ” ” 24° 36´ ”
1818 ” ” 24° 41´ ”
1850 ” ” 22° 29´ ”
1870 ” ” 19° 55´ ”
1873 ” ” 19° 58´ ”
1879 ” ” 19° 7´ ”
In the year 1818 therefore the maximum declination was reached in
London. In Paris the maximum was arrived at in 1814, and was 22° 34´.
The rate of decrease is about 8´ a year, but varies in different
periods, as may be seen. The discovery of the fact that an annual
variation took place in the angle of declination, is attributable to
Cassini, and the diurnal variation was discovered by Graham in 1722.
From 8 o’clock in the morning, when the needle is pointing a little
to the east of its “mean position,” it turns towards the west until 1
p.m. It then returns towards the east again, and passing westerly again
between midnight and three o’clock a.m., settles down till eight a.m.,
when it begins afresh. This variation does not apply to all places.
Magnetic inclination is besides subject to changes. There are also
variations of magnetic force which occur at very irregular periods,
and cannot be said to follow any laws. These disturbances are called
Magnetic Storms, of which the Aurora Borealis is one result.
Professor Faraday in his memorable experiments divided a long list
of different substances into para-magnetic and dia-magnetic bodies.
He classed them under these two heads, according as they took up a
certain position parallel or perpendicular to the axial or equatorial
line. This definition of “dia-magnetic” was “a body through which the
lines of magnetic force are passing, and which does not by their action
assume the usual magnetic state of iron or loadstone.” He concluded
that all bodies were magnetic, and by suspending a great number of
various substances he found they placed themselves axially,—that is,
lying between the poles of the magnet, or equatorially,—viz., at right
angles to that line. If the magnets be suspended at each side the same
bodies will assume a position with their longest diameters between the
poles, while others will be repelled by the magnets even if the poles
be reversed. So those bodies which are attracted and lie in the axial
line are termed para-magnetic; those repelled into the equatorial line
are termed dia-magnetic. In the “Proceedings of the Royal Society for
1846,” Faraday’s account of the various experiments can be studied in
detail. We can only give a brief _resumé_ of them here; and he showed
that the motions displayed by dia-magnetic bodies in a magnetic field
are all reducible to one simple law—viz., that the particles of the
dia-magnetic tend to move into the positions of the weakest magnetic
force.
He experimented upon a large number of bodies and gases; he tested
crystals, metals, liquids, and solids, and proved in whatever state a
body might be in the effect was the same; whether simple or compound,
it made no difference. Of course in a compound the preponderance of the
dia-magnetic or para-magnetic property would influence the result, and
the medium in which the body operated on was placed, was a condition
in the experiment. He proved that if a body be suspended in a medium
or surrounded by a medium whose power either way is stronger than the
body, that body is para-magnetic or dia-magnetic, according as it is
surrounded by a medium whose power is weaker or stronger than the
body itself. The arrangement of the bodies is as follows, from the
para-magnetic to the dia-magnetic, bismuth being the most dia-magnetic
of all:—
PARA-MAGNETIC METALS.
Iron.
Nickel.
Cobalt.
Manganese.
Chromium.
Cerium.
Titanium.
Palladium.
Platinum.
Osmium.
DIA-MAGNETIC.
Bismuth.
Antimony.
Zinc.
Tin.
Cadmium.
Sodium.
Mercury.
Lead.
Silver.
Copper.
Gold.
Arsenic.
Uranium.
Rhodium.
Iridium, etc.
Common air was also discovered to have a magnetic action, and hot air
is more dia-magnetic than cold. Oxygen is as para-magnetic in the air
as iron is on the earth, and this, it was considered, may give rise to
magnetic storms, and account for the declination of the needle.
We may now proceed to consider the Mariner’s Compass. The compass, or
the mariner’s compass, is so common that it is scarcely necessary to
give a long description of it. Its history is unknown. The Chinese
seem to have been aware of its usefulness long before the western
nations adopted it. It was about the time of the Crusades that it was
brought into western prominence, but was not generally known till the
thirteenth century. Chinese writings ascribe to the compass a great
antiquity; they maintain that it was discovered two thousand five
hundred years before the Christian Era, and used for travelling on
land. But, according to other accounts, it was not used at sea till the
year 300 A.D.
The Chinese put the south first when speaking of the points of the
compass, and in the Chinese empire and Thibet west goes likewise before
east. So the imperial edifices in China face the south, and the needle,
in their expression, points south and north—not as we say, north and
south. The antiquity of the compass may be inferred from the recorded
fact in Chinese chronicles, written in the second century before the
Christian Era, that nine hundred years previously to the date of the
chronicle the Emperor gave magnetic cars to certain ambassadors to
guide them home in safety. These cars were fitted with a magnetic
needle which communicated with a figure. Its outstretched hand and
finger followed the compass-direction, and pointed out the way.
The Chinese subsequently (in the twelfth century) suspended the needle
by a thread, and it is said their philosophers at that time noticed
the variation of the needle. But Columbus first, in 1492, and Cabot,
in 1540, certainly remarked it in Europe. It is to Marco Polo that we
are indebted for the direct introduction of the needle into Europe,
although it probably had been in use in the Levant previously, for we
have seen a quotation by an Arab writer, who, in 1242, described the
needle as being used at that time on his voyage from Tripoli in Syria
to Alexandria, two years previously.
Friar Bacon possessed a loadstone, and there are many instances in
which it is referred to in ancient writings. The inventor of the
compass we cannot trace, but no doubt exists as to its being of Chinese
origin.
The ordinary compass is shown in the illustration herewith (fig. 265).
It consists of a magnetized needle, suspended freely, and fixed to a
circular card, which is divided and subdivided into thirty-two points,
as in the cut. This compass is suspended upon gimbals to keep it in
an upright position when the vessel rolls or plunges. The gimbals
are concentric rings, the compass being fastened to the inner one,
and keeps its position in all weathers. It is then enclosed in the
binnacle, a glass receptacle. The card moves with the needle which
points north. There is a dark line (lubber line) which indicates the
ship’s course, and when sailing the steersman must keep that line
opposite the compass direction-point which indicates the course. At
night a lamp is lighted in the binnacle, and the card being transparent
and the points opaque they are easily seen.
The magnetism of iron ships has a tendency to disturb the needle, and
many suggestions have been made and discussed with a view to obviate
this. To put the compass at the mast-head was one, to surround the
compass with “counter-irritants” another. But the usual way is to
“swing the ship,” and so adjust the compass. Swinging the ship means
turning her round point by point, and marking the deflection of the
needle with reference to a certain object. The amount of deflection at
each point is read and noted, and subsequently taken into consideration
when sailing.
The Azimuth Compass is a mariner’s compass fitted with brass uprights
slit through the centre, through which the heavenly bodies may be
seen. These are the _sights_. The card is divided into _degrees_ and
_quarters_. A fine wire is fixed upon one of the sights, and in the
other slit is a prism to reflect the divisions of the card to the eye.
The object—the azimuth distance of which it is desirable to know—is
looked at through the slit, and bisected by the wire. The divisions of
the scale are at the same time reflected, and the number read gives the
azimuth distance required.
[Illustration: Fig. 265.—Compass.]
The compass has led us away slightly from our consideration of the
electro-magnet, but we will now examine it and its effects as briefly
as possible.
An electro-magnet is formed by wrapping a copper wire round a piece
of soft iron shaped like a horse-shoe; the wire should be insulated
with silk. If the wire be wound round the iron in the same direction,
and a current be merely sent through the coil, it will be found that
the horse-shoe iron is highly magnetic, but if the current be stopped
the power is lost. Such magnets will carry weights much heavier than
themselves, and by careful consideration of certain laws, and with
reference to the number of coils and the strength of the current, these
magnets will sustain a weight some thousands of times greater than
their own weight.
[Illustration: Fig. 266.—Electro-Magnet.]
If we cover a non-magnetic piece of iron with a wire coil, and taking
a magnet turn it rapidly beneath the wire-bound iron, so that the
magnetic poles approach each other alternately, an electrical current
will be generated in the wire. The electro-magnetic machine is thus
made; but although strong currents may be generated as a source of
motive power it is a failure.
To Faraday our knowledge of magneto-electricity is due. “He knew” (says
Professor Tyndall in his interesting work, “Faraday as a Discoverer”)
“that under ordinary circumstances the presence of an electrified body
was sufficient to excite by induction an unelectrified body. He knew
that the wire which carried an electric current was an electrified
body, and still all attempts had failed to make it excite in other
wires a state similar to its own.”
But while he was making his experiments on the induction of electric
currents he noticed that at the time the current was passing from the
battery through the coils of wire that no motion was perceptible in
the galvanometer. But when the circuit was opened, and when it was
closed, there was a slight motion of the needle in the galvanometer,
but in different directions. After consideration the philosopher came
to the conclusion “that a battery current through one wire induced a
similar current through the other, but for an instant only.”
Œrsted had already demonstrated that all magnetic effects were
attributable to the attraction and repulsion of electric currents;
and founding his views upon the theory of Ampère, Faraday came to the
conclusion that electricity could be produced from magnetism, or that
the electric current could be obtained from magnets. This he succeeded
in doing. By inserting a steel magnet about half its length into a
coil of wire, Faraday induced a current to pass through the wire
in two directions. Thus he proceeded to solve all the mysteries of
magneto-electricity, and stated that to produce currents it was only
necessary to “cut appropriately the lines of magnetic force.”
The application of the magnet to the machines for electric lighting
will be shown further on. Very powerful currents are obtained by the
induction coil; but the currents would not be of practical service were
it not for the apparatus called a Commutator, or key, which reverses
the connection of the bobbins, and turns the current at every half
revolution. Just as if a current were being sent across and back over
a table, and when the current has reached the end, an instantaneous
_wheel round_, or pivoting of the table, _sends the current on_, in
continuation (but on the table all the time), because of the sudden
change of its position. The back rush being on the table, the movement
of the latter really makes the line continuous, and by quickly breaking
and reversing the current in the commutator, the effect is gained in
the machine.
[Illustration: Electro-magnets and bobbin, etc. (Clarke’s machine)]