Magnetism a Dual Power—Antithetic Character of Paramagnetism and
Diamagnetism—The Earth Paramagnetic—Properties of Paramagnetic
Bodies—Polarity—Induction—Lines of Magnetic Force—Currents of
Electricity induced by them—Proved to be Closed Curves—Analogy and
Identity of Electricity and Magnetism—Terrestrial Magnetism—Mean
Values of the Three Magnetic Elements—Their Variations in Double
Progression proved to consist of Two Superposed Variations—Discovery
of the Periodicity of the Magnetic Storms—The Decennial Period of the
Magnetic Elements the same with that of the Solar Spots—Magnetism of
the Atmosphere—Diamagnetism—Action of Electro-Magnetism on
Paramagnetic, Diamagnetic Bodies, and on Copper, very different—Proof
of Diamagnetic Polarity and Induction—Magnecrystallic Action—Effects
of Compression, Heat, and Cleavage on Magnetic Bodies—Mutual
Dependence of Light, Heat, Electricity, &c. &c.—The Conservation of
Force and the Permanency of Matter Primary Laws of Nature—Definition
of Gravity not according to that Law—Gravity only the Residual Force
of a Universal Power—Magnetism of the Ethereal Medium.
MAGNETISM may be regarded as a new science in consequence of the
profound researches and admirable discoveries of Dr. Faraday. Since the
magnetism of matter is only known by the action of a magnet or of
electricity upon it, by using an extremely energetic magnet or
electro-magnet he has proved that all known substances, whether solid,
liquid, or aëriform, are more or less magnetic, but that the magnetism
is very different in different substances. For example, if a bar of iron
be freely suspended between the poles of a very powerful magnet or
electro-magnet, it will be attracted by both poles, and will set or rest
in the direction of a straight line joining them; but if a similar bar
of bismuth be freely suspended in the same manner, it will rest in a
direction at right angles to that which the iron bar assumed. Thus the
direction in which the iron sets is axial or in the line of force, while
that which the bismuth assumes is equatorial or perpendicular to the
line of force. Substances that are magnetic after the manner of iron are
said to be paramagnetic, those that are magnetic after the manner of
bismuth are diamagnetic. As far as we know, all matter comes under one
or other of these laws. Many bodies are paramagnetic besides iron, as
the loadstone, which consists of the peroxide and protoxide of iron
mixed with small portions of silica and alumina; also some of the gems
and metals, as cobalt, nickel, &c. A substance is often paramagnetic if
it contains only the 130,000th part of its weight of iron; but by far
the greater number are diamagnetic, as all animal and vegetable matter,
acids, oils, sugar, starch, bread, &c., and all the gases except oxygen,
which is highly paramagnetic; and its force increases with its density:
but notwithstanding the predominance of diamagnetic matter at the
surface, the terrestrial globe is paramagnetic—in fact it is a powerful
magnet.
Besides the substances which are paramagnetic naturally, that property
may be imparted by a variety of methods, as by friction with magnets or
even juxtaposition with them; and a bar of hard steel held at the angle
of the dip will become a magnet on receiving a few strokes with a hammer
on its upper end.
Polarity is one of the most distinguishing characters of magnetism: it
is the property which a magnet possesses when freely suspended of
resting spontaneously in the magnetic meridian, or nearly north and
south, and always returning to that position when disturbed in
consequence of the mean magnetic attraction of the earth; yet the magnet
has no tendency to move to the north or south even when floating on
water, because the same pole that attracts one end repels the other.
Both poles of a magnet attract iron, which in return attracts either
pole of the magnet with an equal and contrary force. The action of a
magnet on unmagnetised iron is confined to attraction, whereas the
reciprocal agency of magnets is characterised by a repulsive as well as
by an attractive force; for a north pole repels a north pole, and a
south pole repels a south pole; but a north and south pole mutually
attract one another—which proves that paramagnetism is a dual power in
which the conservation of force is perfectly maintained, for the force
of attraction is exactly equal to the force of repulsion. One kind of
polarity cannot exist without the other: they are absolutely
simultaneous, dependent, and of equal intensity.
Induction is the power which a magnet possesses of exciting temporary or
permanent paramagnetism in such bodies in its vicinity as are capable of
receiving it. By this property the mere approach of a magnet renders
iron and steel paramagnetic, the more powerfully the less the distance,
but the induced force is always exactly equal to the force which
produces it. When the north end of a magnet is brought near to, and in
the line with, an unmagnetised iron bar, the bar acquires all the
properties of a perfect magnet; the end next the north pole of the
magnet becomes a south pole, while the remote end becomes a north pole.
Exactly the reverse takes place when the south end is presented to the
bar, so that each pole of a magnet induces the opposite polarity in the
adjacent end of the bar, and the same polarity in the remote extremity;
consequently the nearest extremity of the bar is attracted, and the
farther repelled; but as the action is greater on the adjacent than on
the distant part, the resulting force is that of attraction. By
induction the iron bar not only acquires polarity, but the power of
inducing paramagnetism in a third body; and although all these
properties vanish from the iron as soon as the magnet is removed, a
lasting increase of intensity is generally imparted to the magnet itself
by the reaction of the temporary paramagnetism of the iron. Iron
acquires the inductive force more rapidly than steel, yet it loses it as
quickly on the removal of the magnet, whereas the steel is impressed
with a lasting polarity.
A certain time is requisite for induction, and it may be accelerated by
anything that excites a vibratory motion in the particles of the steel;
such as the smart stroke of a hammer, or heat succeeded by sudden cold.
A steel bar may be converted into a magnet by the transmission of an
electric discharge through it; and as its efficacy is the same in
whatever direction the electricity passes, the effect arises from its
mechanical operation exciting a vibration among the particles of the
steel. It has been observed that the particles of iron easily resume
their neutral state after induction, while those of steel resist the
restoration of equilibrium, or a return to the neutral state: it is
therefore evident that any cause which removes or diminishes the
resistance of the particles will tend to destroy the paramagnetism of
the steel; consequently the same mechanical means which develop the
power will also destroy it. On that account a steel bar may lose its
paramagnetism by any mechanical concussion, such as by falling on a hard
substance, a blow with a hammer, and heating to redness, which makes the
steel soft. The circumstances which determine whether it shall gain or
lose are its position with respect to the magnetic equator, and the
higher or lower intensity of its previous magnetic state.
A comparison of the number of vibrations accomplished by the same
magnetised needle during the same time at different distances from a
magnet gives the law of paramagnetic intensity, which follows the
inverse ratio of the square of the distance—a law that is not affected
by the intervention of any substance whatever between the magnet and the
needle, provided the substance be not itself susceptible of magnetism.
Induction and the reciprocal action of magnets are therefore subject to
the laws of mechanics; but the composition and resolution of the forces
are complicated in consequence of four forces being constantly in
activity, two in each magnet. Mr. Were Fox discovered that the law of
the paramagnetic force changes from the inverse square of the distance
to the simple inverse ratio when the distance between two magnets is as
small as from the fourth to the eighth of an inch, or even as much as
half an inch when the magnets are large; and in the case of repulsion,
that the change takes place at a still greater distance, especially when
the two magnets differ materially in intensity.
Without assuming any hypothesis of what magnetism is, or how that force
is originated or sustained, Dr. Faraday regards a magnet as a source of
power surrounded by curved lines of force which are not only
representants of the magnetic power in quality and direction, but also
in quantity—an hypothesis which accords perfectly with experiment, and
with the action both of electricity and magnetism. The nature and form
of these lines may be seen by placing a bar magnet upon a table,
spreading a sheet of stiff paper over it so as to be perfectly level and
free from creases, and then sifting very clean iron filings through a
fine sieve equably over it. The filings will instantly assume the form
of the curved lines represented by fig. 1, plate 7, in consequence of
the action of the magnet. These lines are the true representatives of
the magnetic forces, and being related to a polar power, they have
opposite qualities in opposite directions. When a magnet is broken
across the middle, each part is at once converted into a perfect magnet;
the part that originally had a south pole acquires a north pole at the
fractured end; the part that had originally a north pole gets a south
pole; and as far as mechanical division can be carried, it is found that
each fragment is a perfect magnet. Fig. 2, plate 7, shows the lines of
force in a fractured magnet when the ends are not yet separated; fig. 3
shows them when they are.
Currents of electricity are produced in conducting bodies moved across
these lines of magnetic force. If a copper wire at a little distance
above the north pole of a bar magnet be moved from left to right, at any
angle across the lines of magnetic force, they will induce a current of
electricity in the wire flowing from right to left; if the wire be moved
with the same velocity in the contrary direction, the induced current
will be of equal intensity, but it will flow from left to right. Similar
results are obtained from the south pole, and the phenomena are the same
when the magnet is moved and the wire is at rest; in both cases the
intensity is greater the swifter the motion. It appears that the
quantity of electricity induced is directly as the amount of the
magnetic curves intersected, and when a wire is moving uniformly in a
field of equal magnetic force, the current of electricity generated is
proportional to the time, and also to the velocity of motion; for when a
metallic disc is made to revolve through the lines of force, the current
induced is strongest near the edge where the velocity is greatest; and
in different substances moving across the lines of force the intensity
of the induced current is directly as the conducting power of the
substance. Thus bodies moved near a magnet have an electrical current
developed in them, and conversely bodies affected by an electric current
are definitely moved by a magnet near them.
By the preceding experiments it appears that magnetic polarity is
manifested in two ways; in the magnetised needle, by attraction and
repulsion, and in a wire moving across lines of magnetic force it is
shown by the opposite directions in which the induced current flows
according as the body is moved from the right to the left, or left to
right. Hence polarity consists in the opposite and antithetical actions
manifested at the opposite ends or opposite sides of a limited or
unlimited line of force. Antithesis is the true and most general
character of magnetism, whatever may be its mode of action.
It was by the induction of electric currents in copper wires moving
across the lines of magnetic force that Dr. Faraday proved that the
lines of force issuing from a magnet are closed curves which return
again and pass through the interior of the magnet. He placed two bar
magnets of the same length, size, and intensity with their similar poles
together, so that they might act as one magnet. A copper wire was then
passed between their axes, which after extending through half their
length was bent up equatorially and turned back along the outside, so
that the whole wire formed a loop, the two ends being connected with a
galvanometer. When the whole wire was made to revolve, no effect was
produced, although it crossed the lines of magnetic force; but when it
was cut in two, so as to separate the external from the internal part,
electrical currents of equal intensity, but in contrary directions, were
induced in each portion of the wire as they were made separately to
cross the lines of force, for the apparatus was so constructed that that
could be done. The exterior wire crossed the lines of force which issued
from the magnets at right angles to their axes, while the equatorial
part of the interior wire traversed the returning lines of force. It is
evident that these forces neutralized each other when the whole wire
revolved: consequently the internal and external lines of force must
have been of equal intensity and opposite in direction, so as to balance
one another. By this and a very great number of other experiments Dr.
Faraday has proved that the magnetic lines of force are continuous
closed curves alike in shape, size, and power. They extend indefinitely
beyond the magnet, and undergo no change by distance.
Thus the magnetic force pervades the interior of the mass; if
electricity does the same, a compensation must either take place, or it
also must move in lines of force, sensible only at the surface.
Electricity has a perpetual tendency to escape, and does escape, when
not prevented by the coercive power of the air, and other non-conducting
substances. Such a tendency does not exist in magnetism, which never
leaves the substance containing it under any circumstances whatever.
There must be some coercive force, analogous to friction, which arrests
the magnetic forces, so as first to oppose their separation, and then to
prevent their reunion. In soft iron the coercive force is either wanting
or extremely feeble, since iron is easily rendered paramagnetic by
induction, and as easily loses that quality; whereas in steel the
coercive force is extremely energetic, because it prevents the steel
from acquiring the paramagnetic properties rapidly, and entirely hinders
it from losing them when acquired. The feebleness of the coercive force
in iron, and its energy in steel, with regard to the paramagnetic force,
is perfectly analogous to the facility of transmission afforded to
electricity by non-electrics, and the resistance it experiences in
electrics. At every step the analogy between electricity and magnetism
becomes more striking. The agency of attraction and repulsion is common
to both; the positive and negative electricities are similar to the
northern and southern polarities, and are governed by the same
laws—namely, that between like powers there is repulsion, and between
unlike powers there is attraction. Each of these four forces is capable
of acting most energetically when alone; but as the electric equilibrium
is restored by the union of the two electric states, and magnetic
neutrality by the combination of the two polarities, they respectively
neutralise each other when joined. All these forces vary inversely as
the square of the distance, and consequently come under the same
mechanical laws.
A like analogy extends to magnetic and electric induction. Iron and
steel are in a state of equilibrium when neutral; but this equilibrium
is immediately disturbed on the approach of the pole of a magnet, which
by induction transfers one kind of polarity to one end of an iron or
steel bar, and the opposite kind to the other—effects exactly similar to
electrical induction. There is even a correspondence between the
fracture of a magnet and that of an electric conductor; for if an oblong
conductor be electrified by induction, its two extremities will have
opposite electricities; and if in that state it be divided across the
middle, the two portions, when removed to a distance from one another,
will each retain the electricity that has been induced upon it. The
analogy, however, does not extend to transference. A body may transfer a
redundant quantity of positive electricity to another, or deprive
another of its electricity—the one gaining at the expense of the other;
but a body cannot possess only one kind of polarity. With that
exception, there is such perfect correspondence between the theories of
magnetic attractions and repulsions, and electric forces in conducting
bodies, that they not only are the same in principle, but are determined
by the same formulæ. Experiment concurs with theory in proving the
identity of these two influences. Hence, if the electrical phenomena be
due to a modification of the ethereal medium, the magnetic phenomena
must be owing to an analogous cause.
Curved lines of magnetic force issue from every point of the earth’s
surface where there is sensible dip, and bending round enter the earth
again at the magnetic equator. They induce electric currents in
conducting-wires, moving across them exactly the same as in artificial
magnets; and when a hollow helix, or coil of copper wire, whose
extremities are connected with a galvanometer, is placed in the magnetic
dip, and suddenly moved across the lines of force, the needle of the
galvanometer will vibrate through an arc of 80° or 90°, in consequence
of the electric current induced by these lines of magnetic force in the
wire, and the action is greater when a core of soft iron is placed in
the helix, which becomes a temporary magnet by induction. Again, if a
copper plate be connected with a galvanometer by two copper wires, one
from the centre, and another from the circumference, in order to collect
and convey the electricity, it is found that, when the plate is made to
revolve in a plane passing through the line of the dip, the galvanometer
is not affected. But as soon as the plate is inclined to that plane,
electricity begins to be developed by its motion across the lines of
magnetic force; it becomes more powerful as the inclination increases,
and arrives at a maximum when the plate revolves at right angles to the
line of dip. When the revolution is in the same direction with that of
the hands of a watch, the current of electricity flows from its centre
to the circumference; and when the rotation is in a contrary direction,
the current sets the opposite way. Thus a copper plate, revolving at
right angles to the line of the dip, becomes a new electrical machine,
differing from the common plate-glass machine by the copper being the
most perfect conductor, whereas glass is the most perfect non-conductor;
besides insulation, which is essential to the glass machine, is fatal to
the copper one. The quantity of electricity evolved by the metal does
not appear to be inferior to that devolved by the glass, though very
different in intensity. Even a ship crossing the lines of force must
have electric currents running through her. Dr. Faraday observes that
such is the facility with which electricity is generated by the magnetic
lines of force, that scarcely any piece of metal can be moved without a
development of it; consequently, among the arrangements of steam-engines
and metallic machinery, curious electro-magnetic combinations probably
exist which have never yet been noticed. Thus magnetic lines of force
certainly issue from the surface of the globe.
No doubt the earth is a magnet on a vast scale, but it differs from all
others in having four poles of maximum magnetic force of different
intensities, the two in the northern hemisphere having a secular motion
in a contrary direction from the two in the southern. They are not even
symmetrically placed; hence the magnetic intensity varies so much in the
different points on the earth’s surface, that the dynamic equator, or
line passing through all the points of least intensity, is a very
irregular curve surrounding the globe, but by no means coinciding with
the terrestrial equator. In consequence of the mean action of these four
forces, the north end of a magnetised needle, arranged so as to revolve
in a vertical plane, dips or inclines beneath the horizon in the
northern hemisphere, and the south end in the southern. The two
hemispheres are separated by a line encircling the earth, called the
magnetic equator, or line of no dip, in which the dipping or inclination
needle is horizontal. On each side of this line the inclination
increases till at last the needle becomes perpendicular to the horizon
in two points, or rather small spaces, in each hemisphere, known as the
magnetic poles, which are quite different from the poles of the earth’s
rotation. The mean action of the four poles of magnetic intensity causes
the mariner’s compass, or a magnetic needle suspended so as to revolve
in a horizontal plane, to remain at rest when pointing to the two
magnetic poles. It is then in the magnetic meridian of the place of
observation, which is thus determined by the mean action of all the four
magnetic forces.
These mean values of the three magnetic elements, namely, the
declination, inclination or dip, and magnetic intensity, are well known
to be subject to secular, annual, and diurnal variations. The secular
only become sensible after some years, but the annual and diurnal
variations have a double progression—that is to say, two maximum and two
minimum values in their respective periods of a year and twenty-four
hours; for example, the declination needle makes two deviations to the
west and two to the east in the course of twenty-four hours, and that
with great regularity. Now General Sabine discovered that the double
progression arises from two combined or superposed variations having
different hours of maxima and minima, and that they are due to two
distinctly different causes—the one being the difference in the sun’s
position relatively to the place of observation at the different seasons
of the year, and hours of the day and night; the other being a mean
annual and diurnal variation proved by General Sabine to exist in those
great magnetic storms or casual disturbances which affect the magnetic
elements simultaneously over enormously extensive tracts of the globe.
Moreover the General discovered that, besides these annual and diurnal
variations, the magnetic storms have a variation which accomplishes its
vicissitudes in ten or more nearly eleven years, the increase from year
to year being gradual, till its maximum becomes twice as great as its
minimum value. In consequence of this inequality in the storms or casual
disturbances, each of the magnetic elements has a variation of similar
period and similar maxima and minima. Now the number and magnitude of
the spots on the sun had been observed by M. Schwabe, of Dessau, to
increase to a maximum, and decrease again to a minimum, regularly in the
very same period of between ten and eleven years; and General Sabine
found that this variation in the solar spots, and that in the magnetic
elements, not only have the same periods of maxima and minima, but that
they correspond in all their minutest vicissitudes. Thus a very
remarkable and unexpected connexion exists between terrestrial and solar
magnetism. The dual and antagonist principle is perfectly maintained in
the earth’s magnetism, all the phenomena and their variations being in
opposite directions in the two hemispheres. (N. 226.)
No doubt the magnetic lines of force in the earth are closed curves, as
in artificial magnets; but in their circuitous courses they may extend
to any distance in space, or rather in the ethereal medium, even to
thousands or tens of thousands of miles; for the ethereal medium is
permeable to lines of magnetic force, or rather transmits them,
otherwise the solar spots could not affect the variations of terrestrial
magnetism; besides, they pass through the Torricellian vacuum, which is
nearly a void with respect to air, but not to the ethereal medium.
The atmosphere which surrounds the earth to the height of about fifty
miles with sensible density, consists of three and a half parts by
weight of nitrogen gas and one part of oxygen, uniformly mixed. The
nitrogen is neutral whether dense or rare, hot or cold, while the oxygen
is highly paramagnetic; but it loses a great part of its force when
rarefied by heat; consequently the magnetic force of the atmosphere must
increase from the equator to the poles of maximum cold; it must vary
summer and winter, night and day. Its effect upon terrestrial magnetism
is unknown; but it can hardly be without some influence. M. E. Becquerel
observes—“If we reflect that the earth is encompassed by a mass of air
equivalent in weight to a layer of mercury of 30 inches, we may inquire
whether such a mass of magnetic gas, continually agitated, and submitted
to the regular and irregular variations of pressure and temperature,
does not intervene in some of the phenomena dependent upon terrestrial
magnetism. If we calculate, in fact, what is the magnetic force of this
fluid mass, we find that it is equivalent to an immense plate of iron,
of a thickness little more than 1/250 of an inch, which covers the whole
surface of the globe.” Both the conducting power of the air and its
density are increased by cold; and as the sum of the magnetic forces
which issue from the earth on one side of the line of no dip is equal to
their sum on the other side, the intensity and concentration in our
winter are coincident with a diffusion and feebleness in the opposite
hemisphere, so that the line of no dip will move annually from north to
south and back again. The same holds with regard to day and night. Thus
the law of the conservation of force is rigorously maintained; and it is
equally so in the effect of the atmosphere on the magnetic lines of
force, which refracts them as they pass through it, in one direction in
summer, and in the opposite direction in winter—in one direction in the
enlightened hemisphere, in the other in that which is dark. The whole of
the magnetic lines about the earth are held by their mutual tension in
one connected, sensitive system, which feels in every part, even to the
antipodes, a change in any particular place.
It may be mentioned as a well-known fact, that apparent anomalies have
been found in the diurnal variation of the declination in the high
magnetic latitudes of the northern hemisphere when compared with their
great regularity in other parts of the same hemisphere, and that the
magnetic storms are of much greater magnitude there than in lower
latitudes. Moreover, although Captain Maguire’s observations at Cape
Barrow, in the North Polar Ocean, show that the annual and diurnal
variations of the casual disturbances or magnetic storms, as well as
those of the decennial period, are maintained, yet it appears that at
certain hours of the day the disturbance in the declination may be
easterly at Point Barrow, and westerly at the Magnetic Observatory at
Toronto, in Upper Canada, and _vice versâ_: in fact, the magnetic storms
are simultaneous at these two stations, but in opposite directions—a
circumstance not yet accounted for, and may possibly be due to the
increased magnetism of the air in these cold regions. The heat of the
sun has no effect upon terrestrial magnetism unless possibly by its
indirect action on the oxygen of the atmosphere; but hitherto it has
been imperceptible. It is hardly possible that the aurora can be
independent of the magnetic character of the air, since it occurs in the
high latitudes, where the atmospheric magnetism is most powerful.
Captain Maguire remarked that it frequently appeared at Point Barrow
when the magnetic storms were at a maximum.
We are totally ignorant of the cause of terrestrial magnetism, though
the powerful influence of the solar spots renders it highly probable
that it will ultimately be found to originate in the sun himself. Mr.
Barlow’s theory of electric currents revolving round the globe is borne
out by Mr. Fox’s observations in the Cornish mines, which show that
electro-magnetism is extremely active in metallic veins; that not only
the nature of the metalliferous deposits must have been determined by
their relative electrical conditions, but that the direction of the
metallic veins must have been influenced by the direction of the
magnetic meridians, and in fact almost all the metallic deposits in the
world tend from east to west, or from north-east to south-west. However,
these currents of electricity may be regarded as magnetic lines of
force, and are more likely to be the effect than the cause of
terrestrial magnetism. They are found to have a powerful inductive
effect on the Atlantic telegraph, disturbing the needles and
galvanometers at each end of the line to a considerable degree, and on
the night of the 6th of September, 1858, a magnetic storm passed over
the cable, which violently agitated the reflecting galvanometer in
connection with the telegraphic wires.
We are equally ignorant of the cause of the secular magnetic variations,
but we have no reason to believe that the earth is alone magnetic; on
the contrary, the planets are probably magnets, and we know that the sun
and moon are magnetic; hence, as the magnetic, like the gravitating
force, is transmitted through the ethereal medium, the induction of the
sun, moon, and planets, in all their secular and periodic changes, may
cause perpetual variations in terrestrial magnetism, and it may not be
beyond the delicacy of modern observation to ascertain whether a planet,
when nearest to the earth, has any sensible magnetism.
Diamagnetism is also a dual power, but in complete antithesis to
paramagnetism under the same circumstances. Dr. Faraday first discovered
this property in heavy glass, or silico-borate of lead, a piece of which
was repelled by the pole of a powerful electro-magnet, and an elongated
prism of the same heavy glass, when freely suspended between the poles,
set equatorially. He then found that so great a number of substances
followed the same law, that it established the very remarkable fact of a
hitherto unknown force having acted upon the substances submitted to its
influence, a discovery which he subsequently confirmed by many
experiments, all of which proved the antithesis between the two modes of
magnetic action. He also discovered that magnetic bodies differ
exceedingly in their magnetic power: of paramagnetic bodies iron is the
most powerful; then follow nickel, cobalt, and a long gradation down to
osmium and a vacuum. The body that seems to have the lowest diamagnetic
power is arsenic, and the series ascends to heavy glass, antimony,
phosphorus, and bismuth; so iron and bismuth are the most powerful in
their respective classes, and both have a small conducting power for
electricity. It may be presumed that many remarkable instances of
diamagnetism are to be met with in nature; among others, Dr. Faraday has
suggested the idea that Saturn’s ring, from its position, may be
diamagnetic with regard to the planet.
With very powerful magnets or electro-magnets, which are absolutely
necessary for all these experiments, it is found that no _simple_
substance is neutral, but that such may be compounded by mixing in due
proportion a diamagnetic and paramagnetic liquid, as water and
protosulphate of iron.
Professor Tyndall proved diamagnetic polarity by placing two bismuth
bars within two vertical coils or spirals of insulated copper wire,
through which electric currents were transmitted from a galvanic
battery, and caused to act upon a steel magnet freely suspended without
the spirals. Now, when the excited magnetism is merely by induction, the
electric current, being momentary, only causes a shock or momentary
deviation in the magnet, which returns to its original position when the
current ceases. When, on the contrary, the magnetism is permanent, the
suspended magnet does not return to its original position when the
current ceases. In Professor Tyndall’s experiment the deviation was
permanent, and it was equally so when a bismuth bar was freely suspended
and the cores within the spirals were steel magnets. Had the effect been
from currents induced in the mass of the bar of bismuth, division of the
bar would have stopped them, but the result was the same with powdered
bismuth as with the solid mass. Moreover, since the strength of induced
currents depends upon the conducting power of the substance, and as the
conducting power of copper is forty times as great as that of bismuth,
had the polarity been induced and not real, the effect ought to have
been forty times greater when copper instead of bismuth cores were put
in the spirals, whereas it was scarcely sensible. Besides these proofs,
Dr. Tyndall made experiments with eleven different diamagnetic
substances, of which water was one, with similar results. He then
determined the polarity of twelve paramagnetic bodies by the same
method, whence it appeared that the same action which produced a north
pole in the paramagnetic bodies produced a south pole in those that were
diamagnetic, and _vice versâ_, whence he concludes that diamagnetic
polarity is one of the most firmly established truths of science. It
follows from this that, when a man is standing, his head is a north pole
and his feet a south, and the top of an iron railing on which he may be
leaning is a south pole and the lower end a north. Diamagnetic bodies
thus possess a polarity, the same in kind but opposite in direction to
that possessed by paramagnetic ones.[18] They are both dual powers, and
the two diamagnetic forces like the two paramagnetic being coexistent,
simultaneous, and mutually dependent, there can be no doubt that the
diamagnetic forces also are represented, or rather consist of curved and
closed lines of force passing through the interior of the substance. Dr.
Tyndall has proved that the attraction of iron, and the repulsion of
bismuth, are as the square of the electro-magnetic current producing
them, and that diamagnetic substances are capable of induction.
The molecular structure of substances freely suspended between the poles
of a magnet has a decided effect upon the position they assume.
It has already been mentioned that the optic axis is a symmetrical line
in a doubly refracting crystal in which there is no double refraction,
and that in some crystals there are two such symmetrical lines. Now,
Professor Plücker of Bonn discovered, when such crystals are submitted
to powerful magnetic influence, that the single optic axis in the one,
and the resultant or mean line between the double optic axes in the
other, set diametrically or at right angles to the line of magnetic
force; and so powerful did the Professor find the action of magnetism on
crystalline form, that the mineral cyanite, when suspended, arranges
itself so definitely with regard to terrestrial magnetism, that it might
be used as a compass needle.
Dr. Faraday afterwards observed that amorphous substances, cut in the
form of a sphere, have no tendency to set or be attracted or repelled in
one direction in preference to any other; but if the sphere be formed of
a crystallized substance, it is a general fact that, whether it be
paramagnetic or diamagnetic, it is more powerfully attracted or repelled
in one direction than in any other—a property named by Dr. Faraday
magnecrystallic action. For example, a sphere of calcareous spar, which
is a diamagnetic crystal, is most strongly repelled in the direction of
its principal optic axis, and least strongly in the direction of its
least axis. In a sphere of carbonate of iron, which has exactly the same
crystalline form and is highly paramagnetic, the line which in carbonate
of lime sets equatorially, in this case sets axially, and more strongly
in that direction than in any other. The law according to which the
attraction of the carbonate of iron increases from the least to its
greatest or principal optic axis, is precisely the same as that
according to which the repulsion of the calcareous spar increases from
the least to the principal optic axis. These relations are not altered
by the immersion of the spheres in liquids of either magnetism. Dr.
Faraday observed that a line at right angles to the planes of principal
cleavage in crystals takes the axial position, and on that account he
called it the magnecrystallic axis. Its position was proved by MM.
Tyndall and Knoblauch to depend upon the general fact, that the mass is
most strongly repelled in the direction of the planes of principal
cleavage, and that the elective position of crystals depends more upon
the direction of these planes with respect to the electric force, than
upon the optic axis. The planes of principal cleavage set themselves
equatorially in diamagnetic, and axially in paramagnetic substances: it
was thence inferred that the phenomena offered by crystals in the
magnetic field is a particular case of the general law, that the
superior action of magnets upon matter in a particular direction is due
to the particles of the body being closer together in that direction
than in any other: in short, the line of maximum density; the force
exerted being attractive or repulsive according as the particles are
paramagnetic or diamagnetic.
It appears, however, that the set of crystals with regard to the line of
magnetic force does not depend solely upon their density in particular
directions. Professor Matteucci, of Pisa, has proved that the
diamagnetic force is inversely as the conducting power of substances for
electricity, that the conducting power is a maximum in the planes of
principal cleavage, and that a needle of crystallized bismuth, in which
the planes of cleavage are parallel to its length, places itself
equatorially with more force when these planes are vertical, or at right
angles to the force, than when they are horizontal or parallel to it.
Experiments had hitherto been made only with diamagnetic or slightly
paramagnetic bodies, which induced M. le Roux to try the effect of
magnetism on pulverized iron compressed by the hydraulic press, which
reduced the grains of iron to lamellæ equivalent to planes of cleavage.
Cubes of this substance, suspended by a thread over a horseshoe magnet,
oscillated for a longer time when the lamellæ were perpendicular than
when they were horizontal; that is, the force was stronger when the
lamellæ were equatorial than when they were axial, exactly the same
result as in Professor Matteucci’s experiment with the needle of
bismuth. Thus the vertical position of the cleavages, which increases
the diamagnetism of the bismuth, increases also the paramagnetism of the
iron. M. le Roux observes that these results are independent of the
influence of the currents of electricity induced in the oscillating
body, for the fundamental character of the phenomena of Arago’s
discovery of rotation by induction is, that the oscillations diminish
rapidly in extent without any sensible diminution in their duration,
while in his experiments the time of the oscillations varied. He
concludes that the arrangement of the molecules must be intimately
connected with paramagnetism or diamagnetism itself, since the effect of
that arrangement is equally sensible in bismuth and iron, although the
diamagnetism of the former is 25,000 times weaker than the paramagnetism
of the latter.
The diamagnetism of conducting substances and metals, such as gold,
silver, and copper, is augmented by division. Compression has also a
great effect on magnetic action. For example, a bar of soft iron sets
with its longest dimensions from pole to pole of a magnet, but a bar of
compressed carbonate of iron-dust, whose shortest dimensions coincide
with the line of pressure, sets equatorially. A bar of bismuth whose
plane of principal cleavage is parallel to its length sets equatorially,
but a bar of compressed bismuth dust, whose shortest dimensions coincide
with the line of pressure, or a bar of bismuth whose principal planes of
cleavage are transverse to its length, sets with its length axially. The
antithesis is perfect whether the bars are under the influence of a
magnet or electro-magnet. For since the diamagnetic force is inversely
as the conducting power of a body for electricity, and that the latter
is a maximum in the direction of the planes of principal cleavage,
therefore when these planes are parallel to the axis of the bismuth bar
it sets equatorially; but as the conducting power is augmented when the
bismuth dust is compressed in the direction of the force, the
diamagnetic power is diminished, and the bar sets axially. Again, since
the paramagnetic force augments with the conducting power, the action of
the magnet on the iron is antithetic to that on the bismuth.
The action of an electro-magnet on copper is strongly contrasted with
that which it exerts on iron or bismuth. For when a copper bar suspended
by a thread revolves before its pole, it is brought to a dead halt as
soon as the electric current acts upon it, and maintains its position
with considerable tenacity, for it does not return when pushed out of
it, but keeps its new place with stiffness; however, as soon as the
electric current ceases, there is a strong revulsion, the bar revolving
the contrary way. Even when swinging with considerable force it may be
caught and retained in any position at pleasure, but there is no
revulsion when it is arrested either in the axial or equatorial
position; at any angle between these two, but especially midway, the
electricity will make it move towards the axis, but it is arrested
before it comes to it. The action depends much on the form and
dimensions of the bar and the magnetic pole, which ought to be flat. The
phenomena are due to the high electro-conducting power of the copper,
and are met with in some of the other pure metals, though in a far
inferior degree.
Great magnetic power is requisite for all these experiments. Dr. Faraday
employed a magnet that could sustain a weight of 450 lbs. at each pole,
and the poles were either pointed or flat surfaces at pleasure, as the
kind of experiment required.
Heat strongly affects the magnetic properties of bodies. Dr. Faraday
found that, when the temperature of nickel is increased, its magnetic
force diminishes; when that of iron is increased its magnetic force
remains the same, while that of cobalt increases; which seems to
indicate that there is a temperature at which the magnetic force is a
maximum, above and below which it diminishes. Nickel loses its magnetism
at the temperature of boiling oil, iron at a red heat, and cobalt near
the temperature at which copper melts. Calcareous spar retains its
magnetic character at a very high temperature; but the same substance
when it contains iron, and also oxide of iron, loses it entirely at a
dull red heat. A crystal of the ferrocarbonate of lime was absolutely
reversed by change of temperature, for at a low heat the optic axis
pointed axially, and at a high temperature equatorially. With the
exception of these substances, magnecrystals, whether paramagnetic or
diamagnetic, are generally all affected alike by heat. The difference
between the forces in any two different directions, as for instance the
greatest and least principal axes, diminishes as the temperature is
raised, increases as the temperature is lowered, and is constant for a
given temperature. No _unmixed_ or _pure_ substance has as yet passed by
heat from the paramagnetic to the diamagnetic state. No _simple_
magnecrystal has shown any inversion of this kind, nor have any of the
chief axes of power changed their characters or relations to one
another.
It appears that, as the molecules of crystals and compressed bodies
affect magnetism, so magnetism acts upon the molecules of matter, for
torsion diminishes the magnetic force, and the elasticity of iron and
steel is altered by magnetism. M. Matteucci has found that the
mechanical compression of glass alters the rotatory power of a polarized
ray of light transmitted through it, and that a change takes place in
the temper of glass under the influence of powerful magnetism.
Even from the limited view of the powers of nature which precedes, it is
evident that the progress of science based upon experiment tends to show
that the various forces of light, heat, motion, chemical affinity,
electricity, and magnetism will ultimately be traced to one common
origin; that they are so directly related, and mutually dependent, that
they are convertible, motion producing heat, and heat motion; chemical
affinity producing electricity, and electricity chemical action, &c.,
each mediately or immediately producing the other. These forces are
transmitted through substances; they act upon matter, causing changes in
the molecular structure of bodies either momentary or permanent, and
reciprocally the changes indicate the action of these forces. Matter and
force are only known to us as manifestations of Almighty power: we are
assured that we can neither create nor destroy them—that their amount is
the same now as in the beginning. In chemical attraction the powers with
which a molecule of matter is endowed, and which give rise to various
qualities, never change; even when passing through a thousand
combinations, the molecule and its power are ever the same.
Machinery does not create force; it only enables us to turn the forces
of nature to the best advantage; it is by the force of wind or falling
water that our corn is ground, and the steam engine owes its power to
the force of heat and chemical action. As force cannot be created,
neither can it be annihilated. It may be dispersed in various
directions, and subdivided so as to become evanescent to our
perceptions; it may be balanced so as to be in abeyance, or become
potential as in static electricity; but the instant the impediment is
removed the force is manifested by motion; it may also be turned into
heat by friction, but it is never lost. Every motion we make, every
breath, every word we utter, is a force that produces pulsations which
are communicated to continually increasing particles of air, and
conveyed through countless channels so as to become indeed imperceptible
to our senses, yet they are demonstrated to exist as witnesses of the
words we have spoken or the actions we have performed, by analysis, that
all-powerful instrument of human reason.[19]
A body acquires heat in the exact proportion that the adjacent
substances become cold, and when heat is absorbed by a body it becomes
an expansive force at the expense of those around that contract, but it
is not lost. In chemical action at a distance the principle of the
conservation of force is maintained, for a chemical action may be
produced miles away from an electro-magnet, perfectly equivalent to the
dominant chemical action in the battery. The two electricities are
developed in equal proportions, which may be combined so as to produce
many changes in their respective relations, yet the sum of the force of
one kind can never be made in the smallest degree either to exceed or to
come short of the sum of the other. Experimental research proves that
the conservation of force is an unalterable law of nature—“a principle
in physics as large and sure as that of the indestructibility of matter
or the invariability of gravity. No hypothesis should be admitted, nor
any assertion of a fact credited, that denies this principle. No view
should be inconsistent or incompatible with it. Many of our hypotheses
in the present state of science may not comprehend it, and may be unable
to suggest its consequences, but none should oppose or contradict it.”
Having thus expressed his conviction of the truth of this great
principle, Dr. Faraday considers the case of gravity, and concludes that
“the definition of gravity as an attractive force between the particles
of matter varying inversely as the square of the distance, while it
stands as a full definition of the power, is inconsistent with the
principle of the conservation of force.” For while in this definition
the principle is maintained of the constancy of the force _at the same
distance_, it implies a creation of force to an enormous amount when the
distance is diminished, and an equal amount annihilated when the
distance is increased,—“an effect,” he says, “which is equal in its
infinity and its consequences with creation, and only within the power
of Him who creates.” He continues, “It will not be imagined for a moment
that I am opposed to what may be called the _law of gravitating action_,
that is, the law by which all the known effects of gravity are governed;
what I am considering is the _definition_ of the _force_ of gravitation.
That the result of _one_ exercise of a power may be inversely as the
square of the distance, I believe and admit; and I know that it is so in
the case of gravity, and has been verified to an extent that could
hardly have been within the conception of Newton himself when he gave
utterance to the law; but that the _totality_ of a force can be employed
according to that law I do not believe either in relation to
gravitation, or electricity, or magnetism, or any other supposed form of
power. That there should be a power of gravitation existing by itself,
having no relation to the other natural powers, and no respect to the
law of the conservation of force, is as little likely as that there
should be a principle of levity as well as gravity. Gravity may be only
the residual part of the other forces of nature, as Mossotti has tried
to show; but that it should fall out from the law of all other forces,
and should be outside the reach either of farther experiment or
philosophical conclusions, is not probable. So we must strive to learn
more of this outstanding power, and endeavour to avoid any definition of
it which is incompatible with the principles of force generally, for all
the phenomena of nature lead us to believe that the great and governing
law is one. Thus gravitation can only be considered as part of a more
general force whose law has yet to be discovered.”
The definition of the gravitating force immediately suggests the
question of how it is transmitted; the full force of that question was
felt by Newton himself when, in his third letter to Bentley, he wrote,
“That gravity should be innate, inherent, and essential to matter, so
that one body may act upon another at a distance, through a _vacuum_,
without the mediation of anything else by and through which their action
and force may be conveyed from one to another, is to me so great an
absurdity that I believe no man who has in philosophic matters a
competent faculty of thinking can ever fall into it. Gravity must be
caused by an agent, acting constantly according to certain laws; but
whether this agent be material or immaterial I have left to the
consideration of my readers.”
Since Newton’s time the continual decrease in the periodic times of the
comets belonging to our system, and the undulatory theory of light and
heat, have proved the existence of an extremely rare elastic medium
filling space even to the most distant regions of which we are
cognizant. But, rare as it may be, it has inertia enough to resist the
motion of comets, and therefore must be material, whether considered to
be ether or, according to Mr. Grove, the highly attenuated atmospheres
of the celestial bodies. Professor William Thomson of Glasgow has
computed that in the space traversed by the earth in its annual
revolution, a cube whose side is 1000 miles would contain not less than
a pound weight of the ethereal medium, and that the earth, in moving
through it, would not displace the ·250th part of that pound of matter.
Yet that is enormously more dense than the continuation of the earth’s
atmosphere would be in interplanetary space, if rarefied according to
Bayle’s law. But whatever be the density or nature of the ether, there
is every reason to believe that it is the medium which transmits the
gravitating force from one celestial object to another, or possibly it
may possess a higher attribute with regard to gravity than its mere
transmission.
Dr. Faraday, who discovered the magnetism of the atmosphere, is led to
believe that the ethereal medium too is magnetic by the following
experiment. Three solutions of the protosulphate of iron, _l_, _m_, _n_,
the first of which contained 4 grains of the salt dissolved in a cubic
inch of water, the second 8 grains, and the third 16 grains—these were
respectively enclosed in three glass globules, all of which were
attracted by the pole of a magnet. A quantity of the mean solution _m_
was then put into a vessel, and the globule containing the strongest
solution _n_ was immersed in it, which was attracted as before, but the
globule _l_, containing the weakest solution, was repelled when plunged
into the same liquid. Here there was a diamagnetic phenomenon, although
the glass globules and the liquid in which they were immersed contained
iron. The effect was evidently differential, for when the liquid was
less attracted than the globule, the globule approached the pole, and
when the liquid was more attracted than the globule, the latter appeared
to recede from the pole. In fact, the effect is the same as that of
gravity on a body immersed in water; if it be more forcibly attracted
than the water, it sinks; if less forcibly attracted, it rises, the
effect being the same as if it were repelled by the earth. Hence the
question, are all magnetic phenomena the result of a differential action
of this kind, and is the ethereal medium less strongly attracted than
soft iron, and more strongly attracted than bismuth, thus permitting the
approach of the iron, but causing the bismuth to recede from the pole of
a magnet? If such a medium exist, that is, if the ethereal medium be
magnetic, then diamagnetism is the same with paramagnetism, and the
polarity of the magnetic force in iron and bismuth is one and the same.
The ethereal medium may be presumed to transmit the gravitating force;
it transmits the magnetism of the solar spots, its undulations
constitute light, heat, and all the influences bound up in the solar
beam; and the most perfect vacuum we can make is capable of transmitting
mechanical energy in enormous quantities, some of which differ but
little from that of air or oxygen at an ordinary barometric pressure;
and why not thus admit, says Mr. Thomson, the magnetic property, of
which we know so little that we have no right to pronounce a negative?
Mr. Waterstone is also of opinion that it would be taking too narrow a
view if we limited the function of the luminiferous ether to the
conveying of physical pulses only. The atmosphere also conveys physical
pulses, but that is the least important of its functions in the economy
of nature. There is nothing that should hinder us attributing to the
media concerned in the radiation of light and heat the higher functions
of electrical polarity and gravitation. The special dynamic arrangements
by which this is effected may ever elude our research; but as there is
no limit to the vis viva (N. 222) which such media may conserve in their
minutest parts, so there is no physical impossibility in that vis viva
being suddenly transferred to the molecules of ordinary matter in the
proportion and sequence required to carry out the order and system of
nature.
The fundamental principle of action in such media must be in accordance
with _elastic impact_, for upon that the dynamic theory of heat and
conservation of force rests as a foundation. The statical and dynamical
characteristics of gravitation and transfusion of force conform to it,
so that all the forces that hold the molecules of bodies together must
also be in subjection to it.[20]