DIA-MAGNETISM.
At the end of the chapter devoted to the subject of light, will be found
an experiment devised and carried out by Dr. Faraday, in which it is
shown that if a bar of a peculiar glass (called after the inventor,
_Faraday's heavy glass_, or silicated borate of lead) is subjected to
the inductive action of a very powerful electro-magnet, that it has the
power of changing the direction of a ray of polarized light transmitted
through it. This effect is not confined to the poles of an
electro-magnet, but is also perceptible (though in a diminished degree)
with ordinary magnets.
The result of this important experiment was communicated to the Royal
Society by Dr. Faraday on the 27th November, 1845, the enunciation of
the fact by this learned philosopher being, "that when '_the line of
magnetic force_' is made to pass through certain transparent bodies
parallel to a ray of polarized light traversing the same body, the ray
of polarized light experiences a rotation." Now, "_the line of magnetic
force_" means that continual flow of the magnetic current which passes
from pole to pole, and is indicated by iron filings sprinkled on paper
placed above the poles of a magnet, and usually termed _magnetic
curves_, or the curved lines of magnetic force. (Fig. 241.)
[Illustration: Fig. 241. The curved lines of magnetic force.]
The heavy glass already alluded to, upon which the magnet exerts a
certain influence, is called
THE DIA-MAGNETIC;
and by this term is meant a body through which the lines of magnetic
force are passing without affecting it like iron or steel. At page 212
is a picture representing (at Figs. 201 and 202) the direction of the
electricity and that of the magnetic current or whirl at right angles to
it. If, then, Fig. 202 be considered as a piece of glass, the arrow A B
[Page 248] will show "the line of magnetic force," the point B being the
north pole, and the shaft A the south pole of the magnet, and the arrows
traced round will represent direction. This simple drawing expresses the
whole of the law of the action of the magnet on the glass, and if kept
in view, will give every position and consequence of direction resulting
from it.
The phenomenon of the affection of the beam of polarized light is
immediately connected with the magnetic force, and this is supposed to
be proved by the _brightness_ of the polarized ray being developed
_gradually_, as the iron coiled with wire requires about two seconds to
acquire its greatest power after being connected with the battery.
In another experiment of Faraday's, where a beam of polarized light was
sent through a long glass tube containing water, and introduced as a
core _inside_ a powerful electro-magnetic coil, the image of a candle
viewed with a proper eye-piece, appeared or disappeared as the battery
connexion was made or broken with the coil; but this result is not
considered by many philosophers to be conclusive of the action of
magnetism on light, but rather as an alteration of the _refracting_
power of the medium through which the light passes. These experiments
were the precursors of the other effects of magnetism upon different
kinds of matter which Faraday discovered, and he commenced his
examination with a small bar of heavy glass suspended by a filament of
silk between the poles of an electro-magnet, and when the twisting or
effects of torsion had ceased, the battery was connected. Directly the
current passed, Faraday's keen eye detected a movement of the glass, and
on repeating the experiment, he discovered that the movement was not
accidental, but always took place in a certain fixed direction--viz., a
direction at right angles to a line drawn across and touching the two
poles of a horse-shoe-shaped magnet--_i.e._, supposing the feeder or bit
of soft iron usually placed in contact with the poles of the
horse-shoe-magnet to represent the "_axial line_," any line drawn across
it at right angles would be called the _equatorial line_, whilst the
general space included between the poles of the magnet is called "the
_magnetic field_." The movement of the heavy glass was therefore
_equatorial_, and it pointed east and west instead of north and south,
like iron and steel.
[Illustration: Fig. 242. A cube of copper suspended between the poles of
a powerful electro-magnet.]
By the use of the apparatus (Fig. 242) Faraday proved that every [Page
249] substance, whether solid, fluid, or gaseous, was subject to
magnetic influences, assuming either the axial or equatorial position.
The apparatus consists of a prolongation of the poles of a powerful
electro-magnet, between which _the_ cube of copper, weighing from a
quarter to half a pound, suspended by a thread, may be set spinning or
rotating. If the electro-magnet is connected with the battery, the cube
stops immediately, and whilst still in the same position or in the
_magnetic field_, with the magnet in full action, it is impossible to
set it spinning or twisting round again. (Fig. 242.)
A large number of other substances, solid, liquid, and gaseous, were
submitted to the action of the magnet, the liquids and gases being
hermetically sealed in glass tubes, and some of the results are detailed
in the following list:
_Bodies that point axially, or are paramagnetic, like a suspended
needle._
Iron.
Nickel.
Cobalt.
Manganese.
Chromium.
Cerium.
Titanium.
Palladium.
Platinum.
Osmium.
Paper.
Sealing-wax.
Fluor spar.
_Peroxide of lead._
Plumbago.
China ink.
Berlin Porcelain.
_Red-lead._
Sulphate of zinc.
Shell-lac.
Silkworm-gut.
Asbestos.
Vermilion.
Tourmaline.
Charcoal.
All salts of iron, when the latter is basic.
Oxide of titanium.
Oxide of chromium.
Chromic acid.
Salts of manganese.
Salts of chromium.
Oxygen, which stands alone as a paramagnetic gas.
_Bodies that point equatorially, or are diamagnetic, like Faraday's
heavy glass._
Bismuth.
Antimony.
Zinc.
Tin.
Cadmium.
Sodium.
Mercury.
_Lead._
Silver.
Copper.
Gold.
Arsenic.
Uranium.
Rhodium.
Iridium.
Tungsten.
Rock crystal.
The mineral acids.
Alum.
Glass.
_Litharge._
Common salt.
Nitre.
Phosphorus.
Sulphur. [Page 250]
Resin.
Spermaceti.
Iceland spar.
Tartaric acid.
Citric acid.
Water.
Alcohol.
Ether.
Sugar.
Starch.
Gum-arabic.
Wood.
Ivory.
Dried mutton.
Fresh beef.
Dried beef.
Apple.
Bread.
Leather.
Fresh blood.
Dried blood.
Caoutchouc.
Jet.
Turpentine.
Olive oil.
Hydrogen.
Carbonic acid.
Carbonic oxide.
Nitrous oxide (moderately).
Nitric oxide (very slightly).
Olefiant gas.
Coal gas.
Nitrogen is neither paramagnetic nor diamagnetic, and is equivalent to a
vacuum. Magnetically considered, it is like space itself, which may be
considered as zero.
The term _magnetic_ Faraday proposes should be a general one, like that
of _electricity_, and include _all_ the phenomena and effects produced
by the power, and he proposes that bodies magnetic in the sense of iron
should be called _paramagnetic_, so that the division would stand thus:
Magnetic ... { Paramagnetic,
{ Diamagnetic;
and it is this division which has been observed in the preceding tables.
* * * * *
All space above and within the limits of our atmosphere may be regarded
as traversed by lines of force, and amongst others are the lines of
magnetic force which affect bodies, as shown in the table of
paramagnetic and diamagnetic bodies, which have the same relation to
each other as positive and negative, or north and south, in electricity
and magnetism.
The lines of magnetic force are assumed to traverse void space without
change; but when they come in contact with matter of any kind they are
either concentrated upon it or scattered according to the nature of the
matter.
The power which urges bodies to the axial or equatorial lines is not a
central force, but a force differing in character in the axial or radial
directions. If a liquid paramagnetic body were introduced into the field
of force, it would dilate axially, and form a prolate spheroid like a
lemon, while a liquid diamagnetic body would dilate equatorially, and
form an oblate spheroid like an orange. Plücker has demonstrated that if
magnetic solutions are placed in watch glasses across the poles of the
[Page 251] electro-magnet, they are heaped up in a very curious manner.
The poles of the electro-magnet are pieces of soft iron, which may be
drawn away or approached at pleasure, and according as the poles are
nearer or further asunder, the magnetic liquids, such as solution of
iron, are heaped up in one or two directions, as shown at B and C in
Fig. 243.
[Illustration: Fig. 243. Glass dish holding magnetic solution of iron,
and placed in the magnetic field.]
"The diamagnetic power, doubtless," says Faraday, "has its appointed
office, and one which relates to the whole mass of the globe. For though
the amount of the power appears to be feeble, yet, when it is considered
that the crust of the earth is composed of substances of which by far
the greater portion belongs to the diamagnetic class, it must not be too
hastily assumed that their effect is entirely overruled by the action of
the magnetic matters, whilst the great mass of waters and the atmosphere
must exert their diamagnetic action uncontrolled."
Plücker has also announced--what at the time he believed to be true--the
highly interesting and important fact that the optic axis of Iceland or
calcareous spar is repelled by the magnet and placed equatorially--a
fact which Plücker thought true of many other crystals when the magnetic
axis is parallel to the longer crystallographic axis. A piece of
kyanite, which is a mineral composed of sand, clay, often lime, iron,
water, and is used in India, being cut and polished as a gem, and sold
frequently as an inferior kind of sapphire, will, it is said, even under
the influence of the earth's magnetism, arrange itself like a magnetic
needle.
Plücker believed that he had discovered an existing relation between the
forms of the ultimate particles of matter and the magnetic forces, and
he imagined that the results he obtained would lead gradually to the
determination of crystalline form by the magnet. The experiments of
Tyndal and Knoblauch lead, however, to a very opposite series of
conclusions, and by ingeniously powdering the crystals with water, and
making them into a paste, which was afterwards dried and suspended
[Page 252] as a model in "the magnetic field;" also by taking a slice of
apple about as thick as a penny-piece, with some bits of iron wire
through it, in a direction perpendicular to its flat surface, they were
found to set equatorially not by repulsion but by the attraction of the
iron wires; or instead of the iron by placing bismuth wires, the apple
now settled axially, not by attraction but by the repulsion of the
bismuth. Ipecacuanha lozenges, Carlisle biscuits also, suspended in the
magnetic field, exhibited a most striking directive action. The
materials in these two cases were _diamagnetic_; but owing to the
pressure exerted in their formation their largest horizontal dimensions
set from pole to pole, the line of compression being equatorial; and it
is a universal law "_that in diamagnetic bodies the line along which the
density of the mass has been induced by compression sets equatorial, and
in magnetic bodies axial_." Hence they assume, from these and many other
conclusive experiments, that crystallized bodies, such as Iceland spar,
take their position in the magnetic field without reference to the
existence of an "optic axis."
At the conclusion of a brilliant lecture at the Royal Institution by Dr.
Tyndal "On the influence of material aggregation upon the manifestations
of force," in which Plücker's experiments respecting the repulsion of
the optic axis were gracefully discussed and his theory refuted, the
learned doctor said: "This evening's discourse is in some measure
connected with this locality; and thinking thus, I am led to inquire
wherein the true value of a scientific discovery consists? Not in its
immediate results alone, but in the prospect which it opens to
intellectual activity--in the hopes which it excites--in the vigour
which it awakens. The discovery which led to the results brought before
us to-night was of this character. _That_ magnet[E] was the physical
birthplace of these results; and if they possess any value they are to
be regarded as the returning crumbs of that bread which in 1846 was cast
so liberally upon the waters. I rejoice, ladies and gentlemen, in the
opportunity here afforded me of offering my tribute to the _greatest
workman_ of the age, and of laying some of the blossoms of that prolific
tree which he planted at the feet of the great discoverer of
diamagnetism."[F]
[Footnote E: Alluding to a splendid magnet made by Logeman, which was
sent to the Exhibition in Hyde-park in 1851. It could sustain a weight
of 430 pounds, and was purchased by the Royal Institution for Dr.
Faraday.]
[Footnote F: Dr. Faraday.]
It was first observed by Father Bancalari, of Genoa, that when the flame
of a candle is placed between the poles of a magnet it is strongly
repelled. The flames of combustible gases from various sources are
differently affected, both by the nature of the combustible and by the
nearness of the poles. Faraday repeated Bancalari's experiments, and by
a certain arrangement of the poles of this magnet he obtained a powerful
effect in the _magnetic field_, and having the axial line of the
magnetic force horizontal, he found that when the flame of a wax taper
was held near the axial line (but on one side or the other), and about
one-third of the flame rising above the level of the upper surface of
the [Page 253] poles, as soon as the magnetic force was exerted the
flame receded from the axial line, moving equatorially until it took an
inclined position, as if a gentle wind was causing its deflection from
the upright position.
When the flame was placed so as to rise truly across the magnetic axis,
the effect of the magnetism was very curious, and is shown at A, Fig.
244.
On raising the flame a little more the effect of the magnetic force was
to intensify the results already mentioned, and the flame actually
became of a _fish-tailed shape_, as at C, Fig. 244; and when the flame
was raised until about two-thirds of it were above the level of the
axial line, and the poles approached very close, the flame no longer
rose between the poles, but spread out right and left on each side of
the axial line, producing a double flame with two long tongues, as at B,
Fig. 244.
[Illustration: Fig. 244. Effect of magnetism on candle-flame between the
poles of the magnet.]
It was these experiments that led to the important discovery of the
paramagnetic property of oxygen, and proved in a decided manner that
gaseous bodies when heated became more highly diamagnetic. Oxygen, which
(tried in the air) is powerfully magnetic, becomes diamagnetic when
heated. A coil of platinum wire heated by a voltaic current, and placed
beneath the poles of Faraday's apparatus, occasioned a strong upward
current of air; but directly the magnetic action commences the ascending
current divides, and a descending current flows down _between_ the
upward currents.
The discovery, says Silliman, of the highly paramagnetic character of
oxygen gas, and of the neutral character of nitrogen, the two
constituents of air, is justly esteemed a fact of great importance in
studying the phenomena of terrestrial magnetism. We thus see that
one-fifth of the air by volume consists of an element of eminent
magnetic capacity, after the manner of iron, and liable to great
physical changes of density, temperature, &c., and entirely independent
of the solid earth. In this medium hang the magnetic needles used as
tests, and as this magnetic medium is daily heated and cooled by the
sun's rays, its power of [Page 254] transmitting the lines of magnetic
force is then affected, influencing undoubtedly the diurnal changes of
the magnetic needle.
For a complete digest of Faraday's discoveries in diamagnetism the
reader is referred to the second edition of Dr. Noad's comprehensive and
learned work entitled "A Manual of Electricity."
Coming always from the highest walks of philosophy to lower and "_common
things_" one cannot help being reminded of the old-fashioned method of
_drawing up_ a sluggish fire, and the natural query is suggested whether
the poker is to be considered as a weak magnet, and does influence and
draw towards the fire a greater supply of magnetic oxygen gas? (Fig.
245.)
[Illustration: Fig. 245.]
[Illustration: The interior of the optical box at the
Polytechnic--looking towards the screen. The assistants are supposed to
be showing the dissolving views.]
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