Electro-Dynamics—Reciprocal Action of Electric Currents—Identity of
Electro-Dynamic Cylinders and Magnets—Differences between the Action
of Voltaic Electricity and Electricity of Tension—Effects of a Voltaic
Current—Ampère’s Theory—Dr. Faraday’s Experiment of Electrifying and
Magnetising a Ray of Light.
THE science of electro-magnetism, which must render the name of M.
Oersted ever memorable, relates to the reciprocal action of electrical
and magnetic currents: M. Ampère, by discovering the mutual action of
electrical currents on one another, has added a new branch to the
subject, to which he has given the name of electro-dynamics.
When electric currents are passing through two conducting wires, so
suspended or supported as to be capable of moving both towards and from
one another, they show mutual attraction or repulsion, according as the
currents are flowing in the same or in contrary directions; the
phenomena varying with the relative inclinations and positions of the
streams of electricity. The mutual action of such currents, whether they
flow in the same or in contrary directions, whether they be parallel,
perpendicular, diverging, converging, circular, or heliacal, all produce
different kinds of motion in a conducting wire, both rectilineal and
circular, and also the rotation of a wire helix, such as that described,
now called an electro-dynamic cylinder on account of some improvements
in its construction (N. 225). And, as the hypothesis of a force varying
inversely as the square of the distance accords perfectly with all the
observed phenomena, these motions come under the same laws of dynamics
and analysis as any other branch of physics.
Electro-dynamic cylinders act on each other precisely as if they were
magnets during the time the electricity is flowing through them. All the
experiments that can be performed with the cylinder might be
accomplished with a magnet. That end of the cylinder in which the
current of positive electricity is moving in a direction similar to the
motion of the hands of a watch, acts as the south pole of a magnet, and
the other end, in which the current is flowing in a contrary direction,
exhibits northern polarity.
The phenomena mark a very decided difference between the action of
electricity in motion or at rest, that is, between Voltaic and static
electricity; the laws they follow are in many respects of an entirely
different nature, though the electricities themselves are identical.
Since Voltaic electricity flows perpetually, it cannot be accumulated,
and consequently has no tension, or tendency to escape from the wires
which conduct it. Nor do these wires either attract or repel light
bodies in their vicinity, whereas static or ordinary electricity can be
accumulated in insulated bodies to a great degree, and in that state of
rest the tendency to escape is proportional to the quantity accumulated
and the resistance it meets with. In ordinary electricity, the law of
action is, that dissimilar electricities attract and similar
electricities repel one another. In Voltaic electricity, on the
contrary, similar currents, or such as are moving in the same direction,
attract one another, while a mutual repulsion is exerted between
dissimilar currents, or such as flow in opposite directions. Common
electricity escapes when the pressure of the atmosphere is removed, but
the electro-dynamical effects are the same whether the conductors be in
air or in vacuo.
The effects produced by a current of electricity depend upon the
celerity of its motion through a conducting wire. Yet we are ignorant
whether the motion be uniform or varied, but the method of transmission
has a marked influence on the results; for, when it flows without
intermission, it occasions a deviation in the magnetic needle, but it
has no effect whatever when its motion is discontinuous or interrupted,
like the current produced by the common electrical machine when a
communication is made between the positive and negative conductors.
M. Ampère has established a theory of electro-magnetism suggested by the
analogy between electro-dynamic cylinders and magnets, founded upon the
reciprocal attraction of electro-currents, to which he reduces all the
phenomena of magnetism and electro-magnetism, by assuming that the
magnetic properties which bodies possess derive these properties from
currents of electricity, circulating about every part in one uniform
direction. Although every particle of a magnet possesses like properties
with the whole, yet the general effect is the same as if the magnetic
properties were confined to the surface. Consequently, Ampère concludes
that the internal electro-currents must compensate one another, and that
the magnetism of a body must therefore arise from a superficial current
of electricity constantly circulating in a direction perpendicular to
the axis of the magnet; so that the reciprocal action of magnets and all
the phenomena of electro-magnetism are reduced to the action and
reaction of superficial currents of electricity, acting at right angles
to their direction.
Notwithstanding the experiments made by Ampère to elucidate the subject,
there is still an uncertainty in the theory of the induction of
magnetism by an electric current in a body near it. It does not appear
whether electric currents which did not previously exist are actually
produced by induction, or if its effect be only to give one uniform
direction to the infinite number of electric currents previously
existing in the particles of the body, and thus rendering them capable
of exhibiting magnetic phenomena, in the same manner as polarization
reduces the undulations of light to one plane, which had previously been
performed in every plane. Possibly both may be combined in producing the
effect; for the action of the electric current may not only give a
common direction to those already existing, but may also increase their
intensity. However that may be, by assuming that the attractions and
repulsions of the elementary portions of electric currents vary
inversely as the square of the distance, the actions being at right
angles to the direction of the current, it is found that the attraction
and repulsion of a current of indefinite length on the elementary
portion of a parallel current at any distance from it are in the simple
ratio of the shortest distance between them: consequently, the
reciprocal action of electric currents is reduced to the composition and
resolution of forces, so that the phenomena of electro-magnetism are
brought under the laws of mechanics by the theory of Ampère. It appears
that Dr. Faraday’s very remarkable experiment of electrifying and
magnetising a ray of polarized light may possibly afford a demonstration
of the reality of Ampère’s explanation of the ultimate nature of
magnetism.
In this experiment a copper wire 501 feet long was arranged in four
concentric spirals, the extremities of which were connected with the
poles of a powerful galvanic battery, and a polished prism of heavy
glass, or silicated borate of lead, was placed in the axis of the spiral
as a core, through the length or axis of which a ray of polarized light
was sent. This ray, viewed through a piece of tourmaline or a Nichol’s
eye-piece, vanished and reappeared as usual at each quarter revolution
of the eye-piece; but when a current of electricity was sent through the
spiral at the time the ray had vanished, it instantly reappeared, and
remained as long as the electric current continued to flow; but the
instant the electricity ceased the light vanished, and as often as the
electric current flowed through the spiral, or was interrupted, so often
did the polarized ray appear and vanish.
The character of the force thus impressed on the heavy glass is that of
rotation, for the stopping and renewing of the electric current had the
same effect as the revolving motion of the eye-piece in making the light
alternately appear and vanish. Accordingly, Dr. Faraday found that, when
the electricity flowed through the spiral in one direction, the rotation
of the plane of polarization was right-handed; and when it flowed in the
other direction, the rotation of the plane of polarization was
left-handed, the rotation increasing with the length of the prism and
the intensity of the electricity. The same phenomena were produced by a
very powerful magnet when a ray of polarized light was sent through the
heavy glass parallel to the line of magnetic force.
Heavy glass or silico-borate of lead has the property more than any
other substance of making light rotate under electric and magnetic
influence; but many substances have the property more or less, as flint
and crown glass, rock salt, all the fixed and essential oils, water, and
many other liquids, but none of the gases possess it. In those
substances that have the power of circular polarization naturally, the
magnetic and electric influences increase or diminish the rotation
according to its direction.
Polarized heat is made to revolve in the same manner, when the medium
through which it passes is subject to magnetic influence.
Mr. Grove observes that if light and heat be merely modes of force,
which there is every reason to believe that they are, it may be fairly
stated that in these experiments magnetism affects these forces
directly; for light and heat being, in that view, motions of ordinary
matter, magnetism in affecting these movements affects the forces which
occasion them. If, however, this effect of magnetism be a molecular
change of the matter transmitting the light and heat, then it follows
that the light and heat are indirectly affected by the electricity or
magnetism. Dr. Faraday says that the magnetic forces do not act on the
ray of light directly, without the intervention of matter, but through
the mediation of the substance in which they and the ray have a
simultaneous existence; the substances and the forces giving to and
receiving from each other the power of acting on the light. Dr. Thomson
has shown, by a mathematical investigation of the subject, that Dr.
Faraday’s discovery seems to prove the truth of Ampère’s explanation of
the ultimate nature of magnetism. However, in Ampère’s theory, the
current of electricity flowing round the iron makes it a permanent
magnet, but it does not make the heavy glass or the other bodies, which
have the same property, either temporary magnets when the light is
rotating within them, or permanent magnets when the inductive action of
the current of electricity ceases. Hence the molecular condition of the
substances, when the light is rotating in them, must be specifically
distinct from that of magnetised iron: it must therefore be a new
magnetic condition, and the force which the matter in this state
possesses must be a new magnetic force.
After describing his admirable experiment, Dr. Faraday observes that “it
has established for the first time a true, direct relation and
dependence between light and the magnetic and electric forces; and thus
a great addition is made to the facts and considerations which tend to
prove that all natural forces are tied together, and have one common
origin. It is no doubt difficult, in the present state of our knowledge,
to express our expectations in exact terms; and though I have said that
another of the powers of nature is in these experiments directly related
to the rest, I ought perhaps rather to say that another form of the
great power is distinctly and directly related to the other forms; or
that the great power manifested by particular phenomena in particular
forms is here further identified and recognised by the direct relation
of its form of light to its forms of electricity and magnetism. The
relation existing between _polarized_ light and magnetism and
electricity is even more interesting than if it had been shown to exist
with common light only. It cannot but extend to common light; and, as it
belongs to light made in a certain respect more precise in its character
and properties by polarization, it collates and connects it with these
powers in that duality of character which they possess, and yields an
opening, which before was wanting to us, for the appliances of these to
the investigation of the nature of this and other radiant agencies.”
Thus Dr. Faraday’s experiment not only shows the increasing connexion
between the sciences, but the tendency of all the forces of nature to
merge in one great and universal power.
In the action of a magnet upon the stratifications of an electrical
discharge Mr. Gassiot has discovered a new instance of the connexion
between magnetism and light.