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.