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electric currents, expresses the power which these currents possess of inducing any particular state upon matter in their immediate neighborhood, otherwise neutral or indifferent. For example, the connecting wire of a galvanic battery holds iron filings suspended like an artificial magnet, as long as the current continues to flow through it; and the most powerful temporary magnets that have been made are obtained by bending a thick cylinder of soft iron into the form of a horseshoe, and surrounded it with a coil of thick copper wire covered with silk, to prevent communication between its parts. When this wire forms part of a galvanic circuit, the iron becomes so highly magnetic, that a tempoary magnet of this kind made by Professor Henry of the Albany Academy, in the United States, sustained nearly a ton weight. The iron loses its magnetic power the instant the electricity ceases to circulate, and acquires it again as instantaneously when the circuit is renewed. Steel needles are rendered permanently magnetic by electrical induction; the effect is produced in a moment, and as readily by juxtaposition as by contact; the nature of the poles depends upon the direction of the current, and the intensity is proportional to the quantity of electricity.
It appears from what precedes, that the principle and characteristic phenomena of the electro-magnetic science are, the evolution of a tangential and rotatory force exerted between a conducting body and a magnet; and the transverse induction of magnetism by the conducting body in such substances as are susceptible of it.
The action of an electric current causes a deviation of
* Now of Princeton College, New Jersey.
the compass from the plane of the magnetic meridian. proportion as the needle recedes from the meridian, the intensity of the force of terrestrial magnetism increases, while at the same time the electro-magnetic force diminishes; the number of degrees at which the needle stops, and which mark where the equilibrium between these two forces takes place, will indicate the intensity of the galvanic current. The galvanometer, constructed upon this principle, is employed to measure the intensity of galvanic currents collected and conveyed to it by wires. This instrument is rendered much more sensible by neutralizing the effects of the earth's magnetism on the needle, which is accomplished by placing a second magnetised needle sa as to counteract the action of the earth on the first, a precaution requisite in all delicate magnetical experiments.
The science of electro-magnetism which has been under consideration, and 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 posi
tions of the streams of electricity. It appears that 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 and now called an electrodynamic cylinder on account of some improvements in its construction; 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.
The theory of electro dynamics, as well as actual experiment, confirms the identity between the agencies of electro-dynamics cylinders, or helices, and magnets. The law of the reciprocal action of a cylinder and an electric current is precisely the same, and all the experiments that can be performed with the cylinder might be accomplished with a magnet. It has already been observed that the two extremities of an electro-dynamic cylinder or helix exhibit all the properties possessed by the poles of a magnet; that end in which the current of positive electricity is moving in a direction similar to the motion of the hands of a watch, acting as a south pole, and the other end, in which the current is flowing in a contrary direction, exhibiting northern polarity. In conformity with this resemblance, electro-dynamics cylinders act on each other precisely as if they were magnets, during the time the electricity is flowing through them.
The phenomena marks a very decided difference between the action of electricity in motion or at rest, that is, between
voltaic and common electricity; the laws they follow are in many respects of an entirely different nature. 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 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. The common electricity escapes when the pressure is removed, but the electro-dynamical effects are the same whether the conductors be in air or in vacuo.
Although the effects produced by a current of electricity depend upon the celerity of its motion, the velocity with which it moves through a conducting wire is unknown. We are equally ignorant whether it be uniform or varied, but the method of transmission has a marked influence on the results; for when it flows without intermission, it occa sions 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 electric currents, to which all the phenomena of magnetism and electro-magnetism may be reduced, by assuming that the magnetic properties which bodies possess derive these properties from currents of electricity circulating about every part in one uniform direction. It has been observed that, although every particle of a magnet possess like properties with the whole, yet the general effect is the same as if the magnetic properties were confined to the surface: consequently the internal electro-currents must compensate one another, and therefore the magnetism of a body is supposed to 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 the direction of the currents. Notwithstanding some experiments made by M. 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; for it does not appear whether electric currents which did not previously exist are actually produced by induction, or if its effects 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 those undulations of light to one plane which had previously been performed in every plane. Possibly both inay be combined in producing the effect; for the action of an electric current may not only give a common direction to those already existing, but may also