« PreviousContinue »
the agencies of electro-dynamic 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 electrodynamic 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, electrodynamic cylinders act on each other precisely as if they were magnets, during the time the electricity is flowing through them.
The phenomena mark 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 de gree, 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 he same whether the conductors be in air or
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 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 electromagnetism 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 electromagnetism, 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 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 those 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 an 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 attraction and repulsion of the elementary portions of electric currents vary inversely as the square of the distance, the action 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, is 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 dynamics by the theory of Ampère.
From the law of action and reaction being equal and contrary, it might be expected that, as electricity powerfully affects magnets, so, conversely, magnetism ought to produce electrical phenomena. By proving this very important fact from a series of highly interesting and ingenious experiments, Mr. Faraday has added another branch to the science, which he has named magneto-electricity. A great quantity of copper wire was coiled in the form of a helix round one half of a ring of soft iron, and connected with a galvanic battery, while a similar helix connected with a galvanometer was wound round the other half of the ring, but not touching the first helix. As soon as contact was made with the battery, the needle of the galvanometer was deflected, but the action was transitory, for when the contact was continued the needle returned to its usual position, and was not affected by the continual flow of the electricity through the wire connected with the battery. As soon, however, as the contact was broken, the needle of the galvanometer was again deflected, but in the contrary direction. Similar effects were produced by an apparatus consisting of two helices of copper wire coiled round a block of wood, instead