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the union of the two electricities, and magnetic neutrality by the combination of the two polarities, thus respectively neutralizing each other when joined. All these forces vary inversely as the square of the distance, and consequently come under the same mechanical laws. A like analogy extends to magnetic and electrical induction. Iron and steel are in a state of equilibrium when the two magnetic polarities conceived to reside in them are equally diffused throughout the whole mass, so that they are altogether neutral. But this equilibrium is immediately disturbed on the approach of the pole of a magnet, which by induction transfers one kind of polarity to one end of the iron or steel bar, and the opposite kind to the other,-effects exactly similar to electrical induction. There is even a correspondence between the fracture of a magnet and that of an electric conductor; for if an oblong conductor be electrified by induction, its two extremities will have opposite electricities; and if in that state it be divided across the middle, the two portions, when removed to a distance from one another, will each retain the electricity that has been induced upon it. The analogy, however, does not extend to transference. A body may transfer a redundant quantity of positive or negative electricity to another, the one gaining at the expense
of the other; but there is no instance of a body possessing only one kind of polarity. With this exception, there is such perfect correspondence between the theories of magnetic attractions and repulsions and electric forces in conducting bodies, that they not only are the same in principle, but are determined by the same formulæ. Experiment concurs with theory in proving the identity of these two unseen influences.
THE disturbing effects of the aurora borealis and of lightning on the mariner's compass had been long known, but in the year 1819, M. Oersted, Professor of Natural Philosophy at Copenhagen, discovered that a current of voltaic electricity exerts a powerful influence on a magnetised needle, an observation which has given rise to the theory of electro-magnetism, the most interesting science of modern times, whether it be considered as leading us a step farther in generalization, by identifying two agencies hitherto referred to different causes, or as developing a new force unparalleled in the system of the world, which, overcoming the retardation from friction, and the obstacle of a resisting medium, maintains a perpetual motion, often vainly attempted, but which it seems alto
gether impossible to accomplish by means of any other force or combination of forces than the one in question.
When the two poles of a voltaic battery are connected by a metallic wire, so as to complete the circuit, the electricity flows without ceasing; and if a straight portion of that wire be placed parallel to, and horizontally above, a magnetised needle at rest in the magnetic meridian, but freely poised like the mariner's compass, the action of the electric current flowing through the wire will instantly cause the needle to change its position: its extremity will deviate from the north towards the east or west, according to the direction in which the current is flowing; and on reversing the direction of the current, the motion of the needle will be reversed also. The numerous experiments that have been made on the magnetic and electric fluids, as well as those on the various relative motions of a magnetic needle under the influence of galvanic electricity, arising from all possible positions of the conducting wire, and every direction of the voltaic current, together with all the other phenomena of electro-magnetism, are explained by Dr. Roget in some excellent articles on these subjects in the Library of Useful Knowledge.
All the experiments tend to prove that the force
emanating from the electric current, which produces such effects on the magnetic needle, acts at right angles to the current, and is therefore unlike any force hitherto known. The action of all the forces in nature is directed in straight lines, as far as we know, for the curves described by the heavenly bodies result from the composition of two forces, whereas, that which is exerted by an electrical current upon either pole of a magnet has no tendency to cause the pole to approach or recede, but to rotate about it. If the stream of electricity be supposed to pass through the centre of a circle whose plane is perpendicular to the current, the direction of the force exerted by the electricity will always be in the tangent to the circle, or at right angles to its radius; consequently the tangential force of the electricity has a tendency to make the pole of a magnet move in a circle round the wire of the battery. Mr. Barlow has proved that the action of each particle of the electric fluid in the wire, on each particle of the magnetic fluid in the needle, varies inversely as the square of the distance.
Rotatory motion was suggested by Dr. Wollaston; Mr. Faraday was the first who actually succeeded in making the pole of a magnet rotate about a vertical conducting wire. In order to limit the action of the electricity to one pole, about
two-thirds of a small magnet was immersed in mercury, the lower end being fastened by a thread to the bottom of the vessel containing the mercury. When the magnet was thus floating almost vertically with its north pole above the surface, a current of positive electricity was made to descend perpendicularly through a wire touching the mercury, and immediately the magnet began to rotate from left to right about the wire. As the force is uniform, the rotation was accelerated till the tangential force was balanced by the resistance of the mercury, when it became constant. Under the same circumstances, the south pole of the magnet rotates from right to left. It is evident from this experiment that the wire may also be made to perform a rotation round the magnet, since the action of the current of electricity on the pole of the magnet must necessarily be accompanied by a corresponding reaction of the pole of the magnet on the electricity in the wire. This experiment has been accomplished by a vast number of contrivances, and even a small battery, consisting of two plates, has performed the rotation. Mr. Faraday produced both motions at the same time in a vessel containing mercury; the wire and the magnet revolved in one direction about a common centre of motion, each following the other.
The next step was to make a magnet and