ELECTRO-MAGNETISM. When a current of electricity is traversing any substance, or when electricity is in motion magnetism is at the same time developed. This fact was first observed by Professor Oersted, of Copenhagen, and has be come the source of an important series of discoveries included under the above term. The excitation of magnetism de pends upon quantity of electricity, and is best observed in the wire which closes the voltaic circle, especially of one or more pairs of large plates. If a magnetic needle be brought near a wire through which an electrio current is passing, it will immediately deviate from its usual position, and assume a new one, depen dent upon the relative position of the needle and the wire. On placing the electric wire above and parallel to the magnet, the pole next the negative end of the battery always moves to the west ; and when the wire is placed under the needle, the same pole turns to the east. When the electric wire is on the same horizontal plane with the needle, no de clination takes place ; but the magnet shows a disposition to move in a vertical the pole next the negative side of the battery being depressed when the wire is to the west of it, and elevated when it is to the east.
The magnetic phenomena of a wire transmitting electricity are such as ap pear to depend upon the circulation of magnetism at right angles to the electric current, so that if N P represent the wire transmitting a current of elec tricity in the di rection of the horizontal darts, a current of magnetism will be established in the direction of the vertical dart, appearing to move round the axis of the electric current ; hence the term vertiginous or rotary magnetism, applied to these phe nomena; and hence the motion of the pole of the magnet round the electric wire, or of the electric wire round the pole of the magnet, when they respec tively are so arranged as to be able to move freely in any direction. If a steel
needle be placed in contact with the electric wire, and parallel to it, it acquires opposite magnetisms upon its two sides; but if it be placed at right angles to the connecting wire, it becomes polar, and permanently mag netic. If the elec tric wire be twist ed into a spiral, and the steel needle placed within it (as in the cut), it is retained there, i and be comes a more powerful magnet n conse quence of the repetitions and direction of the electric and magnetic currents, as will be evident from the annexed figure, where a represents a glass tube with the wire T conveying the electric current twisted round it ; the darts at the ends of which show the ingress and egress of the electricity, and the transverse darts the direction of the magnetic current. If the cylinder round which the wire conveying the electric current is twisted be of steel, it becomes a permanent meg net ; if of pure soft iron, it becomes a temporary magnet, so long as the electric current is in motion, and a and at are powerfully opposed poles. If the bar be bent, as in the annexed cut, a power ful horse-shoe magnet is ob tained when the ends t Is of the copper wire twisted round it are connected with the vol taic circle ; and a single pair of plates is sufficient for the purpose.