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 Profe or Oersted of Copenhagen, and has become the source of an important series of discoveries included under the above term. The excitation of magnetism depends 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 au electric current la pa ing, it will immediately deviate from its usual position, and a ume a new one, dependent 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 declination takes place; but the magnet shows a disposition to move in a vertical direction, the pole next the negative side of the battery being depre ed 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 appear to depend upon the circulation of magnetism at right angles to the electric current, so that if m N P represent the wire transmit-, c ting a current of electricity in the " E^= 1 -—J1, direction 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 phenomena; 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 respectively are so arranged as to be able to move freely in any direction. If a steel needle be placed!n 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 __i^sS^^S:'5l' permanently magnetic. If the 7H^^^430335^'( ( /■1'.'1 \ ya\T' s spiral,electric and wire the steel be twis,edneedle placed in to a rl i ' ■ within it (as in the cut), it is retained there, and becomes a more powerful magnet in consequence of the repetitions and direction of the electric and magnetic currents, as will be evident from the annexed figure, where o represents a gla tube with the wire tip conveying the electric current twisted round it; the darts at the'ends of which show the ingre and egre of the electricity, and the transverse darts Ihe direction of the magnetic current. If the cylinder round which the wire [s. 405]