Page:Elementary Text-book of Physics (Anthony, 1897).djvu/380

366 within it. If the two circuits are parallel, this current will be in the same sense as that in circuit 1. The current induced in circuit 2 gives rise to tubes of induction which enter circuit 1, and their entrance into circuit 1 is resisted by a current tending to repel them from circuit 1, or to set up tabes of induction in the opposite sense. Thus there will be a small current in circuit 1 in the opposite sense to that originally in it and the current in circuit 1 will therefore diminish more rapidly than if circuit 2 were not present. On the other hand, if neither circuit carries a current, and a current be suddenly impressed on circuit 1, the tubes of induction to which it gives rise will enter circuit 2, and will be resisted by a momentary current in circuit 3 tending to repel them, or to set up tubes of induction in the opposite sense. Thus the induced current in circuit 2 in this case, if the two circuits are parallel, is in the opposite sense to that in circuit 1. This current in circuit 2 will in turn set up tubes of induction which enter circuit 1 and are there resisted by a momentary small current which will be in the same sense as that impressed upon circuit 1. Thus the presence of circuit 2 will temporarily increase the current in circuit 1.

The fact that induced currents are produced in a closed circuit by a variation in the number of lines of magnetic force included in it was first shown experimentally by Faraday in 1831. He placed one wire coil, in circuit with a voltaic battery, inside another which was joined with a sensitive galvanometer. The first he called the primary, the second the secondary, circuit. When the battery circuit was made or broken, deflections of the galvanometer were observed. These were in such a direction as to indicate a current in the secondary coil, when the primary circuit was made, in the opposite direction to that in the primary, and when the primary circuit was broken, in the same direction as that in the primary. When the positive pole of a bar magnet was thrust into or withdrawn from the secondary coil, the galvanometer was deflected. The currents indicated were related to the direction of motion of the positive magnet pole, as the directions of rotation and propulsion in a left-handed screw. The direction of the induced currents