was first discovered by the Danish Physicist Oersted (1777-1851), not by the experiment here described, but in a still more simple way. He found that if a wire carrying a current is placed above and parallel to the needle of a compass, the needle is deflected. The deflection is in one sense with the current flowing one way, and in the opposite sense if the current is reversed. The deflection is increased if the strength of the current is augmented, or the wire brought nearer. There is no deflection if the wire is placed not over, but parallel to and at the side of the needle; and if the wire is shifted from a position parallel to and above the needle to a similar position and distance below the needle, the deflection is reversed. Also, if the wire is not exactly parallel to the direction in which the needle points, there is some deflecting force, though this gets weaker as the angle between wire and needle increases. All these facts the reader may, by the aid of a pocket compass, a voltaic cell and a few feet of wire, find out for himself.
To us in 1912 there is nothing remarkable about such an experiment; but when Oersted first performed it in 1820 it was a revelation of enormous import. The scientific world of
