\u3ci\u3ePhysics\u3c/i\u3e, Chapter 30: Magnetic Fields of Currents

The first evidence for the existence of a magnetic field around an electric current was observed in 1820 by Hans Christian Oersted (1777-1851). He found that a wire carrying current caused a freely pivoted compass needle in its vicinity to be deflected. If the current in a long straight wire is directed from C to D, as shown in Figure 30-1, a compass needle below it, whose initial orientation is shown in dotted lines, will have its north pole deflected to the left and its south pole deflected to the right. If the current in the wire is reversed and directed from D to C, then the north pole will be deflected to the right, as seen from above. In terms of the forces acting on the poles, these forces are clearly perpendicular to the direction of the current and to the line from the nearest portion of the wire to the pole itself