As electricity began to be seriously studied in the 18th century, some scientists speculated that there might be a connection between electricity and magnetism. Interestingly, the link between electricity and magnetism was discovered in the midst of a classroom lecture demonstration in 1819 by the Danish scientist Hans Christian Oersted. Oersted was using a battery—a fairly recent invention—to produce a large current in a wire. By chance, a compass was sitting next to the wire, and Oersted noticed that the current caused the compass needle to turn. In other words, the compass responded as if a magnet had been brought near.

Oersted’s discovery that magnetism is caused by an electric current is illustrated in FIGURE 29.2. Compasses placed around a wire all point north if there’s no current. But a strong current through the wire causes the compass needles to pivot until they are tangent to a circle around the wire Part c of the figure demonstrates an important right-hand rule that relates the orientation of the compass needles to the direction of the current.

FIGURE 29.2 Response of compass needles to a current in a straight wire.

Magnetism is more demanding than electricity in requiring a three-dimensional perspective of the sort shown in Figure 29.2. But since two-dimensional figures are easier to draw, we will make as much use of them as we can. Consequently, we will often need to indicate field vectors or currents that are perpendicular to the plane of the figure. FIGURE 29.3 shows the notation we will use. FIGURE 29.4 demonstrates this notation by showing the compasses around a current that is directed into the figure. To use the right-hand rule, point your right thumb in the direction of the current (into the figure). Your fingers will curl cw, and that is the direction in which the north poles of the compass needles point.

FIGURE 29.3 The notation for vectors and currents perpendicular to the plane of the figure.

FIGURE 29.4 The orientation of the compasses is given by the right-hand rule.