Oersted’s 1820 discovery that a current creates a magnetic field generated enormous excitement. One question scientists hoped to answer was whether the converse of Oersted’s discovery was true: that is, can a magnet be used to create a current?

The breakthrough came in 1831 when the American science teacher Joseph Henry and the English scientist Michael Faraday each discovered the process we now call electromagnetic induction. Faraday—whom you met in Chapter 22 as the inventor of the concept of a field—was the first to publish his findings, so today we study Faraday’s law rather than Henry’s law.

Faraday’s 1831 discovery, like Oersted’s, was a happy combination of an unplanned event and a mind that was ready to recognize its significance. Faraday was experimenting with two coils of wire wrapped around an iron ring, as shown in FIGURE 30.1. He had hoped that the magnetic field generated in the coil on the left would induce a magnetic field in the iron, and that the magnetic field in the iron might then somehow create a current in the circuit on the right.

FIGURE 30.1 Faraday’s discovery.

Like all his previous attempts, this technique failed to generate a current. But Faraday happened to notice that the needle of the current meter jumped ever so slightly at the instant he closed the switch in the circuit on the left. After the switch was closed, the needle immediately returned to zero. The needle again jumped when he later opened the switch, but this time in the opposite direction. Faraday recognized that the motion of the needle indicated a current in the circuit on the right, but a momentary current only during the brief interval when the current on the left was starting or stopping.

Faraday’s observations, coupled with his mental picture of field lines, led him to suggest that a current is generated only if the magnetic field through the coil is changing. This explains why all the previous attempts to generate a current with static magnetic fields had been unsuccessful. Faraday set out to test this hypothesis.

Faraday found that there is a current in a coil of wire if and only if the magnetic field passing through the coil is changing. This is an informal statement of what we’ll soon call Faraday’s law. The current in a circuit due to a changing magnetic field is called an induced current. An induced current is not caused by a battery; it is a completely new way to generate a current.