After Ørsted discovered that electric currents give rise to magnetic effects, it was only natural to ask if the converse of this is also true: can magnetism somehow create electric effects? After nearly a decade of work, Faraday obtained an answer in 1831 when he demonstrated that a time changing magnetic field gives rise to an electric current in a wire loop placed in the region of the changing magnetic field.
As an historical aside, it appears that the American physicist Joseph Henry actually discovered electromagnetic induction in 1830, a full year before Faraday! Henry’s results were not published until after Faraday’s announcement; however, and the discovery of induction is properly credited to Faraday.
Henry is now recognized for his discovery that a current in a loop induces a field in the circuit itself, an effect that is called self-inductance. Henry’s further work in electromagnetism, especially his work on the electromagnetic relay, by which a weak current can operate a powerful local electromagnet over very long distances, contributed to the development of the electrical telegraph. Among other honors, Henry served as the first secretary of the Smithsonian Institute between 1846 and 1878.
We develop Faraday’s idea of magnetic induction in this chapter. For example, by rotating a magnet in the vicinity of a wire coil, or a wire coil near a magnet, electric current is induced to flow. Now mechanical energy can be converted into electrical energy! This is essentially how Hoover Dam works to deliver 4.2 trillion watt-hours of energy every year to hundreds of thousands of homes in the U.S. Southwest, and the alternator in your car delivers power to recharge the car’s battery and power its electrical system.
 The figure shows a painting titled “Professor Henry Posts Daily Weather Map in Smithsonian Institution Building, 1858,” which was commissioned for the 1933 Chicago Century of Progress Exposition (Hoover, Louise Rochon, 1933, Smithsonian Archives – History Div, 84-2074 and 31052-E and 94-12563).