After the discovery in 1819 by Oersted that an electric current produces a magnetic field, scientists turned their attention to search for the inverse effect, that is, the production of an electric current by a magnetic field. In 1831, Faraday in England and Henry in the United States showed that a varying magnetic field or more generally a varying magnetic flux in a circuit induces an electric current. Faraday was the first to publish his results and the discovery of this induction law was attributed to him. Henry later discovered self-induction, i.e. the induction of an electromotive force (emf) in a circuit if its own magnetic flux varies. The discovery of induction had many important applications. One of them was the large-scale production of electricity, which led to a new technological era.
The induction phenomenon is quite complicated, because there are really two kinds: Neumann’s induction, which appears even in vacuum if the magnetic field varies in time, and Lorentz induction, which appears in moving conductors in a constant magnetic field. In this chapter we will study induction and some of its applications.
8.1. Induction due to the variation of the flux, Faraday’s and Lenz’s laws
Faraday’s historic experiment showed that a current is induced in a circuit (without generators) if the magnetic flux through this circuit varies. The flux may be varied by displacing a magnet or by varying the field of a nearby electromagnet by varying its current. The ...