# Chapter 8

# Induction

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 ...