Introduction

By the early nineteenth century, Oersted, Ampere and others had shown that an electric current produces a magnetic field around it. This chapter builds the reverse chain of reasoning that Michael Faraday (and, independently, Joseph Henry) worked out in 1831: can a changing magnetic field, or a changing magnetic flux, itself produce an electric current? Faraday's careful experiments with a magnet, a coil and a galvanometer, and later with two coils, established that it can -- this phenomenon is called electromagnetic induction, and the current it produces in a closed circuit is the induced current.

Faraday summarised his observations as follows: (i) moving a magnet's pole towards a stationary closed coil produces a current in the coil, called the induced current; (ii) withdrawing the magnet produces a current again, but in the opposite direction to (i); (iii) it is the RELATIVE motion that matters -- sliding the coil towards or away from a stationary magnet gives exactly the same effect as moving the magnet towards or away from a stationary coil; (iv) reversing the magnet's polarity reverses the direction of the induced current; (v) the size of the induced current grows with the relative speed between coil and magnet, and with the number of turns in the coil; and (vi) the induced current exists only so long as the relative motion continues -- once the magnet and coil are at rest with respect to each other, the current stops.

Faraday then repeated the experiment with two coils instead of a magnet and a coil: a primary coil, wired to a battery, rheostat and key, and a secondary coil, wired only to a galvanometer, with the two coils' planes facing each other. Three further observations emerged: closing or opening the primary circuit gives a momentary galvanometer kick, in opposite directions for make and break; moving either coil relative to the other (with both circuits closed) induces a current in the secondary for as long as the relative motion lasts; and simply changing the primary current's magnitude (by sliding the rheostat) -- with both coils held fixed in place -- is by itself enough to deflect the galvanometer connected to the secondary coil.

Taken together, these observations point to one single underlying cause: whenever the magnetic flux linked with a coil changes -- whether because a magnet moves, because a nearby current-carrying coil's field changes, or because the coil's own area or orientation changes -- an emf is induced in that coil, and the direction of this induced emf reverses when a growing flux turns into a shrinking one (or vice versa).