The effect that an electromotive force is generated in an electrical field if the magnetic field associated with it is changed.
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The movie below shows the effect of a magnet being moved in and out of an electric coil. A current is induced, indicated by the instrument, when the magnetic field to which the coil is exposed changes. No current is observed when the magnet and with it the magnetic field are at rest.
The animation below uses equipotentials to show the change of the field during the phase when the magnet is removed. A current flows only when field lines cross the coil (move from the coil's inner side to its outside) and is proportional to the number of field lines that pass.
Belcher, J. W. (2004) Teaching Electromagnetism Using Advanced Technologies, Massachsetts Institute of Technology Open Courseware, http://web.mit.edu/jbelcher/www/inout.html (accessed 1 June 2004).
Movies: © J. Belcher, MIT Open Courseware; reproduced by permission.