感应

Electromagnetic induction is produced when an electrically conducting material is exposed to a time-varying magnetic field.

For example, consider a bar magnet placed close to a coil. The galvanometer connected to this coil shows zero current passing through it.

If the bar magnet moves toward the coil, a current flows through the coil. Furthermore, if the bar magnet is moved away from the coil, current flows in the opposite direction.

Similarly, if the bar magnet is kept fixed and the coil is moved in either direction, then current flows through the coil.

Suppose a bar magnet is replaced with a second coil connected to a battery. If one of the coils is moved, a current flows through the other coil. 

If a switch controls the current passing through the one coil, then an instantaneous current pulse is induced in the other coil while opening or closing the switch.

Here the current flowing through the other coil is called an induced current, and the corresponding emf is called an induced emf.