Electromagnetic induction is the production of an electromotive force across a conductor when it is exposed to a varying magnetic field.
Two different coils of copper wire having large number of turns (say 50 and 100 turns respectively) are taken. They are now inserted over a non-conducting cylindrical roll, as shown in figure below.
The coil-1, having larger number of turns, is connected in series with a battery and a plug key. Also, the other coil-2 is connected with a galvanometer as shown.
The key is initiated and the galvanometer is observed. We will observe that the needle of the galvanometer instantly jumps to one side and then quickly returns to zero, indicating a momentary current in coil-2.
The coil-1 is now disconnected from the battery. Now we will observe that the needle momentarily moves, but to the opposite side. It means that now the current flows in the opposite direction in coil-2.
From these observations, we conclude that a potential difference is induced in the coil-2 whenever the electric current through the coil 1 is changing (starting or stopping). As the current in the first coil changes, the magnetic field associated with it also changes. Thus the magnetic field lines around the secondary coil also change. Hence the change in magnetic field lines associated with the secondary coil is the cause of induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in
another conductor, is called electromagnetic induction.
In practice, we can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient in most situations to move the coil in a magnetic field.