Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.

Explanation

The phenomenon where a changing magnetic field in a conductor induces a current in another conductor is called electromagnetic induction.
Experiment to show that a current is set up in a closed loop when an external field passing through the loop increases or decreases :
1] Wind insulated copper wire on an insulating cylinder (made of cardboard or thick paper) as two coils - coil 1 and coil 2 having large number of turns (say 100 and 50) as shown.
2] Connect coil 1 (having more number of turns) to a battery and plug key, and coil 2 to a galvanometer.
3] Insert the plug key and observe the deflection in the galvanometer .It shows momentary deflection in one direction .
4] Now remove the plug key. Galvanometer again shows deflection but in the opposite direction.
Inference : Whenever electric current through coil 1 is changing (increasing or decreasing) , magnetic field associated with it also changes accordingly. This cause changes in the magnetic field experienced by coil 2 and hence a current is induced in it which is observed as a deflection in the galvanometer. Thus, changing magnetic filed induces a current in a closes loop.