In the arrangement shown in given Figure, there are two coils wound on a non-conducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then

This is due to the phenomenon of electromagnetic induction. Changing magnetic field in a conductor induces a current in another conductor. When key is plugged the galvanometer shows deflection and then returns to zero. Again when key is unplugged deflection is observed but in opposite direction as current flows in opposite direction.