Mutual Induction 

Mutual Induction
Make a coil with insulated conducting copper wire at one end of a soft iron core and connect its ends to a torch cell through a switch Make another coil at the other end of the same soft iron core and Connects the ends of the coil into a galvanometer. Of these, the circuit which is connected to the battery is called primary circuit and that connected to the galvanometer is called secondary circuit. Here, there is no interconnection between primary and secondary connections.
When we switch on, the galvanometer deflects to one side and returns back to its mean position. When we switch off, the galvanometer deflects to opposite direction and come back to original position.
Now repeat the experiment connecting a 6V AC supply instead of the DC in the primary coil and 6V bulb instead of the galvanometer in the secondary, the bulb starts glowing continuously. How does it happen?
When ac passes through a solenoid, a fluctuating magnetic field is produced around it.
When the strength of the current in one of the two adjacent coil changes, a change take place in the magnetic field around. As a result of this, an emf is induced in the second coil. This phenomenon is mutual induction.
The emf produced due to mutual induction and the number of turns in the coil are inter related .If we increase the number of coils in the secondary, the intensity of the bulb also will increase. If we cut down the number of turns into half, the intensity of the glow also decreases. From this, we can conclude that there is a relation between the emf produced by mutual induction and number of turns.