Mutual Induction: Fig. 17.19 shows two coils X and Y, placed close to each other, the coil X has been connected with a battery and a switch, whereas a sensitive galvanometer is connected with the coil Y. we observe that as soon as the switch of the coil X is closed, the galvanometer connected with the coil Y shows a momentary deflection indicting a momentary flow of current through the coil Y in spite of the fact that the two coils are quite separate having no electrical connection between them.
Similarly at the instant when the flow of current in the coil X is stopped by putting its switch off, the galvanometer connected with the coil Y again shows a deflection but this time its direction is opposite to that of the previous case.
We can explain this observation on the basis of magnetic flux passing through the two coils.
As soon as the switch connected with coil X is closed, a current begins to flow in it and with it, and its magnetic field begins to build up (fig. 17.19). some of the lines of force of this field start passing the neighboring coil Y, which results in the change of magnetic flux to take place through the coil Y due to which a current is induced in its circuit.
As soon as magnetic field of coil X attains its constant value, it stops increasing due to which the increase in flux through the coil Y also stops. Hence the induced current reduces to zero. The magnetic field of coil X attains its constant value within a few moments after the closure of its switch, therefore the current induced in coil Y is also momentary.
Similarly when the switch of the coil X is opened, the flow of current through it stops and in few moments its magnetic field which current is again induced in it. As this current is induced due to the decrease in flux, so its direction is opposite to the direction of the previous momentary current because that current was induced in the coil Y due to increase in the value of magnetic flux.
If a current is induced n a circuit due to a change of current in another circuit, this phenomenon is known as mutual induction.
The coil in which the change in current produces induced current in another coil is known as primary coil and the coil in which current is induced is known as a secondary coil.