MUTUAL INDUCTION
Let us consider two coils P and S placed close* to each other. A battery and a tapping key K are connected to the P-coil, while a galvanometer is connected across the S-coil [Fig. ]. The coils P and S are respectively known as primary and secondary coils.
As soon as the key K is pressed, the current in the P-coil starts growing. As the current grows, the magnetic flux starts building up with the coil P and due to self induction, an induced e.m.f. is produced in it. The induced e.m.f. opposes the growth of current in the P-coil. Since the S-coil is placed close to the P-coil, the magnetic flux building up with the S-coil and hence an induced e.m.f. is produced in the S-coil also. This induced e.m.f. in the S-coil also opposes the growth of current in the P-coil.
Similarly, when the key is released, the induced e.m.f. is produced in the primary as well as the secondary coil. The sign of the induced e.m.f. produced in this case will be opposite to that produced when the key is pressed.
Thus, during both growth and decay of the current in the primary coil, an opposing induced e.m.f. is produced in the secondary coil.
The phenomenon according to which an opposing e.m.f. is produced in a coil as a result of change in current or magnetic flux linked with a neighbouring coil is called mutual induction.