The flux due to the current in the coil links that coil and if the current changes, the resulting flux change induces an emf in the coil itself.

This effect is called self induction. The coil is said to have self inductance, (L) and the coil is said to be an inductor. The induced emf tends to oppose the growth of current in the coil.

* Demonstration of self induction *

L and R have the same resistance. L_{1 }and L_{2 }are identical lamps. When the current is switched on, lamp L_{2} lights up a second or two later after lamp L_{1} has lit. This is because the induced emf in L opposes the current flowing through lamp L_{2}. Therefore the growth of current in L_{2} to its steady value is delayed.

*Variation of current with time through L _{1 }and L_{2} *

If the 3V d.c is replaced by a 3V a.c, the lamp in series with L does not light because the induced emf, due to self induction in L, opposes the applied p.d continuously.

* Question: *Why L

_{2}light when the iron core is removed in the coil?

Self inductance, L, of a coil is the ratio of the induced emf to the rate at which the current in

**THIS VIDEO EXPLAINS MORE ABOUT SELF INDUCTANCE**

If two coils i.e. the primary and the secondary coils, are near each other, and the current in the primary coil is changed, an emf is induced in the secondary coil. This process is called *mutual induction.*

*Demonstration of mutual induction*

When switch is closed, the galvanometer, G, deflects momentarily, indicating flow of current in secondary coil. This is because the current flowing through the primary coil induces a magnetic field which links up with the secondary coil. When this magnetic field changes, an emf is induced in the secondary coil.

When the switch is opened, the galvanometer deflects momentarily in the opposite direction. This because the induced flux reduces to zero and hence an emf is induced in a direction according to Lenz’s law.

Mutual inductance, M between two coils is the ratio of the induced emf in the secondary coil to the rate of change of current in the primary coil.

Hence mutual inductance is the magnetic flux linkage in the secondary coil when the primary current is 1A.

Unit of mutual inductance is the Henry(H).

Mutual inductance of a solenoid and a coil.

Consider a short secondary coil of N_{s} turns , area A wound round the middle of a long primary solenoid of n turns per metre.

**Calibration of a Ballistic galvanometer using a standard mutual inductance **

A known mutual inductance can be used to calibrate a ballistic galvanometer so that its sensitivity C can be known.

**THIS VIDEO IS ABOUT MUTUAL INDUCTANCE**