An inductor or an induction coil is a tightly woven coil of wire. Now, you would not expect an ordinary piece of wire to show any special property on passage of current through it. A coil with several loops or turns however, exhibits a remarkable property when current passes through it. The current through the coil creates a magnetic field in the immediate space surrounding the coil. The field stores electrical energy during the passage of current and for a very short while, even if you cut off the current.
Another amazing fact of an inductor coil is that if you place the coil in a varying magnetic field, a current starts to flow through it. The amount of current depends upon the rate at which you change the field.
Bulb and Coil Experiment
You can make out this amazing property of an inductor coil from a simple experiment. Consider a simple circuit with a battery, bulb and a switch. The bulb glows when you close the switch while it stops glowing the moment you open or release the switch.
If you now include a coil of wire wound around an iron bar across the bulb, the bulb will light up as you close the switch. However, instead of glowing at a constant brightness, the intensity of the light changes from bright to dim. If you now open the switch, the bulb does not turn off immediately as you would expect. Instead, the brightness gradually decreases before turning off completely.
Explaining the Observations
You can attribute this curious behaviour to the inductor coil placed across the bulb. When you close the switch, current flows from the battery through the bulb, causing it to glow. At the same time, current flows through the inductor coil too. This generates a magnetic field in the space surrounding the coil. The magnetic field varies in the short time the current builds up. The changing magnetic field induces a current to flow through the coil. However, according to the rules of electricity, this current is opposite to the original current sent by the battery. Hence, the effective current through the coil increases with time, while decreasing that passing through the bulb. This causes the bulb to reduce its glow from bright to dim.
When you open the switch, the magnetic field falls. During the fall of the field, the induced current causes the voltage across the inductor to rise for a moment. This causes the bulb to brighten up briefly. When the current reduces to zero, the bulb turns off.
The physical quantity associated with this property is called inductance. The value of this quantity is measured in Henrys. Inductance depends upon four features, which include the number of turns in the coil, the degree of overlap, area of the cross section of the wire and the material of the core inside the coil.
You can increase the inductance by increasing the number of turns and the cross section area of the coil. You may also increase the value by increasing the degree of overlap i.e. by using a tightly wound coil.
Uses of Inductors
You must have wondered how traffic signalling works. Traffic light sensors make use of inductors, which form filter circuits along with capacitors. Inductors are essential components in electronic circuits and devices like receivers, transmitters, oscillators and voltage regulators, as well.