The air core inductor may be nothing more than a coil of wire, but it is usually a coil formed around a hollow form of some nonmagnetic material such as cardboard. This material serves no purpose other than to hold the shape of the coil. View B shows an air core inductor and its schematic symbol. Several physical factors affect the inductance of a coil. They are the number of turns, the diameter, the length of the coil conductor, the type of core material, and the number of layers of winding in the coil.
Inductance depends entirely on the physical construction of the circuit.
It can only be measured with special laboratory instruments. The first factor that affects the inductance of the coil is the number of turns. Figure 6-6 shows two coils. Coil A has two turns, and coil B has four turns. In coil A , the flux field setup by one loop cuts one other loop.