Practical  inductors

Obviously it is not easy to make wire loops even approaching a million miles in circumference. But the length of wire is coiled up. When this is done, the magnetic flux can be increased several times for a given length of wire compared with flux produced by a single turn loop. This is the way inductors are made in the practical electrical and electronic devices.

For any coil, the magnetic flux density can be multiplied when the ferromagnetic core is placed within the coil of wire. The increase in flux density has the effect of multiplying the inductance of a coil, so that it is many times greater with a ferromagnetic core than with an air core.

The current that an inductor can handle depends on the size of the wire. The inductance does not; it is a function of the number of turns in the coil, the diameter of the coil, and the overall shape of the coil.

Generally inductance of a coil is directly proportional to the number of turns of wire. Inductance is also directly proportional to the diameter of the coil. The length of a coil, given a certain number of turns and a certain diameter, has an effect also: the longer the coil, the less the inductance.