Practical inductors

Of course, it’s not easy to make wire loops even approaching a million miles in circumference. But lengths of wire can be coiled up. When this is done, the magnetic flux is increased many times for a given length of wire compared with the flux produced by a single-turn loop. This is how inductors are made in practical electrical and electronic devices.
For any coil, the magnetic flux density is multiplied when a ferromagnetic core is placed within the coil of wire. Remember this from the study of magnetism. 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.
In general, 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.