PRACTICAL INDUCTORS

In practice, you cannot make wire loops anywhere near 10⁶ km 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.
For any coil, the magnetic flux density is multiplied when a ferromagnetic core is placed within the coil of wire. You may 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 inductance also depends on 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 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 such that the longer the coil is made, the lower the inductance becomes.