An electromagnet is a coil or a group of coils that create a magnetic field when a continuous current is applied to them. The obtained magnetic field is linked to the shape of the coil, the number of turns and the current flowing through. To remain simple, we could build an electromagnet with one turn of electric wire. But, in order to have the same magnetic field, it will require 100 times more current than in a coil of the same size, but with 100 turns.
The limit of these two options is the fact that the wire heats, because of either a too strong current or the electric resistance because the wire is too long. This is called Joule effect. If we use superconductors, the Joule effect disappears and it enables the creation of very compact electromagnets that can provide very strong magnetic fields.
