Magnets produce magnetic fields that can be strong but that strongly weaken when moving away. In order to get a strong field on a big volume, electromagnets are required, i.e. a metal wire coil in which flows an electric current. The current that flows in circle creates a magnetic field perpendicular to the section of the coil and in all its volume.
To get a strong field, a high electric current is required. But when there is a current, there is resistance, and where there is resistance, there is heating, because of the Joule effect. If the current is too strong, the coil will melt. To avoid this problem, we can either cool the wire with water (very expensive and not very convenient) or use a superconducting wire, because the latter does not resist and hence does not heat. Magnetic fields of several teslas (1 tesla is equal to about 10 000 times the earth magnetic field) can be obtained; using coils with several thousands turns of superconducting wires plunged in liquid helium. These wires are often made from niobium and titanium alloys (NbTi) or niobium and tin (Nb3Sn). These coils are often called “superconducting magnets” by misuse of language.
These strong magnetic fields are used for MRIs in hospitals, for Nuclear Magnetic Resonance (NMR) in chemistry and in physics, in physics laboratories to study the effect of fields on solids, in particle accelerators to make particles turn, to store electricity, to make the plasma used in ITER for the fusion project levitate, but also on board the Maglev train to make it levitate.
