This is the phenomenon whereby metals (elements, as well as compounds and alloys) have no electrical resistance. Superconductivity usually only occurs at temperatures close to 0° K (-273 ° C), but has also been known, for some metals, to occur even at liquid nitrogen temperatures (77° K). The temperature below which substances are known to superconduct is called the transition temperature, or critical temperature. Superconductivity allows for the creation of extremely strong electromagnets, because a current can be kept circulating through coils indefinitely, as no E.M.F. is needed if there is no resistance. Another kind of superconductivity, high-temperature superconductivity, is said to occur at temperatures as high as 100 ° K, but at this stage remains only theoretical.
The equation above is also important in that it is where magnetic field strength, B, is defined
Magnetic flux, f , measured in webers (Wb) can also be quantified. Precisely, magnetic flux is the number of particles flowing per unit of area, in a cross section of a beam of particles. It is a measure of the amount of magnetism which takes into account the strength and extent of the magnetic field. Magnetic field strength, B, measured in Teslas (T), also called flux density, and the cross-sectional area of flux, A, define magnetic flux: