Bohr–Van Leeuwen theorem - a theorem of classical statistical physics, according to which the magnetisation of a system of electrons in a constant external magnetic field under conditions of statistical equilibrium is equal to zero; it was proved in 1911 by the Danish physicist N. Bohr and generalised in 1919 by the Dutch physicist Johanna van Leeuwen. The Bohr–Van Leeuwen theorem shows that within the framework of the classical statistical mechanics of charged particles it is impossible to explain ferromagnetism, paramagnetism and diamagnetism. As was shown later, the magnetism of substances is due to the quantum properties of the particles that make up the substance.