The quantum Hall effect is a quantum mechanical version of the Hall effect, which is the production of transverse (perpendicular to the main current) conductivity in the presence of a magnetic field. The quantization of the Hall effect formula_10 at integer multiples (the "Landau level") of the basic quantity formula_11 (where "e" is the elementary electric charge and "h" is Planck's constant). It can usually be observed only in very clean silicon or gallium arsenide solids at temperatures around and very high magnetic fields.