An increase in the resistance of a metal due to the presence of a magnetic field, which alters the paths of the electrons. At normal temperatures the change in resistance resulting from the magnetic field is small but at very low temperatures the increase is considerable. The theory of magnetoresistance is too complicated to be explained quantitatively by the simple model of electrical conductivity in metals, which assumes that it results from the movement of free electrons. To obtain a quantitative explanation, it is necessary to take into account the energy-band structure of metals. Usually magnetoresistance is a fairly small effect (about 5%). However, under certain circumstances much larger effects are possible. See colossal magnetoresistance, giant magnetoresistance.