The change of electrical resistance produced in a current-carrying conductor or semiconductor on application of a magnetic field H. Magnetoresistance is one of the galvanomagnetic effects. It is observed with H both parallel to and transverse to the current flow. The change of resistance usually is proportional to H² for small fields, but at high fields it can rise faster than H², increase linearly with H, or tend to a constant (that is, saturate), depending on the material. In most nonmagnetic solids the magnetoresistance is positive.