Magnetoresistance is the tendency of a material to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance: some occur in bulk non-magnetic metals and semiconductors (e.g. geometrical magnetoresistance, Shubnikov de Haas oscillations or the common positive magnetoresistance in metals); others in magnetic metals (e.g. negative magnetoresistance in ferromagnets or AMR); or finally in multicomponent or multilayer systems (e.g. magnetic tunnel junctions, GMR, TMR, EMR). The first magnetoresistive effect was discovered by William Thomson (better known as Lord Kelvin) in 1851, but he was unable to lower the electrical resistance of anything by more than 5%. Nowadays, systems are known (e.g. semimet