Magnetoresistance is the property of some conductive materials to gain or lose some of their electrical resistance when placed inside a magnetic field. The effect was first discovered by William Thomson Kelvin who managed to reduce the resistance of some materials by five percent. Subsequently, giant magnetoresistance (GMR) produced reductions of up to 50% in resistance, and the 1993/1994 discovery of colossal magnetoresistance (CMR) in perovskite can allow its resistance to be reduced 10,000 times without supercooling.

The effect was inexplicable until the development of quantum theory sometime after its discovery; the alignment of ions in a magnetoresistant substance and the fact that in some of these substances not all of the ions are identical causes the effect.

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