Magnetostriction is a property of ferromagnetic materials to undergo a change of their physical dimensions when subjected to a magnetic field. This effect was first identified in 1842 by James Joule when observing a sample of nickel.

This property, which allow magnetostrictive materials to convert magnetic energy into kinetic energy and conversely, is used for the building of both actuation and sensing devices. It is often quantified by the magnetostrictive coefficient, L, which is the fractional change in length as the magnetization of the material increases from zero to the saturation value. The effect is responsible for the familiar "electric hum" ( Listen^?) which can be heard near transformers and high power electrical devices (depending on country, either 100 or 120 Hz, plus harmonics).

The reciprocal effect, the change of the susceptibility of a material when subjected to a mechanical stress, is called the Villari effect. Two other effects are related to magnetostriction: the Matteucci effect is the creation of a helical anisotropy of the susceptibility of a magnetostrictive material when subjected to a torque and the Wiedemann effect is the twisting of these materials when an helical magnetic field is applied to them.

External links

• Magnetostriction
• Magnetostriction(2)
• Electricity is supposed to be the "silent servant". So why do transformers hum?