Magnetostriction

Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of material's magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.

This effect causes losses due to frictional heating in susceptible ferromagnetic cores. The effect is also responsible for the high-pitched buzzing sound that can be heard near transformers on alternating current carrying pylons.