Magnetostriction's a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified in 1842 by James Joule when observing a sample of nickel. (Compare with electrostriction) This effect can cause losses due to frictional heating in susceptible ferromagnetic cores. Explanation Internally, ferromagnetic materials have a structure that's divided into domains, each of which's a region of uniform magnetic polarization.… (More on Magnetostriction)