If we have to strain a ferromagnetic crystal along certain direction, we would change the inter atomic spacing not only along this direction but also in other direction. And hence the exchange interactions between the atomic spins are also changed. This would lead to a change in the magnetization property of crystal. When an iron crystal is placed in a magnetic field along an easy direction, it gets longer along this direction but contracts in the transverse directions. The longitudinal strain $l/l along the direction of magnetization is called magnetostriction constant denoted by λ. When the crystal reaches saturation magnetization, λ also reaches saturation called saturation strain. The crystal lattice strain energy associated with magnetostriction is called magnetostrictive energy. Magnetostriction is responsible for transformer hum noise.