The magnetostriction constant for alternating magnetic fields

A magnetostriction constant K for the Joule effect is defined by the equation p = KH where p is the alternating mechanical stress produced by a small alternating magnetic field H superposed on a steady magnetic field H 0 . It is shown that the alternating intensity of magnetisation I produced by an alternating strain δ l / l is given by the equation I = K δ l / l , K being the same constant in both equations. These equations are used to calculate the motional impedance of a laminated ring toroidally wound and vibrating in its fundamental radial mode. K has been measured for various values of H 0 with a ring of the alloy Fe 60 per cent., Ni 40 per cent. It is shown that the magnetostriction effect for steady magnetising fields can be inferred from the measurements with alternating magnetic fields, if the reversible permeability and the differential permeability of the alloy are known.