Double-exposure holographic interferometry is applied to measure the "linear" or "longitudinal" magnetostriction constant of a soft-ferrite rod. This high-accuracy measurement is done indirectly, by measuring the small rotations of a lever in contact with the rod using double-exposure holographic interferometry implemented with a robust "arrowhead"-shaped holographic setup. Ferrite devices are used in a large number of well-known technologies and hold a promise for more advanced applications, so the study of ferrites properties is therefore very important. The experiments and methods described here and the magnetic phenomena studied are suitable for an undergraduate project or an open-ended experiment. (Contains 5 figures.)