Abstract: A method and device are described for measuring a change in the mechanical state aiming at detecting the disturbance the change in state has on a magnetic flux (.phi.). This flux is brought to flow through at least a part of the body (1) under the influence of a driving magnetizing force (H.sub.0).In order to achieve an unambiguous measuring result the average length (l) of the magnetic circuit is kept constant and independent of the change in the mechanical state. The flux (.phi.) consists partly of a main flux (.phi..sub.0) having a definite direction and being of such a magnitude that the magnetic properties of the body depart from the region of irreversibility and partly of an alternating, gradually vanishing flux (.phi..sub.v) superimposed upon the main flux. The alternating flux must have such an initial magnitude that saturation is obtained in both directions of the alternating flux (.phi..sub.v). After the vanishing of the alternating flux (.phi..sub.