PURPOSE: To measure easily the strain of metals and rocks by pasting a magnetic tape which is beforehand magnetized with magnetic grids of a specific wavelength as a strain measuring element on a strain measuring object such as a rock, detecting the change in the wavelength of the magnetic grid from the reproduced voltages obtd. with playback heads which rub the surface thereof and measuring the strain.

CONSTITUTION: The wavelength of the magnetic grids magnetized on a magnetic tape 2 is designated as λ. The mounting space (d) between A and B of playback heads 3 is adjusted to integer times of λ. The deviation corresponding to the value wherein the change contained in each wavelength λ is accumulated between the heads A and B is generated in the position of the magnetic grids facing respectively oppositely to the heads A and B; therefore, a relative time difference in time (t) is contained in the reproduced voltages of the heads A and B by the rubbing movement of a pair of the head A and B groups, that is, a phase difference appears. The figure shows the case in which the head speed increases during the rubbing movement. An increase in the reproduced voltages and a reduction in the periods, that is, an increase in the frequency are resulted and the time is reduced to t1 as a compared to the time difference (t). Therefore, if the phase difference between both voltage is sensitively detected, a micro strain is measurable.