An apparatus employed for studying light‐modulated changes in the magnetic susceptibility of semiconductors is described. Its chief parts are a flat reed spring from which the specimen is suspended, an electrostatic counter‐balancing system, a frequency modulation phase sensitive motion detector, and a modulated light source. The light modulation operates at the mechanical frequency of reed and specimen, whose forced harmonic motion is proportional to the light‐induced susceptibility changes. The chief advantages of this new method are the elimination of drifts due to changes in the nonmodulated part of the susceptibility, the excellent nonsynchronous noise elimination, and the high detection sensitivity, which was 10−11cgs mass‐susceptibility units at a value ofHgradHof 107gauss2/cm. Another incidental advantage is a simplicity and ruggedness not generally encountered in high sensitivity instruments.