An alternating or a modulated magnetic field of subsonic frequency exerts on a small specimen a periodic force which is communicated to a non-resonant reed, which is thereby forced to vibrate, the motion being indicated by a selective microvibration detecting arrangement. An opposing electrostatic field of the same frequency can be suitably applied to the reed so as to nullify its vibrations, and may thus give a measure of the magnetic force. A force whose amplitude is only 0.01 dyne can be readily measured to an accuracy of about 1%.
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Y L Yousef (1953) studied this question.