in the nmr apparatus the resonant conditions may be kept constant by adjusting the strength of the static magnetic field . the echo signal that is induced under the specific conditions is processed so as to obtain the deviation of the resonant frequency in the correction circuit . the shift of the given static magnetic field is calculated based upon the deviation of the resonant frequency . the dc energizing current for the h o power supply is varied based upon the magnetic field shift so as to maintain the nmr phenomenon at the desirable resonant conditions .