Abstract: A nuclear magnetic resonance magnetometer is provided comprising two semi-coils which pick up precessional signals of the nuclear spins of two samples. A capacitor of a battery of capacitors forms a resonating circuit with the two semi-coils. The signal at the terminals of the capacitor is amplified in a differential amplifier, phase-shifted in a phase-shifter introducing a constant phase-shift of 90.degree., and reinjected into the semi-coils through a potentiometer. The frequency of the oscillations is measured by a frequency meter. A circuit controls the connection of the capacitors of the battery is response to the valve of the frequency measured so as to maintain the parasite phase-shifts between signals picked up and reinjected signals less than a threshold.