A principle for the construction of a vectorial Earth-field magnetometer is described. The electron spin resonance (ESR) in an organic radical is obtained with a polarization field much higher than the field to be measured which thus acts by its projection on this field. The pulsation of the polarization field between two opposite values gives the access to two error quantities used first to lock the magnetometer at the resonance and to construct a control loop to cancel the field to be measured. These two loops can be used jointly with the knowledge of the resonant frequency to calculate the absolute value of the measured field, independently of environmental parameters.
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