A magnetometer determining the intensity of a magnetic field by measuring the frequency (Larmor precession frequency) with which atomic electrons in the field precess. The electrons in an atom move in orbits about the nucleus. As moving charges, they experience a force, in a magnetic field, proportional to the intensity of the field and in a direction perpendicular to the plane of the orbit. The orbit therefore precesses about the nucleus at a rate proportional to the intensity of the field. The frequency is measured by passing light of the proper frequency through a gas and measuring the frequency (equal to the Larmor frequency) at which the light is strongly absorbed when modulated by a weak magnetic field applied to the gas.