The proton (hydrogen nucleus) has a magnetic moment because of its spin. The nuclei precess about the Earth’s magnetic field H at a frequency fL (Larmor frequency),
where is the gyromagnetic ratio ( Hz/nanotesla for protons). Precession of polarized nuclear spins induces a voltage at the precession frequency in a measuring coil. The induced frequency is measured by a counting arrangement (Figure P-16) to determine the value of the Earth’s magnetic field. For the normal Earth field of about 50 000 nanotesla, fL=2100 Hz. As the protons gradually relax into random orientation, the induced-field strength drops to zero. The rate of drop depends on interatomic forces and hence on molecular structure. Achievable accuracy: 0.1 nT. Also called proton-resonance magnetometer. Compare optically pumped magnetometer.