(lar’ mor) The frequency with which gyromagnetic moments precess in a magnetic field. Atoms and nuclei possess magnetic moments because of their spin and precess like small gyroscopes about the direction of an externally applied steady magnetic field (such as the Earth’s field). Radio-frequency energy at right angles to the steady field will be absorbed because of resonance when the RF-frequency equals the precession frequency. This principle is involved in proton-precession magnetometers and optically pumped magnetometers and in the nuclear-magnetic resonance log (see italicized entries). Named for Sir Joseph Larmor (1857–1942), English mathematician.