FIG. P-16. Proton-resonance magnetometer. Proton spin axes precess about a magnetic field. A polarizing field normal to the Earth’s field is impressed for a short time to polarize the nuclei and is then removed. The nuclei, now oriented together, precess about the Earth’s magnetic field at the Larmor frequency, inducing this frequency in a measuring coil. The induced frequency controls the length of time that a gate is open, the time being measured by counting cycles of a standard frequency, and the time duration is a measure of the Earth’s field.