FIG. F-16. Fluxgate magnetometer. Two permalloy cores approach saturation in the weak magnetic field of the Earth. A 1000 Hz cyclic field superimposed by a coil around the core completes saturation. The place in the energizing cycle at which saturation is reached is a measure of the strength of the Earth’s field. A secondary coil detects the changes in flux. Two parallel cores are used with windings in opposite directions and the difference is measured. A current through an additional winding nullifies most of the background magnetic field so that the magnetometer is sensitive to small changes in the Earth’s field. The current through the nulling coil is a measure of the magnetic field strength.