An instrument for measuring magnetic-field strength. Ground magnetometers sometimes measure the vertical component of the magnetic field, sometimes a horizontal component, sometimes the total field. Most airborne magnetometers are of one of three types: (a) fluxgate, (b) proton-precession, or (c) optically pumped (see individual entries); all measure the total-field intensity and cesium magnetometers measure the absolute field and achieve 0.001 nT accuracy. Vector and vertical-component airborne magnetometers are used occasionally. See also variometer and squid magnetometer.