instrument for measuring the strength and sometimes the direction of magnetic fields, including those on or near the Earth and in space. Magnetometers are also used to calibrate electromagnets and permanent magnets and to determine the magnetization of materials.

Magnetometers specifically used to measure the Earth's field are of two types: absolute and relative (classed by their methods of calibration). Absolute magnetometers are calibrated with reference to their own known internal constants. Relative magnetometers must be calibrated by reference to a known, accurately measured magnetic field.

The simplest absolute magnetometer, devised by C.F. Gauss (Gauss, Carl Friedrich) in 1832, consists of a permanent bar magnet suspended horizontally by a gold fibre. Measuring the period of oscillation of the magnet in the Earth's magnetic field gives a measure of the field's strength.

A widely used modern absolute instrument is the proton-precession magnetometer. It measures a voltage induced in a coil by the reorientation (precession) of magnetically polarized protons in ordinary water.

The Schmidt vertical-field balance, a relative magnetometer used in geophysical exploration, uses a horizontally balanced bar magnet equipped with mirror and knife edges.