An instrument for measuring the magnitude, and sometimes the direction, of a magnetic field. Absolute magnetometers measure the field without reference to a standard magnetic instrument. The most widely used are the vibration magnetometer, the deflection galvanometer, and the more modern nuclear magnetometer. The vibration instrument was devised by Gauss in 1832 and depends on the rate of oscillation of a small bar magnet suspended in a horizontal plane. The same magnet is then used as a fixed deflector to deflect a second similarly suspended magnet. The deflection galvanometer uses a Helmholtz coil system of known dimensions with a small magnet suspended at its centre. The deflected magnet comes to rest at a position controlled by the earth’s magnetic field, the coil’s magnetic field, and the angle through which the coil must be turned to keep the magnet and the coil in alignment. The sensitive nuclear magnetometers are based on measuring the audiofrequency voltage induced in a coil by the precessing protons in a sample of water. Various relative magnetometers are also in use, especially for measuring the earth’s magnetic field and in calibrating other equipment.