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.