Small magnetometers like these measure magnetic strength on Earth and in the atmosphere to determine how much of the Sun's magnetic energy has been transferred to Earth's magnetosphere. Images credit: Glassmeier, et al.
A magnetometer is a device for measuring the strength and direction of a magnetic field. Magnetometers typically detect the strength of magnetic fields in three planes. Various types exist, exploiting, for instance the oscillation rate of a small, freely suspended bar magnet or the deflection of a magnet against its suspension or a reference field.
The simplest type consists of a short, bar magnet with a nonmagnetic pointer attached to its center so that it is at right-angles to the axis of the magnet. The magnet is pivoted, like a compass needle, at its center and the pointer travels over a calibrated scale.
Sensitive magnetometers used in space research include the proton precession magnetometer and the helium magnetometer.