This has been the most commonly used magnetic instrument for prospecting on land. It consists of a magnet pivoted near but not at its center of mass, so that the magnetic field of the earth creates a torque around the pivot that is opposed by the torque of the gravitational pull upon the center. The angle at which equilibrium is reached depends on the strength of the field. Readings are taken through an eyepiece by comparing a scale reflected from a mirror on the magnetic element with a fixed scale. The balance may be either the horizontal or vertical type.