A flat coil is suspended in a magnetic field so that in equilibrium it is coplanar with the field and can oscillate about an axis in the plane of the coil perpendicular to the field. When an alternating current is passed through the coil the system behaves as an inductance in series with a condenser of capacity inversely proportional to the square of the magnetic field. The principle has been applied to the measurement of magnetic field. The instrument is capable of a precision of one part in 102with simple auxiliary apparatus.The application to a particle analyzing magnet is described. With some elaboration the magnetic field can be measured and stabilized to one part in 104in the range 1–12 kilogauss.