The paper describes the apparatus used to stabilize the magnetic field of a large electromagnet. A rotating coil placed in the magnetic field generates an alternating voltage which is proportional to the strength of the field. Variations in this rotor voltage are used to control the current supplied to the electromagnet in such a manner as to keep the field constant. By means of a calibrated held control the magnetic field strength may be adjusted to any value between 500 and 6000 gauss. The current for the electromagnet is obtained by rectifying 60 cycle alternating current in a full wave, thyratron, rectifier circuit. This current is controlled by a pulse circuit which governs the fraction of each cycle during which the thyratrons are conducting. The stabilization at any held setting is better than 1 part in 1000.