An instrument is described which has the property of accurately measuring the magnitude of magnetic field to 0.05 &mgr;gauss at the center of a set of Helmholtz coils while simultaneously reducing by a factor of 600 stray fluctuations of the field caused by drift in the earth's magnetic field, power supply anomalies, and external magnetic noise generated by the motion of magnetic material. The device utilizes an optically pumped metastable helium magnetometer which senses the total magnetic field. The output signal of the magnetometer is used to control the current in the Helmholtz coils. The new features of this device are (a) its ability to discern and hence control magnetic field variations at the microgauss level and (b) its scalar character enabling the field control to be accomplished with a single set of coils