A magnetic field discriminator is described which uses the nuclear induction signal from protons to give a ±75‐volt change of d.c. voltage for a ±0.3‐gauss change in magnetic field without using direct coupled amplifiers.An ordinary degenerative current regulator with the addition of information from the magnetic field discriminator will control a magnetic field to within 0.02 gauss for a 10 percent line voltage change or for normal changes in magnet geometry.The magnitude of the magnetic field is maintained in terms of a frequency, the value of which gives the absolute magnitude of the magnetic field.