A magnetic body having nonlinear permeability is influenced by a magnetomotive force, the magnitude of which is to be measured or controlled. An electric energy source is connected to a winding that is magnetically coupled to the magnetic body. The electric energy source generates an oscillating output so as to cause the magnetic flux within the magnetic body to oscillate at a predetermined frequency. The oscillating flux is associated with an exciting current and excitation voltage, both oscillating at the predetermined frequency. The nonlinear permeability of the magnetic body causes the waveform of the exciting current to have different symmetry than the waveform of the excitation voltage. The difference of symmetry is indicative of the polarity and average value of magnetomotive force experienced by the magnetic body. The difference in symmetry is used to measure the average magnetomotive force experienced by the magnetic body. Alternatively the difference in symmetry may be used as an input to a control system that controls the average magnetomotive force experienced by the magnetic body. When applied to current transformers, the invention enables ordinary current transformers to operate with a-c and d-c primary currents while coupling very little noise to the primary circuit.