Slip Rings and Brushes Current to create the magnetic field is supplied to the coil from one of two sources, the battery or the AC generator itself. In either case, the current is passed through the AC generator’s voltage regulator before it is applied to the coil. The voltage regulator vories the amount of current supplied .Increasing field current through the coil increases the strength of the magnetic field. This , in turn. Increases AC generator voltage output . Decreasing the field voltage to the coil has the opposite effect. Output voltage decreases.