A highly stable DC transformer system has been developed to control large currents to 1 p.p.m. It uses a resistive current divider and two magnetic-modulator ampere-turn comparators with feedback. The comparators are operated just below flux density saturation. This prevents input transients from disrupting the feedback control. The main comparator has a relatively small core operating at low magnetomotive force due to a small current, provided by the resistive divider. The subcomparator corrects changes in the small current by changing the resistor values. A 20-A regulated current source achieves 1 p.p.m./6 h for ambient temperatures controlled to +0.25 degrees C. >