The Requirements and Design for a D-C Null Detector
Frederick L. Maltby · Transactions of the American Institute of Electrical Engineers · 1951
Modern instrumentation requires the measurement of many variables that are expressed most readily as d-c voltages. In the apparatus described here, a reversing synchronous contactor is used to invert the error signal into an alternating potential of the power frequency. This alternating potential in turn is amplified and impressed upon the control winding of a 2-phase motor which has its reference winding continuously energized at the power frequency.