Resistance comparison means having a modulated source and a detector sensitive to the modulating signal

Claim 1

RESISTANCE COMPARISON MEANS FOR COMPARING FIRST AND SECOND RESISTANCES WITH EACH OTHER BY COMPARING THE POTENTIAL ACROSS THE FIRST RESISTANCE WITH THE POTENTIAL ACROSS THE SECOND, WHICH APPARATUS COMPRISES AN ALTERNATING CURRENT SOURCE, FIRST AND SECOND CURRENT TRANSFORMERS EACH HAVING A PRIMARY WINDING CONNECTED IN SERIES WITH EACH OTHER AND WITH SAID ALTERNATING CURRENT SOURCE AND EACH HAVING A CURRENT OUTPUT PROPORTIONAL IN A KNOWN RATIO TO THE CURRENT FLOWING IN ITS PRIMARY WINDING, MODULATING MEANS CONNECTED TO SAID ALTERNATING CURRENT SOURCE FOR MODULATING THE SUPPLY OF CURRENT FROM SAID ALTERNATING CURRENT SOURCE TO THE SERIALLY CONNECTED PRIMARY WINDINGS OF THE CURRENT TRANSFORMERS AT A FREQUENCY OF A LOWER ORDER THAN THE FREQUENCY OF SAID ALTERNATING CURRENT SOURCE, A FIRST RECTIFYING CIRCUIT CONNECTED TO THE SECONDARY WINDING OF SAID FIRST CURRENT TRANSFORMER, A SECOND RECTIFYING CIRCUIT CONNECTED TO THE SECONDARY WINDING OF SAID SECOND CURRENT TRANSFORMER, MEANS CONNECTING THE FIRST RESISTANCE TO SAID FIRST RECTIFYING CIRCUIT SO THAT THE RECTIFIED CURRENT FROM SAID FIRST RECTIFYING CIRCUIT FLOWS THERETHROUGH, MEANS CONNECTING THE SECOND RESISTANCE TO SAID SECOND RECTIFYING CIRCUIT SO THAT THE RECTIFIED CURRENT FROM SAID SECOND RECTIFYING CIRCUIT FLOWS THERETHROUGH, THE CURRENT FLOWING THROUGH EACH OF SAID RESISTANCES BEING IN KNOWN RATIO TO EACH OTHER BY REASON OF THE SAID PRIMARY TO SECONDARY CURRENT RATIO OF EACH TRANSFORMER, AND A PHASE SENSITIVE NULL DETECTOR CONNECTED TO SAID RESISTANCES FOR COMPARING THE POTENTIAL AT THE MODULATION FREQUENCY ACROSS THE FIRST RESISTANCE WITH THAT ACROSS THE SECOND RESISTANCE, SAID MODULATING MEANS BEING CONNECTED TO SAID DETECTOR TO PROVIDE A REFERENCE SIGNAL THERFOR, WHEREBY SAID DETECTOR PROVIDES A COMPARISON, AND ALSO A DIRECTION OF ANY DEVIATION, OF THE POTENTIAL ACROSS THE FIRST RESISTANCE WITH THAT ACROSS THE SECOND RESISTANCE.