High precision four-terminal resistance bridge utilizing a SQUID null-detector

A practical method which may be used to measure small four-terminal resistances at liquid helium temperatures with a precision of 0.1 ppm is described. Sensitivity sufficient to do this with a measuring current of 30 mA in a 500-μΩ resistor is achieved by the use of a SQUID null-detector in a circuit with a high precision MacMartin–Kusters type direct current comparator. A major problem is the production of suitable comparison resistors to use as standards at low temperatures. The choice of material and construction is discussed for a pair of resistors with which it was possible to obtain noise voltages in liquid helium at 4.2 K that were near the Johnson noise in value, and to obtain resistance ratio measurements that were independent of current from 30 to 80 mA within 0.1 ppm. The method may be extended to resistances from 1 mΩ to 10 μΩ in value and even lower if thermoelectric voltages can be suitably reduced.