A simple high-resolution cryogenic resistance bridge using a SQUID null detector

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Abstract
A simple and inexpensive resistance bridge is described which can monitor small changes in resistance with very high resolution. It has been used to measure the temperature dependence of (four-terminal) resistances with values in the range 10-8-10-5 Omega . The sensitivity is limited only by Johnson noise generated in the sample, comparison and contact resistances; with a sample current of 100 mA the resolution is 1 in 106 to 1 in 107. The highest temperature at which the bridge can be used is set by the superconducting transition temperature of the leads to the sample, 9K in this case. The circuit uses an RF SQUID as a null detector, and the only critical components are the resistors for the arms of the bridge; kept constant by maintaining them at liquid helium temperature. A current supply of the requisite moderate stability (about 1 in 105) is also described.