AN AUTOMATED CRYOGENIC CURRENT COMPARATOR RESISTANCE RATIO BRIDGE

The method of comparing two-terminal cryogenic and four-terminal room temperature resistors by using a dc cryogenic current comparator bridge is well established [1]-[4]. To make it suitable for routine calibration work, our bridge is fully automated with a computer controlling the measurement current and recording the readings of the bridge imbalance current. The bridge measures integer ratios of between 1:1 and 1:25 of four-terminal resistors at room temperature in the range 1-OMEGA to 10 k-OMEGA. An overall accuracy of 3-10 parts in 10(9), depending on the resistance values, has been achieved. The precautions taken to achieve fully automatic operation in an ordinary laboratory environment of an interference sensitive circuit involving a SQUID are described.