Direct-Current Comparator Bridge for Resistance Thermometry

The design, construction, and performance of a dc bridge that covers the complete range of both platinum-and germanium-resistance thermometry in four ranges, 0-11 up to 0-11 000 ohms, is described. The bridge uses a potentiometric method based on the current comparator. In the design, emphasis has been placed on reducing the noise level, on obtaining a single-balance operation, and on providing a recorder output. Detector sensitivity and noise level (with a three-second time constant) permit full eight-decade bridge resolution to be achieved at a thermometer current of I mA on all ranges except the lowest, which requires 5 mA. Bridge errors are believed to be less than 10-7 of a reading or one step on the last dial, whichever is larger. A facility is provided for measuring the self-heating effect of the thermometer, even if the measured temperature is not constant. The bridge is also suitable for the measurement of four-terminal standard resistors over the same ranges.