A Self-Balancing Bridge with Range and Precision Adequate for a Platinum Thermometer

Calorimetry at NBS needs automation of bridge readout and bridge balancing. Bridge-balancing logic allows it to track the thermometer with progressively decreasing deviations as thermometer temperature approaches equilibrium. A step in the next decade does not interrupt since carry is automatic. Thermometer lead resistance is compensated as lead unbalance changes. Readout, at appropriate times, is into punched cards. With thermometer current only 0.002 ampere (to limit heating), the unbalance detector must function with only 0.02 microvolt available for the minimum step (10 microhms), in spite of much larger disturbing voltages, notably, thermal agitation, thermal EMF, and power frequency pickup.