Direct Reading Resistance Thermometer Bridge. Part I

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Abstract
The paper describes the basic circuitry required for a high precision direct reading resistance thermometer with dials calibrated in degrees C and any standard platinum resistance thermometer as sensing element. A circuit has been developed which, through linear variations of the resistance elements, gives a quadratic law having the form R=R0(1+AT+BT2), where B is negative, as is required for a platinum thermometer. At the same time it allows separate adjustments for slope A and curvature B or B/A of the resistance vs temperature relation of the particular thermometer being used and also for the ice point resistance R0. The auxiliary circuitry is given for comparing the computing circuit with the thermometer and for maintaining a constant recorder sensitivity of all temperatures; stability requirements for work to 0.001°C over a 700°C range are discussed.

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