An Absolute Noise Thermometer for High Temperatures and High Pressures

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Abstract
A null device for determining the ratio of two absolute temperatures with an accuracy of 0.1 percent is described. This device balances the mean‐square fluctuations in voltage across the terminals of two resistors arising from thermal agitation at the temperatures to be compared. The ratio of the resistances, when the noise voltages from the two resistors are equal, determines the ratio of their absolute temperatures. The limitation on the ultimate accuracy of the ratio of temperatures so determined is imposed by the relative value of the first and second moments of the voltage fluctuations observed, which in turn depends on the length of time utilized in averaging the fluctuations. With the apparatus described below, an observation time of two minutes is adequate to guarantee the accuracy claimed above for high temperatures, provided certain elementary precautions in constructing the noise thermometer are observed. Numerous tests have been conducted in the neighborhood of 1000°K which substantiate these claims. Extrapolation of the performance obtained at such temperatures indicates that material improvements in the accuracy of the absolute temperature scale above 2000°K may be obtained. The calibration of the thermometer is independent of the chemical composition and past physical history of the material from which it is constructed, in addition to being independent of the absolute pressure.
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