Abstract
Two small thermostats for use in the temperature ranges −40°C to −70°C and −70°C to −150°C are described. The controlling element in both cases is a thermocouple used in conjunction with a potentiometer and galvanometer-photomultiplier amplifier. The higher-temperature thermostat uses an acetone bath as the immersion medium while the lower temperature one uses a metal block for this purpose. Temperature control to within 0.01°C may be attained with both thermostats if reasonable care is taken in correcting for the normal drift in the potentiometer calibration which occurs over periods of time greater than an hour.

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