ABSTRACT
Because the analysis of the Carnot cycle makes use of the ideal gas equation of state, it follows that the kelvin scale and the ideal gas thermometer scale must be the same. An ideal gas thermometer can be used to measure absolute temperatures provided they are not too low. Helium-4 (4He) gas liqueˆes at 4.2 K, at a pressure of one atmosphere, and other gases liquefy at higher temperatures at atmospheric pressure. Transition to another phase therefore limits the range of use of gas thermometers. Secondary thermometers such as resistance thermometers, which use metals or semiconductors as sensors, are generally used at temperatures down to 0.05 K (or 50 mK). A variety of other thermometers, such as ideal paramagnetic thermometers, have been developed for use at still lower temperatures.
