| | Different devices and materials can be used to measure temperature. Basically, whenever we have a process or a material property dependent upon temperature or temperature differences, these can be exploited to build thermometers. Simple combinations of two metals—so-called thermocouples, which are actually thermoelectric generators—can be used to measure temperature differences (Fig. 1). The resistance of electric resistors commonly depends upon temperature (Fig. 2, left), so we can build resistive thermometers. An important type of thermometer is made from a simple gas such as air whose pressure depends upon its temperature (Fig. 2, right).
Interpretation Using the gas thermometer, the pressure of a dilute gas is measured at constant volume. It is found that—if the temperature is not too low—the data points follow a linear relationship. This relation is extrapolated backwards to a pressure of zero. Since the same temperature is found for a zero value of pressure independent of the type of substance that is in a gaseous state, it is assumed that this value constitutes the lowest possible temperature that can be attained (it is – 273°C). It turns out that a temperature scale that starts here with a value of zero serves as a measure of the thermal potential (thermal level). In contrast to electricity—but in analogy to hydraulics—the thermal potential is absolute: It has an absolute zero value. | 
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Investigation 3 |
