Empirical means of measuring temperature such as considered have inherent limitations. For example the tendency of the liquid in a liquid-in-glass thermometer to freeze at low temperatures imposes a lower limit on the range of temperatures that can be measured.

At high temperatures liquids vaporize, and therefore these temperatures also cannot be determined by a liquid in-glass thermometer. Accordingly, several different thermometers might be required to cover a wide temperature interval.

In view of the limitations of empirical means for measuring temperature, it is desirable to have a procedure for assigning temperature values that does not depend on the properties of any particular substance or class of substances.

Such a scale is called a thermodynamic temperature scale. The Kelvin scale is an absolute thermodynamic temperature scale that provides a continuous definition of temperature, valid over all ranges of temperature.

Empirical measures of temperature, with different thermometers, can be related to the Kelvin scale.

To develop the Kelvin scale, it is necessary to use the conservation of energy principle and the second law of thermodynamics. after these principles have been introduced. However, we note here that the Kelvin scale has a zero of 0 K, and lower temperatures than this are not defined.

The Kelvin scale and the gas scale can be shown to be identical in the temperature range in which a gas thermometer can be used. For this reason we may write K after a temperature determined by means of constant-volume gas thermometry.

Moreover, we assume that all temperatures referred to in the interim are in accord with values given by a constant-volume gas thermometer.

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