The thermodynamic temperature scale
One of the difficulties in the accurate measurement of temperature is that the various types of thermometer, e.g., mercury-in-glass, platinum resistance, therm electric, and so on give different readings when used to measure the same temperature, because
the value obtained depends on the properties of the substance used in the thermometer. It was not until the middle of the nineteenth century that the theory of temperature measurement was placed on a firm basis by Lord Kelvin, who devised an absolute scale called the thermodynamic scale, which is quite independent of the properties of any substance used in thermometers. In practice, however, the thermometer whose readings come closest to the thermodynamic scale is the constant volume gas thermometer (Fig. 16.7), a simple form of which is shown on page 176. Temperatures on the thermodynamic scale are not measured in degrees but in units called kelvins. They are so-named in honor of Lord Kelvin and are denoted by K not OK. Moreover, the thermodynamic scale has been defined in such a way as to make kelvins exactly the same size as degrees Celsius. On the thermodynamic scale the melting point of ice is 273 K, so the boiling point of water is 373 K. A full discussion of the theory of the thermodynamic scale is outside the scope of this book, but we shall have more to say about it in chapter 16 when it will be explained why 0 °C is equal to 273 K.