A number of empirical scales of temperature have been in use: the Celsius scale is widely used for many purposes and in certain countries the Fahrenheit scale is still used. These scales both rely on the use of fixed points, such as the freezing point and the boiling point of water, and the division of the fundamental interval between these two points into units of temperature (100 degrees in the case of the Celsius scale and 180 degrees in the Fahrenheit scale).
However, for scientific purposes the scale in use is the International Practical Temperature Scale (1968), which is designed to conform as closely as possible to thermodynamic temperature and is expressed in the unit of thermodynamic temperature, the kelvin. The eleven fixed points of the scale are given in the table, with the instruments specified for interpolating between them. Above the freezing point of gold, a radiation pyrometer is used, based on Planck's law of radiation. The scale is expected to be refined in the late 1980s.
Subjects: Physics — Chemistry.