Practical and Absolute Temperature Scales

Thus thermometers are calibrated according to IPTS, and then give good approximations of the Absolute temperature. These temperatures are expressed either as °C or as K, depending on whether 0 or 273 (more exactly 273.15) is assigned to the ice point. In 1948, the name of the Centigrade scale was officially changed to the 

Celsius scale, retaining the °C symbol, after Anders Celsius, a Swedish astronomer who first used (in 1742) 1(X) divisions between the freezing and boiling points of water as a temperature scale. The fact that he used 100 for the ice-point and zero for the boiling point was perhaps eccentric, but no more or no less logical or fundamental than the reverse. 

This situation was rectified in 1954 when by international agreement the definition of the temperature scale was changed so that now the triple point of water is fixed by definition at 273.16 on the Kelvin scale. The Kelvin scale now has therefore one labeled fixed point instead of two unlabeled fixed points with 100 divisions in between. (Alternatively you could think of the new scale as having two fixed points, one at absolute zero and the other at the triple point of water, with 273.16 divisions in between, but since absolute zero is unattainable it is hardly a fixed point in the usual sense.) 

In summary, then, the Kelvin temperature scale in use today has one fixed pomt, the triple point of pure water, assigned a value of 0.01 degrees Celsius and 273.16 degrees Kelvin (more correctly called 273.16 Kelvins). Since there is experimentally 0.01 degree between the triple point and the freezing point of water at atmospheric 

The International Practical Temperature Scale consists of a number of equilibrium states which have been assigned temperatures on the Celsius scale, to facilitate the measurement of temperatures everywhere. Since the number of degrees between the triple point of water and any other equilibrium state, such as the freezing point of molten zinc, is a matter of experimental determination, the temperature values assigned to the chosen fixed points are changed from time to time as techniques improve. 

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