The temperature of a system can be determined by bringing a 2nd body, a thermometer, into contact with the system and allowing thermal equilibrium to be reached. The value of temperature is found by measuring some temperature dependent property of thermometer. Any such property is called as thermometric property.
To allocate numerical values to the thermal state of the system, it is required to establish a temperature scale on which temperature of system can be read. This requires selection of basic unit and reference state. Thus, the temperature scale can be established by assigning numerical values to certain easily reproducible states. For this purpose it is probable to use the following 2 fixed points:
(1) Ice Point: It is equilibrium temperature of ice having air saturated water at standard Atmospheric pressure.
(2) Steam Point: The equilibrium temp. of pure water having its own vapour of standard atmospheric pressure.
Compansion of references on different scales