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Zeroth Law of Thermodynamics

 The property which distinguishes thermodynamics from other sciences is Temperature. One might say that the temperature bears as important a relation to thermodynamics as force does to statics or velocity does to dynamics. Temperature is associated with the ability to distinguish hot from cold. When two bodies at different temperatures are brought into contact, after some time they attain a common temperature and are then said to exist in thermal equilibrium.




 When a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in Thermal equilibrium with each other. 

This is known as the zeroth law of thermodynamics. It is the basis of temperature measurement.

In order to measure Quantitative measure of temperature, a reference body is used, and a certain physical characteristic of this body which changes with temperature is selected. The changes in the selected characteristic may be taken as an indication of changes in temperature. The selected characteristic is called the thermometric property, and the reference body which is used at the determination of temperature is called the thermometer. A very common thermometer consists of a small amount of mercury in an evaluated capillary tube. In this case the extension of mercury in the tube is used as the thermometric property.


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