Zeroth law of thermdynamics

If two bodies
are in thermal equilibrium with a third body, they are also in thermal
equilibrium with each other.

 

This obvious fact cannot be concluded from the other laws of thermodynamics, and it serves as a basis of temperature measurement.

 

By replacing the third body with a thermometer, the zeroth law can be restated two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact

The zeroth law was first formulated and labeled by R.H. Fowler in 1931.

We cannot assign numerical values to temperatures based
on our sensations alone. 

Furthermore, our senses may be misleading. Several properties of material changes with temperature
in a repeatable and predictable way, and this forms the basis of accurate
temperature measurement.

The commonly used mercury-in-glass thermometer for
example,  is  based on
the  expansion of mercury with temperature.

Temperature is
also measured by using several other temperature dependant properties.

Two bodies (eg. Two copper blocks) in contact attain
thermal equilibrium when the heat transfer between them stops. The equality of
temperature is the only requirement for thermal equilibrium.