Zeroth Law Of Thermodynamics

Consider three systems, namely, object 1, object 2 and object 3, as shown in figure above. If object 3 is in Thermal equilibrium* with object 2 and object 2 in in thermal equilibrium with object 1, then we can say that object 1 is in thermal equilibrium with object 3.

This law explains about the term 'Equilibrium'.

* Thermal Equilibrium :- 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.