Let us learn Zeroth Law of ThermodynamicsLet us consider two systems A and B separated from each other by thermally insulated wall (one that conducts no heat), but each being in contact of a third system C through a (diathermic) wall which permits heat to pass. Then the system A will be in thermal equilibrium with system C and similarly system B will also be in thermal equilibrium with system C.
Now if the adiabatic wall between systems A and B is replaced by a diathermic wall, experiments show that no further change occurs in a system A and B indicating thereby the system A was also in thermal equilibrium with B, This important experimental fact leads to the following general conclusion. you can also get help with density of metals . This statement is called the zeroth law of thermodynamics and forms the basis of concept of temperature. All these three systems can be said to possess a property that ensure their being in thermal equilibrium with one another.