The zeroth law of thermodynamics is a generalization principle of thermal equilibrium among bodies, or thermodynamic systems, in contact.
The zeroth law states that if two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.
Systems are said to be in thermal equilibrium if they are able to transfer heat between each other (for example by conduction or radiation) but do not do so. Systems can also be said to be in thermal equilibrium if they are not able to transfer heat to each other, but would not do so if able. The law implies that thermal equilibrium between systems is a transitive relation, which affords the definition of an empirical physical parameter, called temperature. The temperatures are equal for all systems in thermal equilibrium. The law permits the construction of a thermometer to measure this property.
Read more about Zeroth Law Of Thermodynamics: Zeroth Law As Equivalence Relation, Thermal Equilibrium Between Many Systems, Foundation of Temperature, History
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“Faith, I have been a truant in the law,
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And therefore frame the law unto my will.”
—William Shakespeare (1564–1616)