• id: JTHERM-01-N03
• title: Zeroth Law — Temperature as a Concept
• tags: zeroth-law, thermal-equilibrium, temperature

The Zeroth Law states: if A is in thermal equilibrium with C, and B is in thermal equilibrium with C, then A is in equilibrium with B. This transitive property of thermal equilibrium allows us to define temperature — C can be a thermometer. Without the Zeroth Law, the concept of temperature has no thermodynamic basis. It was formulated after the First and Second Laws but is more fundamental, hence "zeroth."