What does the zeroth law of thermodynamics state?

The zeroth law of thermodynamics establishes a fundamental principle of thermal equilibrium among systems. It states that if two systems are each in thermal equilibrium with a third system, then those two systems must also be in thermal equilibrium with each other. This principle allows us to define temperature in a consistent manner, as it implies that two systems at the same temperature will not transfer heat when brought into contact.

This foundational idea becomes essential when measuring temperatures and comparing thermal states, as it underpins the concept of thermometers and the standardization of temperature scales. Essentially, the zeroth law serves as a basis for establishing the relationship between temperature and thermal equilibrium, facilitating a coherent approach to thermodynamics in various applications and scientific investigations.