Zeroth Law of Thermodynamics

The zeroth law of thermodynamics is one of the fundamental principles in thermodynamics.  When two thermal systems are in equilibrium and they with a third, then all are equal to each other, meaning  \(A=B\)  and  \(B=C\)  then  \(A=C\).  If a system does not transfer heat, it is in thermal equilibrium even though it can transfer heat.  It establishes the concept of temperature and thermal equilibrium. 

In simpler terms, it means that if two objects A and B are separately in thermal equilibrium with a third object C, then A and B will be in thermal equilibrium with each other when brought into contact, meaning they will have the same temperature.  This principle allows us to define and measure temperature and enables the construction of reliable thermometers.

The Zeroth Law is called "zeroth" because it was added after the first and second laws of thermodynamics had already been established.  It is considered "zeroth" in order to preserve the logical sequence of the laws.