Structural Biochemistry/Zeroth Law
The Zeroth law (also called equilibrium law) states that "if objects A and B are separate in thermal equilibrium with a third object C, then A and B are in thermal equilibrium with each other" (Jewett, Serway. Physics for Scientists and Engineers - 6th ed).
The Zeroth Law of thermodynamics focuses on the thermal equilibrium of two connected bodies in the same system. This thermal equilibrium is made apparent when an object of higher temperature transfers heat to an object that is of lower temperature. Eventually, both bodies reach the same temperature where the change in heat between the two is no longer measurable. They reach a constant temperature which exists between the two starting temperatures.
To take this definition one step further, let's consider a hypothetical situation of three systems, where system A is in contact with system B only, and system B is only in contact with system C, and we assume that A,B, and C are in thermal equilibrium then all three objects have the same temperature
This law was named and further studied by Ralph H. Fowler, a British physician and astronomer who contributed widely to physical chemistry and statistics. The Zeroth Law was developed long after the first three thermodynamic laws had been established. Scientists did not realize the law's extreme importance long after its discovery. Because the law was basic and laid the foundation for the rest of the thermodynamic laws, it was called the Zeroth Law rather than the Fourth Thermodynamic Law.
This law allows us to make quantitative measurements about the temperature of one system by relating it to the temperature of other systems.