The short answer: Ralph H Fowler gave the term “Zeroth law of thermodynamics” in 1935.
In 1935, Fowler was reading the texts of two physicists Meghnad Saha and B N Srivastava and he came up on the text that “Every physical quantity must be measurable in numeric terms”.
They wrote in their text “Any of the physical properties of A which change with the application of heat may be observed and utilized for the measurement of temperature.” Source: Wikipedia).
In this postulate, they didn’t use the term “Zeroth law of thermodynamics”.
There were many similar postulates mentioned in the previous physics texts published by authors before 1935, but they didn’t use the term “Zeroth law” in their texts.
Then in 1935, Ralph Fowler framed the term “Zeroth law of thermodynamics” and along with his co-author he gave the postulate: “If two assemblies are each in thermal equilibrium with a third assembly, they are in thermal equilibrium with each other.”
They concluded that if two or more assemblies are in thermal equilibrium with each other, then there must be some physical quantity which will be common in those assemblies.
This physical quantity is termed as “Temperature”.
Thus they proved the existence of physical property named “Temperature” through this Zeroth law of thermodynamics.
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