Which law of thermodynamics was first formulated by Sir Ralph Fowler?

Sir Ralph Howard Fowler (17 January 1889 – 28 July 1944), a British physicist and astronomer, worked on thermodynamics and statistical mechanics, at Trinity College Cambridge. In 1931, he was the first to formulate and label the zeroth law of thermodynamics, long after the first, second, and third laws had been widely recognised.

The first law of thermodynamics is a formulation of the law of conservation of energy. The second law of thermodynamics states that heat always flows spontaneously from hotter to colder regions of matter. The third law of thermodynamics states that the entropy (the “randomness”) of a closed system at thermodynamic equilibrium approaches a constant value when its temperature approaches absolute zero.

Fowler showed that there was one more principal law of thermodynamics. Because it is logically prior to the other three it was called the “zeroth” law (not the fourth). The zeroth law of thermodynamics states that if two thermodynamic systems are both in thermal equilibrium with a third system, then the two systems are in thermal equilibrium with each other.