Who first named the zeroth law of thermodynamics?

Ralph Fowler first named the zeroth law of thermodynamics. The law states that if two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with one another.

The idea itself was not new in the 1930s. Earlier physicists, including James Clerk Maxwell, had discussed thermal equilibrium and the common nature of heat. What Fowler contributed was the memorable name and its placement before the already established first, second, and third laws. Renumbering the existing laws would have caused confusion, so the new principle became the “zeroth” law.

The exact date is sometimes reported differently because historians distinguish between Fowler’s early formulation, the first printed use of the name, and its later textbook adoption. Fowler’s 1936 writing is commonly cited as the first published use of “zeroeth” or “zeroth law,” while Fowler and Edward Guggenheim helped establish the formulation in their 1939 book. These dating details do not change the standard quiz answer: Ralph H. Fowler is credited with naming the law.