Zeroth law of thermodynamics

The zeroth law states that thermal equilibrium is transitive: if body A is in equilibrium with body B, and B with C, then A and C are in equilibrium with each other. Though it sounds trivially obvious, it is a genuine physical postulate, and without it the notion of temperature as a single consistent number would collapse.

Its practical content is the justification for thermometry. A thermometer (B) can be used to compare two bodies (A and C) that never touch: if B reads the same against each, then A and C are at the same temperature, guaranteed by transitivity. The law therefore underwrites the assignment of a unique numerical temperature to every body in equilibrium.

The peculiar name reflects history rather than logic. The first, second, and third laws had already been numbered and named when Ralph Fowler, in the 1930s, recognised that this more fundamental fact had been assumed all along. Numbering it 'zeroth' acknowledged its logical priority while preserving the established names of the others.

Articulated and named by Ralph Fowler and Edward Guggenheim in the 1930s, formalising an assumption that had been taken for granted since the beginnings of thermometry.