Abstract

In our derivation of the second law of thermodynamics from the relation of adi-abatic accessibility of equilibrium states we stressed the importance of being able to scale a system’s size without changing its intrinsic properties. This leaves open the question of defining the entropy of macroscopic, but unscalable systems, such as grav-itating bodies or systems where surface effects are important. We show here how the problem can be overcome, in principle, with the aid of an ‘entropy meter’. An entropy meter can also be used to determine entropy functions for non-equilibrium states and mesoscopic systems. 1