Thermodynamics and Statistical Mechanics

The science of thermodynamics introduces a new concept, that of temperature) it is absent from classical mechanics, as well as from the theory of electricity and magnetism and from atomic physics (with the exception of Joule heat, intensity of spectral lines conceived as interactions between a large number of material particles). Our sense of heat furnishes a qualitative measure, and a quantitative measure, albeit fortuitous to a certain extent, is given by any thermometer. A body which is in thermal equilibrium has the same temperature everywhere. The same is true of two bodies which have remained in thermal contact for a sufficiently long time. Equality of temperature is a necessary condition of thermodynamic equilibrium.