An elementary introduction to entropy via irreversibility
An outline is given of an approach to thermodynamics in which entropy is derived directly, from a simple postulate of direct experimental significance, without reference to temperature or thermal equilibrium The author first shows how the irreversibility of a natural process may be quantitatively measured; entropy is then defined so that its increase in any process equals the irreversibility. Lastly equilibrium states are defined and absolute temperature is derived from entropy by differentiation.