With the practice of using reversible processes, Sadi Carnot in 1824 advanced the study of the second law by disclosing a principle consisting of the following propositions.

1. No engine can be more efficient than a reversible engine operating between the same high temperature and low temperature reservoirs. Here the term heat reservoir is taken to mean either a heat source or a heat sink.
2. The efficiencies of all reversible engines operating between the same constant temperature reservoirs are the same.
3. The efficiency of a reversible engine depends only upon the temperatures of the heat source and heat receiver.