In the Carnot cycle, the working fluid must always remain constant temperatures, as the heat reservoirs they are in contact with and that they are separated by adiabatic transformations which prevent thermal contact. The efficiency was first derived by William Thomson in his study of an unevenly heated body in which the adiabatic partitions between bodies at different temperatures are removed and maximum work is performed. It is well known that the final temperature is the geometric mean temperature T H T L so that the efficiency is the Carnot efficiency for an engine working between T H and T H T L .