Thomson's dilemma
APPLICATION of the second law of thermodynamics in its classical form requires that the heat of each process be convertible into work; that is, each process must be reversible. Since no real system meets this criterion, analysis is then made in terms of some imaginary limiting case in which the parameters have been adjusted to make the irreversible effects negligible. It is sometimes impossible, however, to imagine a limit in which the irreversible processes can be disregarded. This problem was first faced by William Thomson (later Lord Kelvin) in his thermoelectric researches. His dilemma was this: Even if he did ignore the irreversible processes, he found he could still apply the second law with what seemed to be very satisfactory results. This dilemma was sufficiently interesting to excite some of the best theoretical and experimental minds over the century that was to follow Thomson's work.