Engines as Thermometers
William Thomson (1824–1907)—later known as Lord Kelvin—was born in Belfast, Northern Ireland.
Kelvin is best known for his work on the development of an absolute temperature scale whose units bear his name. What is the difference between an absolute and a practical temperature scale? Practical temperature scales, such as the one developed in 1742 by Swedish astronomer Anders Celsius, are defined in terms of the physical properties of a particular substance, such as the melting and vaporization points of water. Is it possible to conceive of a temperature scale which in no way references the particular properties of any substance? Kelvin said yes. Inspired by the work of Sadi Carnot,7 Kelvin recognized how the principles of operation of heat engines—in particular the efficiency of the idealized Carnot cycle—might be used to define a temperature scale which is independent of the properties of any particular substance. His ideas were originally published in 1848 in the June 5 Cambridge Philosophical Society Proceedings and also in the October Philosophical Magazine. The following reading selection is from an edited version of this paper which was republished by Kelvin in 1881 in his collection of Mathematical and Physical Papers.