4 Dugald Clerk, The Work and Discoveries of Joule 



an absolute zero of temperature. Thomson accordingly 

 defined equal temperature differences thus : — 



" Equal temperature differences are to be differences 

 between the temperatures of the source of heat and the 

 refrigerator, when the proportion of work produced from 

 a given quantity of heat is the same." 



Thermometers graduated in this way could be treated as 

 instruments based on definite principles and so rendered 

 independent of the particular properties of any material. ■' 



Unfortunately the material theory of heat was maintained, 

 and accordingly heat was supposed to pass through the 

 Carnot ideal engine without change ; that is, although work 

 was performed by the engine, the refrigerator received the 

 same amount of heat from the engine as was applied at the 

 source. All that was supposed to happen was a fall of a 

 given quantity of heat from a higher temperature to a lower, 

 which was conceived to perform work as water does in a 

 water wheel in falling from a higher to a lower level. This 

 fallacy introduced an important error into the absolute scale, 

 and the definition required that the proportion of work 

 performed by the given quantity of heat was to be the same 

 for each degree. This gave a scale greatly differing from that 

 of mercurial, air, and other thermometers ; the degrees defined 

 by it corresponding to larger and larger intervals on the air 

 thermometer as temperature increases. Professor Tait pointed 

 out also that on such a scale of temperature the temperature 

 of a body totally deprived of heat is negative — infinite. 



In a Paper published at the Royal Society of Edinburgh 

 in 1851, entitled " On the Dynamical Theory of Heat," 

 Thomson accepted the Joule determinations of the mechanical 

 equivalent of heat and absolute zero, and cleared his mind 

 completely from the errors inherent in the material theory, 

 and he proposed a second absolute thermometric scale deduced 

 from the Carnot cycle as modified by allowing for the dis- 

 appearance of heat in the process of producing mechanical 

 work. Thomson now defined temperature thus : — 



u The temperatures of two bodies are proportional to the 

 quantities of heat respectively taken in and given out in 

 localities at one temperature and at the other respectively, 

 by a material system subjected to a complete cycle of 

 perfectly reversible thermodynamic operations, and not 

 allowed to part with or take in heat at any other tempera- 



