Kelvin had previously proposed to define an absolute scale of temperature independent of the properties of any particular substance in terms of Carnot's function by making F'(t) constant.
But this procedure in itself is not sufficient, because, although it would be highly probable that a gas obeying Boyle's law at all temperatures was practically an ideal gas, it is evident that Boyle's law would be satisfied by any substance having the characteristic equation pv = f (0), where f (0) is any arbitrary function of 0, and that the scale of temperatures given by such a substance would not necessarily coincide with the absolute scale.
This unit is taken as being 4.180 joules per gramme-degree-centigrade on the scale of the platinum thermometer, corrected to the absolute scale as explained in the article Thermometry, Which Has Been Shown To Be Practically Equivalent To The Hydrogen Scale.
With Regard To The Scale Of Temperature, There Is Very General Agreement That The Absolute Scale As Realized By The Hydrogen Or Helium Thermometer Should Be Adopted As The Ultimate Standard Of Reference.
