Absolute Zero of Temperature

  To arrive at the Dulong and Petit empirical formula for the velocity of cooling one does not need to know the absolute temperatures, just changes in temperature relative to the zero of the temperature scale one is using. They did however assume that the rates of emission and absorption of heat were the same function of the absolute temperature. Their use appears to have been influenced by Dalton's discussion of the existence of an absolute temperature in 1808. Dalton states the difference from absolute zero may be approximately 1500 °F below the zero of the Fahrenheit scale but one can interpret this as the heat content of a body expressed in terms of the standard unit of heat based on heat capacity which is not constant for all materials. In the Dulong and Petit empirical formula with the equivalence of emission and absorption the absolute temperature of the thermometer scale used is a common factor that can be removed from the exponential factor and included in the common coefficient.

In 1848 Thomson (Kelvin) proposed an absolute temperature scale based on Carnot's work on steam engines. Joule and Thomson worked on this and a few years later arrived at a value of -273.7 °C for the temperature of absolute zero.

Supplemental (Apr 1): In 1854 Thomson and Joule used the coefficient of expansion of air for an estimate of the absolute zero of temperature equal to 272.85 °C and also indicated where the 273.7 °C value came from. A value for the thermal coefficient of expansion for air, α, equal to 0.0036623/°C had been published prior to this by Regnault in 1842. The formula for the relative expansion is 1 + αΔT so for a temperature change of 100 °C a change of 100α is observed.