 Then the quantity of entropy taken in from the hot reservoir when the working body is heated is equal to that passed to the cold reservoir when the working body is cooled.Then the absolute or thermodynamic temperatures, and , of the reservoirs are defined so that to be such that
The zeroth law of thermodynamics allows this definition to be used to measure the absolute or thermodynamic temperature of an arbitrary body of interest, by making the other heat reservoir have the same temperature as the body of interest.Kelvin's original work postulating absolute temperature was published in 1848.