Real gases have nonzero Joule–Thomson coefficients. Depending on the identity
of the gas, the pressure, the relative magnitudes of the attractive and repulsive
intermolecular forces, and the temperature, the sign of the coefficient may be either
positive or negative (Fig. 2.30). A positive sign implies that dT is negative when dp is
negative, in which case the gas cools on expansion. Gases that show a heating effect
(μ < 0) at one temperature show a cooling effect (μ > 0) when the temperature is
below their upper inversion temperature, TI (Table 2.10, Fig. 2.31). As indicated in