The analysis of the Joule–Thomson coefficient is central to the technological problems
associated with the liquefaction of gases. We need to be able to interpret it physically
and to measure it. As shown in the following Justification, the cunning required to
impose the constraint of constant enthalpy, so that the process is isenthalpic, was
supplied by Joule and William Thomson (later Lord Kelvin). They let a gas expand
through a porous barrier from one constant pressure to another and monitored the
difference of temperature that arose from the expansion (Fig. 2.26). The whole
apparatus was insulated so that the process was adiabatic. They observed a lower temperature
on the low pressure side, the difference in temperature being proportional to
the pressure difference they maintained. This cooling by isenthalpic expansion is now
called the Joule–Thomson effect.