The property measured in the experiment is the ratio of the temperature change to
the change of pressure, ΔT/Δp. Adding the constraint of constant enthalpy and taking
the limit of small Δp implies that the thermodynamic quantity measured is (∂T/∂p)H,
which is the Joule–Thomson coefficient, μ. In other words, the physical interpretation
of μ is that it is the ratio of the change in temperature to the change in pressure when
a gas expands under conditions that ensure there is no change in enthalpy.