The purpose of the original Joule (as opposed to JT) expansion experiment was to assess intermolecular interactions in real gases. In that experiment, Joule immersed twin glass globes into a water bath. The globes were connected by a closed valve. One globe held a pressurized gas, while the other was evacuated. Joule then opened the valve, allowing the gas in the filled globe to expand into the evacuated one. Since the gas is expanding into (approximately) a vacuum, it does no work (Pext = 0), and thus any temperature change is due to changes in intermolecular potential energy resulting from changes in intermolecular separation (and conservation of energy). This "free expansion" is thus called a "Joule expansion". Joule's experiment failed to show a temperature change, however, because he was measuring the temperature change of the water bath, and the latter's heat capacity far exceeded that of the gas. [I recall reading that modern calculations of Joule's experiment estimate the actual temperature change was ~0.001C, too small for Joule to measure.]

The obvious improvement would be to insulate the glass globes and measure the temperature of the gas directly. Joule didn't do this, however, because he realized that even the heat capacity of the glass globes would be too much greater than that of the gas, precluding him from accurately measuring a temperature change (i.e., perfect adiabatic walls don't exist). [This is explained in Reif, Fundamentals of Statistical and Thermal Physics]

According to Reif, the solution was the JT experiment, which was designed to allow a steady-state flow of gas. Such a flow allowed for the temperature of the insulated apparatus to equilibrate with that of the gas. Since the container is now at the same temp as the gas, the high heat capacity of the container walls is not an issue, i.e., heat is no longer flowing between the gas and the container. This equilibration is not possible with a single-event experiment in which gas is expanded into a vacuum, but it is possible if you have a high-P tank with a pressure regulator, venting gas continuously into the atmosphere. That's what the JT set-up gives you. It so happens that this set up results in an isoenthalphic, rather than isoenergetic, process.

QUESTION: Did the fact that the JT experiment is isoenthalphic result purely from the need to have a continuous flow (i.e., was its isoenthalpic nature just an incidental consequence of the design), or was it specifically designed to be isoenthalpic because Joule and Thomson gained specific insights into intermolecular interactions because of its isoenthalpic nature -- insights not possible from a free expansion? [I don't think the latter is the case, since one can get inversions in both free and JT expansions and, when it comes to gaining molecular insights, the JT expansion muddies the waters vs. a free expansion.] Yes, this experiment gives us $(\partial H/\partial P)_T$, but that can also be obtained from 𝛽: $(\partial H/\partial P)_T$ = V(1- 𝛽T). [Or is 𝛽 actually measured from $𝝻_{JT}$?].

My suspicion is they designed it to be continuous for the reasons I described above (steady-state temp drop), and its isoenthalphic nature was a happy (but important) bonus.


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