 Joule's free-expansion experiment

In his paper published in 1845, Joule connected two copper receivers with stopcocks, filled one with compressed dry air, and evacuated the other with an air pump. He immersed the connected receivers in water in an insulated arrangement, stirred the water, and read a sensitive thermometer. He then opened the stopcocks, let the air redistribute, stirred again, and compared the water's temperature with its initial value. Alternating control trials accounted for effects such as stirring and evaporation. The observation was no detectable net cooling of the common bath, rather than a record of the gas temperature at every instant. ^{[ref] [ref]}

Joule also immersed the receivers and their connecting piece in separate water vessels. The source receiver cooled while the receiving side warmed, with nearly cancelling thermal effects. ^[ref]

This distinguishes local transient changes from the final observation for the combined apparatus. Insulating its outer boundary suppresses external heat exchange; it does not prevent heat exchange between the gas, the receivers and the water inside that boundary.

An idealized modern version can use two rigid chambers joined by a valve, with thermometers in the chambers and on the walls. Prepare one chamber with gas and the other evacuated, disconnect the pump, close the apparatus to matter exchange, and let the initial temperature settle. Record initial and final equilibrium pressure and temperature, along with the wall response. Repeated closed-valve runs can reveal drift and valve-actuation effects; independent heating calibrations can estimate the heat capacity of the participating apparatus. This is a proposed measurement protocol, not a claim of a particular measured result. Fast local readings can characterize the release, but they cannot replace the final equilibrium readings in the endpoint internal energy balance.