What is Joule-Kelvin Effect

In the mid-19th century James Prescott Joule and William Thomson (Lord Kelvin) conducted experiments in which they allowed gases contained in a cylinder to expand through a porous plug into a second, empty cylinder.
This set-up insulated the gas against transfer of heat or other energy from outside the cylinders.
Their finding: Generally speaking, gases cool as the expand.
Put another way, pressure affects temperature.
Such vital modern technologies as refrigeration and air conditioning are based on this insight, manipulating gas pressure to affect temperature.
As gas molecules become more spread out, the fast-moving particles slow down, and their kinetic energy decreases.
Temperature, a measure of of kinetic energy, also decreases.
But energy cannot be destroyed, only transformed.
So where does it go?
It takes the form of potential energy—the pull among spread-apart gas molecules.