The process begins with gas under some pressure, P i , at temperature T i , confined to one half of a thermally isolated container (see the top part of the drawing at the beginning of this article). The gas occupies an initial volume V i , mechanically separated from the other part of the container, which has a volume V 0 , and is under near zero pressure. The tap (solid line) between the two halves of the container is then suddenly opened, and the gas expands to fill the entire container, which has a total volume of V f = V i + V 0 (see the bottom part of the drawing). A thermometer inserted into the compartment on the left (not shown in the drawing) measures the temperature of the gas before and after the expansion.