English: A schematic of Clusius-Dickel apparatus for the separation of isotopes. A fluid is introduced between the hotter and colder tube walls and undergoes convection; the optimum arrangement depends significantly on the temperature gradient, the convective flow and the size of the annulus. The tubes can be many metres long and the arrangement of hot and cold walls can be reversed. The process can also be carried out in a flat cell instead of an annular one.