Color separation in an argon-helium arc due to radiative properties and demixing

Free-burning arcs in argon-helium mixtures containing a large proportion of helium show a separation between the pink color characteristic of helium arcs and the violet color typical of argon arcs. Two effects are shown to contribute to this; first, demixing, which concentrates helium in the high-temperature region near the center of the arc, and second the fact that the radiative emission from helium, relative to that from argon, increases with temperature.