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