This Letter reports a new class of arc columns in the torr‐pressure range which in a wide range of voltages are voltage‐determined. The arc columns consist of mixtures of some noble gas seeded with tiny amounts of alkali vapor (pressure about 3 × 10−4torr). At a certain critical current a very strong rise, by a factor of 4 or more, in column gradient occurs, accompanied by very striking visual changes in the column. An explanation in terms of the concept of ``donor depletion'' is given, the ``donor'' atoms being the alkali atoms. Possibilities for practical applications are mentioned.