1. A well-known phenomenon is the establishment of a potential difference between an electrically isolated probe or wall in contact with a plasma [for example, see L. B. Loeb,Basic Processes of Gaseous Electronics(University of California Press, 1955), pp. 336–338]. The latter effect results essentially from the large mean velocity of free electrons compared to the mean velocity of positive ions. The establishment of such a negative potential and the refractory dust absorption of free electrons are closely related phenomena.