Some basic points about normal cathode fall in a glow discharge

In normal glow discharge, the normal cathode fall Vn from a planar electrode remains constant when discharge current varries in a certain range, even by several order of magnitude. Hittorf first observed this phenomena in the condition that the discharge does not cover the entire cathode, nor is confined to a small area in the center. As matter of fact, the normal current density at the cathode remains constant (Hehl's law), and the area of the cathode covered by discharge is proportional to the total current. Many explanations in the literature are given from the minimum energy principle, assuming that the normal cathode fall is the least potential maintaining the necessary current through the discharge, and providing the space charge necessary for the cathode fall. So physical descriptions of the processes by which this constancy is maintained are found