Measurement of constant potential distribution within a sheath is a fairly complicated task because of the small sheath thickness, especially at high frequencies. More accurate measurements showed the plasma potential to be quite large and the sheaths to be as well-defined as at low pressures. This chapter discusses some experimental evidence for the normal current density effect in some discharge modes. A direct way based on discharge cross section measurement at various currents is often hard to employ, because the sheath contours are far from being regular, especially in the γ-mode. The current-voltage characteristic of an RF discharge is a major experimental parameter, whose theoretical interpretation may provide an insight into the discharge behavior. When a discharge is probed by direct current applied to RF electrodes, the resistance to this current is largely exerted by the electrode sheaths, which possess a lower active conductivity than the plasma.