English: Graph showing the characteristic (current vs. voltage) curve of a gas discharge in neon gas at a pressure of 1 torr, between two planar electrodes separated by 50 cm, showing the different discharge regimes.

• If the voltage applied across the tube is lower than the the breakdown voltage of the gas, around 650V in this tube, a "dark discharge" occurs, in which ions created by external radiation such as photoionization or radioactivity are multiplied in a Townsend discharge. The current through the tube stays low and depends on external ionization.
• When the applied voltage across the tube reaches the breakdown voltage of the gas (D), electrons are accelerated to a high enough velocity that each liberates on average one additional electron through collisions. The current through the tube is no longer limited by the voltage and increases, and a glow discharge starts, causing the gas to emit light. The tube voltage drops to a lower level called the "sustaining voltage" (G or H).
• When the current through the tube reaches a high enough level (I) that it heats the neutral gas to the point where it creates additional ions through collisions, the voltage drops again and the discharge becomes an electric arc.

Original gave source of data as: C. F. Gallo, Coronas and Gas Discharges in Electrophotography: A Review, IEEE Transactions on Industry Applications, Vol.IA-13, No. 6, p.739 (1975)