File:Glow-discharge-tubes.gif

Figure 1 shows what can happen when an electric current passes through a gas discharge tube. A number of visual phenomena may appear, depending on the gas pressure, voltage across the tube, tube length, gas type, and other factors. It should be noted that at the cathode (the source of electrons), the electron density is the same as at the anode.

Figure 2 shows a hypothetical "funnel-shaped" glow discharge tube, in which the cathode is much larger than the anode; the electron density increases towards the anode.

Figure 3 shows a spherical glow discharge tube. The cathode has become a "virtual cathode", and some the glow discharge phenomena have disappeared. The electron density at the anode is much greater than at the virtual cathode. Image created by Ian Tresman on July 13th, 2005, based on an images by Hannes Alfvén in his book Cosmic Plasma (1981), p.55