This figure supplements the book’s Figure 42. The book’s figure describes the features that appear in the glow discharge. However, we felt that a color picture is required as a cross-check to help you correctly set up your own system.
Nothing happens at atmospheric pressure, since the voltage of the power supply is too low to bridge through approximately 25 cm of air inside the column. However, a glow discharge in the form of a thin streamer forms between the electrodes once the pump is turned on and the pressure drops to around 100 Torr. As the pressure continues to drop, the streamer becomes more vivid, radiating with a pretty pink/violet glow.
When the pressure is lowered to around 5 Torr, the pink glow pulls away from the cathode, leaving a bright bluish glow next to the cathode and a dark space (known as the Faraday space) between the pink and blue light areas. At around 1 Torr, the blue negative glow separates from the cathode. At the same time, the pink positive column breaks up into a series of separate stripes (striations). Below that pressure, the various interesting features of the glow discharge become very pronounced as described in the book and shown in the color pictures above.