English: A classic glow discharge in a Crookes tube, a glass gas discharge tube evacuated to a pressure of around 10^-5 - 10^-6 torr, with a metal electrode inside at either end. The glow is due to a voltage of several thousand volts applied between the electrodes, which causes the gas inside the tube to ionize, allowing a current to pass through it. The cathode (negative electrode) is on the left, the anode (positive electrode) is on the right. The regions of the discharge from left to right are:

• Cathode glow - the first purplish bright region on the left next to the cathode
• Cathode dark space - the darker band between the two brighter region next to the cathode.
• Negative glow - the second larger purplish bright region next to the cathode
• Faraday dark space - the wide area without a glow near the center of the tube
• Positive column - the pinkish glowing column filling the right side of the tube. The column shows a common feature called striations, alternating bright and dark bands, caused by an instability in the plasma.
• Anode glow - the especially bright layer adjacent to the anode on the extreme right.

Alterations to original image: cropped