meh I can't be bothered explaining. Look at HSC Online
http://www.hsc.csu.edu.au/physics/core/implementation/9_4_1/941net.html#net1
"
Using discharge tubes
A common piece of apparatus used for this investigation is a set of glass discharge tubes at different pressures, arranged side-by-side on a board. The tubes have been sealed after having had varying amounts of air pumped out of them (the more air pumped out, the lower the air pressure).
Each tube contains an electrode at each end to allow the application of a large voltage, which is provided by an induction coil. The high voltage causes an electrical discharge through the air in the tube, causing the air to glow. Different discharge patterns are formed at different pressures.
Sample observations
* At 5% of atmospheric pressure, long, thin red-purple streamers appear between the two electrodes.
* At lower pressure, these streamers give way to a soft red glow.
* Upon further pressure reduction, the glow is broken into striations, bands of light and dark. The amount of dark space between the glowing bands increases with further reductions.
* At 0.01% of atmospheric pressure, the dark space extends throughout the tube. At this very low pressure, the glass near the anode glows a yellow-green colour."
and follow the links there for pictures of discharge tubes as well.
