A purposly simple apparatus shows that when an electric discharge occurs in a "rarified" gas, visible light is emitted.

Airglow apparatus (see figure)
2000 V neon sign transformer
Alligator clips for connecting above items
Portable vacuum pump
Vacuum hose with standard taper joints for connection to the apparatus

This demonstration is best done as part of an introduction to atomic structure, atomic emission spectra, atomic energy levels, etc. The discovery that "rarified" air (pressure 1 torr or so) conducted electricity and that a discharge of visible light occurred under these circumstances led to the discovery of the electron and the eventual invention of neon signs and the like. The apparatus is shown below:

Explain to the class that air at normal atmospheric pressure is not a conductor of electricity. It will only conduct when an arc can jump between two high voltage electrodes in close proximity. Show the effect by connecting the two wires to the transformer, turning it on, and moving the alligator clips to within one inch of each other. An electric arc will jump between the metal clips.

Connect the clips to the nail electrodes in the apparatus, connect the hose to the vacuum pump, turn on the transformer, and start the pump. Initially, nothing happens, but when the pressure in the tube drops to just a few torr, a blue-white glow begins to appear. Turn off the room lights at this point. The airglow will increase in brightness as the pressure drops.

Ordinary nails were purposely used for the apparatus to take the mystery out of the idea of an "electrode," which is nothing more than a conductor of some sort. Nothing fancy.

Point out that the color of the light is characteristic of the components of air, and that other gases, like neon or argon, emit other characteristic colors. Point out also that this airglow phenomenon relates to the aurora borealis or "northern lights" phenomenon seen at night in places far north of Tucson.

Hold the electrodes by the insulated parts only!!!!