Cathode ray
Gas discharge tubes
### Gas discharge tubes

[thumb\|upright\=1\.4\|[Glow discharge](/wiki/Glow_discharge "Glow discharge") in a low\-pressure tube caused by electric current.](/wiki/File:Glow_discharge_regions.jpg "Glow discharge regions.jpg")

In 1838, [Michael Faraday](/wiki/Michael_Faraday "Michael Faraday") applied a high voltage between two metal [electrodes](/wiki/Electrode "Electrode") at either end of a glass tube that had been partially evacuated of air, and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end at the [anode](/wiki/Anode "Anode") (positive electrode).Michael Faraday (1838\) ["VIII. Experimental researches in electricity. — Thirteenth series.,"](https://books.google.com/books?id=ypNDAAAAcAAJ&pg=PA125) *Philosophical Transactions of the Royal Society of London*, **128** : 125\-168\. In 1857, German physicist and glassblower [Heinrich Geissler](/wiki/Heinrich_Geissler "Heinrich Geissler") sucked even more air out with an improved pump, to a pressure of around 10−3 [atm](/wiki/Atmosphere_%28unit%29 "Atmosphere (unit)") and found that, instead of an arc, a glow filled the tube. The voltage applied between the two electrodes of the tubes, generated by an [induction coil](/wiki/Induction_coil "Induction coil"), was anywhere between a few [kilovolts](/wiki/Kilovolt "Kilovolt") and 100 kV. These were called [Geissler tubes](/wiki/Geissler_tube "Geissler tube"), similar to today's [neon signs](/wiki/Neon_sign "Neon sign").

The explanation of these effects was that the high voltage accelerated free [electrons](/wiki/Electron "Electron") and electrically charged [atoms](/wiki/Atom "Atom") ([ions](/wiki/Ion "Ion")) naturally present in the air of the tube. At low pressure, there was enough space between the gas atoms that the electrons could accelerate to high enough speeds that when they struck an atom they knocked electrons off of it, creating more positive ions and free electrons, which went on to create more ions and electrons in a chain reaction, known as a [glow discharge](/wiki/Glow_discharge "Glow discharge"). The positive ions were attracted to the cathode and when they struck it knocked more electrons out of it, which were attracted toward the anode. Thus the ionized air was electrically conductive and an electric current flowed through the tube.

Geissler tubes had enough air in them that the electrons could only travel a tiny distance before colliding with an atom. The electrons in these tubes moved in a slow [diffusion](/wiki/Diffusion "Diffusion") process, never gaining much speed, so these tubes didn't produce cathode rays. Instead, they produced a colorful glow discharge (as in a modern [neon light](/wiki/Neon_light "Neon light")), caused when the electrons struck gas atoms, exciting their orbital electrons to higher energy levels. The electrons released this energy as light. This process is called fluorescence.


What effect occurred when air pressure was reduced to 10^-3 atm?