Ignition. If a current is passed between the two
electrodes of the lamp a flux of electrons appears between the cathode and
anode. Although the pressure of the gas inside the lamp is very low, some
electrons find atoms of the supporting gas (usually Ar or Ne) to collide with.
In such collisions some of these atoms lose electrons.
The hit atoms become
very mad - you cannot imagine how mad an atom of rare gas can be when it loses
an electron. So, it looks around and observes that the cathode is the one who
throws the electrons and seeks revenge for the lost electron. It hits the
cathode with all the energy it can get from the electric field between the
electrodes and some atoms of the cathode material are ejected. This process is
known as sputtering.
Further work. The atmosphere becomes unbreathable: it is very hot inside the cathode. The electrons hit atoms, some of the atoms lose electrons and become ions, the ions hit the cathode and other atoms are sputtered, more and more free atoms, ions, and electrons appear inside the cathode. There is something like a war inside the cathode. In all this time the fundamental processes discussed in the introductory part occur, that is, because of the heat the atoms become excited and emit electromagnetic radiation.
How long does a lamp last ? You must believe what I tell you because I was in a lamp once and I didn't know what to do to get out of there. After they were sputtered some of my luckier mates didn't come back on the cathode. They found some more confortable seats on the glass of the lamp. There they absorbed some atoms of the supporting gas until there was no gas inside the lamp for ignition. It took us about two years to destroy the lamp but we did it and that's how we escaped from there.
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