A light bulb "runs out" because atoms from the tungsten filament
gradually evaporate (actually, "sublimate" may be a better word) from
the filament when it is hot. The filament becomes progressively thinner
and weaker (and sometimes you can see a dark splotch of the atoms
accumulating on the inside of the glass). The filament usually
oscillates with the power frequency a bit, it may get shaken around if
the bulb is moved, and it undergoes thermal expansion and contraction
when it heats up and cools down when switched on or off. This is
eventually too much for the poor filament and it will break.
Not so with a plasma ball! There is no filament, merely a
spherical electrode in the middle. There may be special coatings on the
electrode which may evaporate away with time, I don't know exactly.
Small neon bulbs, the kind used in night-lights, have two electrodes
with a coating that does in fact degrade with time (either the coating
degrades or deposits from impurities in the gas collect on the
electrodes). Sometimes you'll find an old one of these neon bulbs which
only lights up when you shine light on it. That's because the "work
function" (how much energy it takes to free an electron from the
electrode) has gone up as it got old, and shining light on it adds
energy the electrons can use to help escape from the electrode. It
could well be that plasma balls don't need any special coating on the
electrode.
They still aren't expected to last forever, though. Plasma balls
require a source of high voltage which alternates from positive to
negative and back rapidly. Power supplies don't live forever, and I
suspect that even those which come in the bases of plasma balls will
eventually stop working.
Tom
(published on 10/22/2007)
