Plasma balls work by electrical discharges through low-pressure gases.
Voltage differences between the central electrode and the outer glass
sphere are created by cycling the voltage on the central electrode
rapidly from large negative voltages to large positive voltages, taking
advantage of the capacitance between the central electrode and the
inner surface of the glass sphere.
Electrons travel from the electrode outwards half of the time,
feeling the push of the electric field. The energy they get after
traveling a short distance depends on the local electric field and the
distance they manage to travel before colliding with a gas molecule
(this is called the "mean free path" and depends quite a lot on the
pressure of the gas inside the ball. The lower the pressure, the bigger
the mean free path.)
Different gases are used inside plasma balls, and these gases glow
differen colors. Neon has the famous bright orange glow, argon is a
deep purple, nitrogen is a reddish purple. Other gases glow with
different colors. Why, you may ask? The reason has to do with the
different energy levels of the electrons in orbit around these atoms
(or molecules, as in the case of nitrogen). An atom changes its
electronic energy configuration most often when the energy of the
electron striking it is "just right" (this is called the Franck-Hertz
effect). The energies of electrons bound inside atoms and molecules is
quantized, that is, it can only take on certain values.
If the mean free path is more or less the same everywhere in the
gas, then the speed of the electrons flowing through the plasma depends
on the local field strength. Slower electrons will preferentially
collide and change the energy state with atoms which prefer exactly
that electron energy. Electrons may still collide with all the other
gas components, but they will not change the atomic energy state if the
energy is wrong.
That way, if a mixture of gases fills the plasma ball, then
different gases will glow in different parts due to the falling
electric field strength as you go away from the central electrode.
(republished on 08/02/06)