Hi Neal,
The discharges you see in glowing plasma balls occur because of
the voltage difference between the metal ball at the center and the
inside of the glass holding the gas inside. Glass is an electrical
insulator, and so it would charge up rapidly, equalizing the voltage
with the inner ball if the voltage were applied as a constant. To keep
the discharges going, the inner ball alternates rapidly between
positive and negative high voltages, which is the reason why these
things hum when they're switched on (it's probably the alternating
power supply and also the alternating heating of the gas inside as the
current switches on and off rapidly). The pretty glow comes from
electrons knocked off of gas molecules which grab electrons back to
become electrically neutral. Neon and Argon and Nitrogen are common
gases with pretty colored glows.
The thin tubes of discharge stay more or less in the same place
even though the current switches on and off rapidly because the heated
gas has a lower resistance than unheated gas. This is also why the
discharge filaments tend to rise slowly over time, because the hot gas
is less dense than the colder remaining gas.
The reason why discharge filaments head for your hand when you
place it in contact with the outside of the glass is capacitive
coupling between the inside of the glass and the outside and the fact
that your hand supplies the other lead of the capacitor, allowing
mobile charges in your body (mostly made of saltwater) to flow to
cancel charge buildup on the outside of the glass. As charge builds up
on the inside of the glass, an equal but opposite charge builds up on
the outside because the glass polarizes electrically. The canceling
charge on your hand reduces the energy necessary to accomplish this,
and so it is energetically favored to put more charge on the inside of
the glass near your hand than anywhere else.
The glass cannot conduct a steady current, so there is a limit to
how much current you can get in your body and so you are pretty safe
doing this. With higher voltages higher frequencies, more energy is
deposited in your fingertips however.
Tom
(published on 10/22/2007)
