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[TCML] Geissler tube or lighting effects
Hi All,
There seems to be some interest in plasma tubes on this list, so I'll try
posting my answers in a way that relates to Tesla coiling. After all, it's the
plasma that we're all after, isn't it? I'll do my best to share what I know
about this fascinating fourth state of matter until it drifts too far off
topic.
Sincerely,
Tony Greer
**********
Terry Oxendale asks:
>Could you explain the practical difference between a Geissler and a
>Plasma tube?
A plasma tube is a loose term that can mean just about any sealed vessel
that contains a gas or vapor, usually at less than atmospheric pressure, and
which is ionized by an electrical potential going through it. Neon tubes,
fluorescent lamps, mercury vapor lamps, and even xenon flash tubes are all
considered plasma tubes.
A Geissler tube is one type of plasma tube, maybe the first, produced in
about 1857 by Heinrich Geissler, a German glassblower who experimented with
passing high voltages through partially evacuated glass tubes and vessels. Soon
after, he developed a market for ornamental flameworked gas discharge tubes,
usually small and very intricately bent tubing with an electrode on each end
and powered by a spark coil. These became quite popular and were known as
Geissler tubes, which had no other function than their aesthetic value. They
usually contained low pressure air, or sometimes nitrogen or possibly CO2.
>What kind of pressures are required for optimum output?
Different pressures will give different modes of discharge, and so will the
gas that is used. Low pressures, like in a neon tube which is filled to about
10 Torr (10 mmHg), will produce a glow discharge, but as the pressure is
increased it becomes a linear discharge, like a Tesla coil's streamers or in a
plasma globe.
>What are the tricks of the trade in construction?
The tricks of the trade in making neon tubes and sealed plasma tubes are
very involved. Glass to Glass or Glass to Metal seals are the only thing that
will work for extended periods, and it's very difficult to remove enough air and
get the inside of the glass to release all the impurities that bond so
tightly to it. Heat, vacuum, electron bombardment and a 'getter' coating in the
electrodes are all used to get the tube clean enough to remain stable in color
and have a long life. I can suggest a good book or two on neon and cold
cathode lighting if anyone is interested. But there's still a lot you can do to
experiment with plasma without getting too technical or expensive.
>I will be doing this with a 5" diameter (1/8" thick glass) tube. What would
be your
>recommendation for sealing the ends?
I've made a plasma chamber from a large Pyrex tube too, and I used 1/4"
acrylic for the end pieces. I used carriage head bolts for the electrodes on each
end, and just hot glued everything together. A small tube that can be
connected to a vacuum pump exits one end, and one on the other end will allow the
admission of a gas. You might try using phenolic or something else that is
more heat resistant, and epoxy it in place over the ends. This kind of seal is
okay for an actively pumped tube or for gas flow-through experiments.
>What kind of power requirement will
>I need for a 4-5 foot long tube (kV and power)?
The voltage required will depend on the gas or gases and the "pressure"
inside the tube (relative to a vacuum), and the current will determine the
brightness. The lower the pressure, the less voltage it will take, down to about 1
Torr anyway. Below that, the glow discharge fades and cathode rays start to
show up. The inert gases ionize more easily than the non inert ones, with
xenon (the heaviest) being the easiest and helium (the lightest) the hardest.
>If I use the output of the TC on this tube, what effect does the higher
frequency have on >the display?
Higher frequencies are more efficient at lighting a plasma tube and produce
some really cool effects. The type of gas or gases, the pressure, the
frequency, the current and the physical size and shape will all effect the way the
plasma looks.
>Can I use the pig only as a power supply
The pig would be overkill unless it was just for a few seconds at low power.
The 60 Hz output would just tend to make it glow like a big neon tube. The
TC would make some interesting effects at different pressures below
atmospheric, but it might get hot. A neon transformer, an electronic neon power supply
or a flyback transformer should also light it up, but not as brightly.
If anyone's interested, here's some pix of my shop and house and some of the
things I've made:
http://imageevent.com/thepacratz/glassblowingtopicspix
And here's some more plasma sculptures and other weird stuff:
http://www.glassartists.org/Gal17749_Tony_Greer_Special_FX_Neon_s_Gallery.asp
Best Regards,
Tony Greer
Special Effects Neon
**************It's only a deal if it's where you want to go. Find your travel
deal here.
(http://information.travel.aol.com/deals?ncid=aoltrv00050000000047)
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