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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  




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