Fun with Discharge Tubes

June 2010

Some time back I was reading about discharge tubes (aka glow tubes, Geissler tubes), and thought it would be interesting to try to make them myself. I've since discovered it's somewhat easier said than done. Having said that, over this weekend I successfully got a glow out of a glass tube of my own making! I still have much to improve, but it's a start. Click on the pictures for full size view.

I used Borosilicate glass tubing (aka hard glass, chemistry glass, Pyrex). This glass has a higher melting point than the more common soda-lime soft glass, so you must use a burner that can reach the required temperature. I use a Nortel Minor lampworking torch, burning a mixture of Propane and Oxygen. Instead of using bottled Oxygen, I use an old medical Oxygen concentrator I picked up on eBay for a few hundred bucks. Although Borosilicate glass has a higher melting point than soft glass, it is more resilient and less prone to cracking from stresses in the glass that build up from working it.The metal electrodes must be of a metal that won't readily melt in the high temperature flame, when you're inserting it into the molten glass. It must also expand/contract at a similar rate to the surrounding glass, in order to make a vacuum-tight seal, and reduce stresses in the surrounding glass. I used pure Tungsten wire purchased from smallparts.com. Getting the glass-to-metal seal to be reliably vacuum-tight is quite difficult. I'm still working on developing techniques for this and on improving my glassworking skills generally.The glass tube must be evacuated, down to a few Torr if possible. I bought a rotary oil-sealed vacuum pump from an eBay vendor here in Sydney. These are normally used for evacuating air-conditioning systems. I also needed hoses and fittings. After some digging around, I discovered the suppliers used by air-conditioning technicians have all the right stuff. These places have a whole range of hoses, fittings, gauges and good quality vacuum pumps! I also bought a very accurate and sensitive digital vacuum gauge from a US eBay vendor. Using this, I discovered that my cheap pump and plain air-conditioning hoses can get down to a few Torr of vacuum. Better than I needed.Connecting the vacuum system to the glass takes some thought. For my first tube shown here, I got lucky because a standard air-conditioning charging hose, fits very snugly over a 6mm (outside diameter) glass tube. I also need to work on this area some more, so I can cleanly separate the glass tube and seal it off, while retaining the required level of vacuum. In the pictures here, I cheated a little because the vacuum system remains connected while the power is applied to the tube.

Then comes the power supply. To make the rarefied air in the tube glow, you need high voltage. My first attempt was to make a supply using an automotive ignition coil controlled by a digital switching circuit. It didn't work very well. Then I discovered this has all been done before, and you can buy small, cheap, solid state, gas discharge lamp power supplies online. The one I used here converts 12 volts DC into 2,000 volts AC at 10mA, 30kHz. It cost $US20 and works a treat. It is known as a "MiniMax3" and is available from Information Unlimited via their website.

One question that came to mind was, what does this "wild thing of nature" emit while it's operating? I waved a Geiger counter around the tube while it was operating, and the counter made some weird whooshing noise in certain positions relative to the electrodes. I'm wondering if this thing emits some x-rays or other dangerous stuff. Next experiment will be to find some way of detecting x-rays. I read somewhere that solid state diodes can do it.

Contact Me

My real name is Greg Newton and I'm in Sydney, Australia. You can contact me at: greg dot newton dot oz at gmail dot com (written like that to avoid spam bots).