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Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
joe doh wrote ...
I think the americium 241 would ionise the air in the gap, the radiation just does not pass through glass, so no wonder it doesn't work with neon bulbs. I don't see how this would help you though, you would just have to increase the gap distance, or is that what you want?
Increasing the gap to get a longer spark should provide a bit more light. I am a bit reluctant to just up the voltage because of all the insulation and corona issues--and working with these things in cramped conditions does not help either. I will try the americium 241 as soon as I have scrambled together a couple of old fire detectors.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Okay I'm not trying to dispell great thinking but taking apart smoke detectors to get at the Am ionisation source is not legal. It purposley protected and while in its protection is exempt IAEA regulations. People in the USA can and will be prosecuted if found to be tampering with these units. A warning of how quick things get out of control was on the home fuser net recently.
but you can improve your gap with out the radioactive souce by making all the electrodes sharp to enhance field emission. The best materials for the sparkgap are high melting point metals such as Tantalum or Tungsten - with sharp edges. Of course multiple edges are best. Remember that your anode terminal will erode the fastest and does not require the sharp edges as you have to enhance electron flow from the cathodes and these will not erode as quickly.
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
johnf wrote ...
Okay I'm not trying to dispell great thinking but taking apart smoke detectors to get at the Am ionisation source is not legal. It purposley protected and while in its protection is exempt IAEA regulations. People in the USA can and will be prosecuted if found to be tampering with these units. A warning of how quick things get out of control was on the home fuser net recently.
Thanks for the advice, but considering how often these are disposed of in the garbage, I couldn't take such law seriously. At least not as long as smoking is allowed. Now how many people get cancer from taking apart their fire detectors each year---or maybe they don't because it's illegal Hopefully the law enforcement in my country don't put smoke-detector-opening hobbyists high on their wanted lists.
johnf wrote ...
but you can improve your gap with out the radioactive souce by making all the electrodes sharp to enhance field emission. The best materials for the sparkgap are high melting point metals such as Tantalum or Tungsten - with sharp edges. Of course multiple edges are best. Remember that your anode terminal will erode the fastest and does not require the sharp edges as you have to enhance electron flow from the cathodes and these will not erode as quickly.
I haven't had much luck with sharp edges in spark gaps because of corona issues. Agreed on the excellence of materials such as Tungsten for this purpose, too bad it's not possible to machine these very hard materials at home with common tools. Brass will have to do for now.
Registered Member #87
Joined: Thu Feb 09 2006, 01:36PM
Location: San Jose
Posts: 191
I have some 1/8 inch tungsten, and I could make a pair of 'trodes. I am an ultra skilled dremel master, with such an assortement of bits, I need an excuse to wear them out. If 1/8 dia. is good, I'll donate 1/2 inch length to the cause, so two 1/4 inch long electrodes that can be clamped, with any sort of tip you want. I'm really liking that photography. As for your americium ideas, I have yet to try placing it near a gap, but it should work in air. Perhaps the reason it didn't help for the neon bulbs is the alpha rad. couldn't get through the glass. I wouldn't worry too much about the feds knocking on your door for taking apart a smoke detector, just like you don't get ticketed for j-walking...
Registered Member #74
Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99
Excelent photos...
Our local university have a triggered spark gap. The main gap is made from two hemispheres of what looks like polished steel (stainless?) . The trigger gap is a much smaller sphere about 1/2" from one of the other electrodes.
The main gap is around 30mm wide and has about 25Kv across it, the trigger is a large strobe trigger transformer, giving a short pulse of around 4Kv.
I would stick with hemispheres rather than having pont's as the points will have to take the whole force of the breakdown and will wear and they wont have as higher hold off voltage.
the trigger makes the "spike" of ionised air to one hemesphere to lower the holdoff voltage and arc across the gap.
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
Do you happen to know about the polarities, is the trigger closer to the + or - side of the gap, and is the trigger pulse negative or positive? From what I've gathered from different sources, the best configuration is generally considered to be having the trigger close to the positive side of the gap and giving it a positive pulse. This presumably gives the most reliable triggering and least jitter (ref: ), I don't really know why, though.
I also believe it is wise to stay with convex shaped electrodes, spark gaps with sharp points don't seem to work well in practice.
Registered Member #74
Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99
No, I have no ides about the polarities..
My understanding is that the pulse should be in the same polarity as the field between the two electrodes,effectivly 'warping' the field to cause breakdown.
Of course with a cap discharge through an ignition coil, you get a large swing one way followed by a slightly smaller reverse one, so it probably dosen't matter. Plus once you have a cloud of ioised air from a discharge that cloud is conductive itself.
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
Self_Defenestrate wrote ...
I have some 1/8 inch tungsten, and I could make a pair of 'trodes. I am an ultra skilled dremel master, with such an assortement of bits, I need an excuse to wear them out. If 1/8 dia. is good, I'll donate 1/2 inch length to the cause, so two 1/4 inch long electrodes that can be clamped, with any sort of tip you want. I'm really liking that photography. As for your americium ideas, I have yet to try placing it near a gap, but it should work in air. Perhaps the reason it didn't help for the neon bulbs is the alpha rad. couldn't get through the glass. I wouldn't worry too much about the feds knocking on your door for taking apart a smoke detector, just like you don't get ticketed for j-walking...
Thanks for the offer, I already obtained some broken glass drill bits (tungten carbide) which I may use. What's the best procedure for grinding them with a dremel, just using the regular cutting disks and lots of patience, or something else? Diamond?
For ionizing gas inside a glass vessel from the outside, the alpha rad cannot be used. Alpha particles only travel a few centimeters in open air and are stopped by a thin paper. However, the gamma rad could possibly be used, but I think it is too low power from those small Am buttons.
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Triggered spark gaps
Hopefully the law enforcement in my country don't put smoke-detector-opening hobbyists high on their wanted lists. 
