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Graphite for spark gaps?

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Proud Mary

Sun Feb 17 2008, 11:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

jpsmith123 wrote ...

as I presently envision the design, there will be some investment in tooling to shape the graphite into a spherical or hemispherical shape. Thus I thought I'd ask first and try to get as much anecdotal information as possible before spending time and money.

Have you thought about using steel M10 or M12 dome nuts? You can stick a piece of threaded rod in one of these, mount it in a drill chuck, and so grind off the corners without difficulty. They will conduct heat away famously, and when they get pitted and worn, you can just unscrew em and screw on new ones. They're always readily available, and they're cheap as chips on ebay:

jpsmith123

Mon Feb 18 2008, 12:31AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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I may end up doing that, Harry, or possibly using round drawer knobs from Lowe's or Home Depot, etc.

Being somewhat of a perfectionist though, what intrigues me about graphite is that it doesn't make toxic metal or metal oxide dust and/or vapor, which may then deposit on any insulating surfaces nearby, but instead it makes CO2 (if operated in air, that is). Moreover, as per the papers I've seen, it apparently has a relatively low rate of erosion as well.

Proud Mary

Mon Feb 18 2008, 09:56AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

jpsmith123 wrote ...
what intrigues me about graphite is that it doesn't make toxic metal or metal oxide dust and/or vapor, which may then deposit on any insulating surfaces nearby, but instead it makes CO2

I'd be surprised if there wasn't some kind of sputtering activity with the release of particulates due to thermal stress fractures on the electrode surface. And perhaps you'll even make a few carbon nanotubes on the way! If you look at photomicrographs of even low power spark strikes on metal foils, there are often ejecta lying beyond the lip of the crater, so I wouldn't be surprised if your graphite was rearranged in this way too.

ramses

Wed Feb 20 2008, 09:22PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

keep in mind though with the electrode erosion information, that graphite is a LOT less dense than tungsten, copper, or stainless. you may loose more mass, but more volume, and thus length. additionally, the graphite in the commercial graphite electrodes may be compressed a lot more than normal. if you can use replacement electrodes from those manufactures, great, otherwise, go with some other material like previously suggested. the CO2 may leave a more conductive path that might make it harder to quench the arc.

Sulaiman

Wed Feb 20 2008, 09:40PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I am using Tungsten rods from eBay like these

(item # 130197505228) and I can thoroughly recommend them.

Weston

Wed Feb 20 2008, 10:48PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

the reason the pencil leads burened up is because they contain plastic. i have 2 1/4 inch carbon rods that i used to try to make a latern batery arc lamp. after 20 min. i lost only like a mg almost an almost unnoticible amount. i will test them out for the primary of my tesla coil if you like for mass loss.

Proud Mary

Thu Feb 21 2008, 12:23AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

As Marx generators are more devices for individual shot firing, I'm not sure how much concerns over ablation of the electrodes should concern you. It is quite a different situation to a Tesla spark gap, firing at 50Hz or 60Hz.

I have looked at the specifications of some very large Marx generators, and with some of these even very costly capacitors often have expected life-times of only a few hundred shots.

If you use stout welding carbons, I doubt that a (relatively) low-power Marx will make that much impression on them with the low number of shots likely to be fired, when compared with the incessant activity at a Tesla spark gap.

Weston

Thu Feb 21 2008, 02:15AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

has any one used brass? for the very high amperage coin crushers they use brass rods and a RC servo for switching. it is too large for a scr, about 1,000,000,000 amps pulse for the largest.

ramses

Thu Feb 21 2008, 08:09PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

i kind of doubt that coin shrinkers put out 1000 MEGA AMPS...

Weston

Thu Feb 21 2008, 08:14PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

pulse. only for like a nano second

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