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Dr. ISOTOP

Thu Jan 06 2011, 02:59AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Anyone know of a good source of ~1" (or larger) conducting spheres for spark gaps? I need them for my Marx generator.
I've been thinking about using dome nuts, but the 1" variety is rather expensive.
Preferably, the spheres should be hollow, since it makes them cheaper, lighter, and easier to drill.

Patrick

Thu Jan 06 2011, 05:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

3/4 " ball bearings are what I use there easy to find , (car junk yard) (and hardware store), but there solid and HEAVY. I use a disc sander to grind a 1/4" diameter flat, then coat the whole thing in flux, and solder or weld a steel ( or solder a copper) rod to it.

This is what I do for 1/4, 3/8, 1/2, 3/4 " inch ball bearings, it works well. However, they can get hot enough for solder to melt.

I dont know if this helps but I thought I would post it for all to see. I have always wanted a source as you suggest but not found one that is high quantity, or consistent. (have looked for several years).

Dr. ISOTOP

Thu Jan 06 2011, 05:42AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Hmm...bearings look nice...a 50-pack of 3/4" bearings is only $15 on McMaster-Carr.
I'm wonder whether there's a household item that is a hollow metal sphere. Perhaps some sort of ornamental/decorative device?

Proud Mary

Thu Jan 06 2011, 02:14PM

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

Dome nuts are often a good solution to spark gap needs. They are readily available in sizes up to M24, (and probably larger) and have the advantage of being threaded, allowing for simple, stable, reproducible adjustment, which can be locked in place by a lock nut once the required setting is found.

Patrick

Thu Jan 06 2011, 03:14PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

Dome nuts are often a good solution to spark gap needs. They are readily available in sizes up to M24, (and probably larger) and have the advantage of being threaded, allowing for simple, stable, reproducible adjustment, which can be locked in place by a lock nut once the required setting is found.

Yes, but are they cheaper then another kind of round, like ball bearings ?

HV Enthusiast

Thu Jan 06 2011, 04:41PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

bwang wrote ...

Anyone know of a good source of ~1" (or larger) conducting spheres for spark gaps? I need them for my Marx generator.
I've been thinking about using dome nuts, but the 1" variety is rather expensive.
Preferably, the spheres should be hollow, since it makes them cheaper, lighter, and easier to drill.

The best bet, or maybe the easiest bet, would be to just go to your local Home Depot or Lowes Hardware store and get some of the spherical drawer knobs. These are really nice and work extremely well for Marx Generators. Just screw them in and you get a nearly perfect 1" spherical surface for spark gap. They could be pricey though. Maybe $2-$3 bucks each, or more.

However, if you need a lot of them, you'll have to go to several Lowes or Home Depots and they usually don't carry too much stock at one place.

darkspeed

Thu Jan 06 2011, 05:18PM

Registered Member #2958 Joined: Sat Jul 03 2010, 02:36AM
Location:
Posts: 8

mcmasterkarr has threaded metal ball handels for equipment - they work great

HV Enthusiast

Thu Jan 06 2011, 07:04PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

darkspeed wrote ...

mcmasterkarr has threaded metal ball handels for equipment - they work great

Wow. They look really nice and are very inexpensive as well.

Proud Mary

Thu Jan 06 2011, 07:15PM

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

Patrick wrote ...

Yes, but are they cheaper then another kind of round, like ball bearings ?

I've not tried machining a plane surface on a ball bearing and aligning a tapped hole with the centre of the sphere because it would need more skill than I have to hand. Welding an accurately aligned rod or threaded bar to a solid steel sphere looks no less daunting when you think of the need to set gaps with a micrometer to ensure reliable operation.

Dome nuts are readily replaced and re-set where pitting/ablation has occurred.

But my mince pies were said to be very good this year!

quicksilver

Thu Jan 06 2011, 07:22PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I've used PM's dome nut idea and it's a fair bet easier to adjust and work with. There are also dome nut production pieces in very heavy black chrome which resists pitting a bit better than others, etc.

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