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New Van de Graaff gen

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Nickel

Mon Feb 06 2012, 08:16PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

Hi Folks. I've finally got around to signing up. I am after a bit of advice about my new-build VDG before I start.
Last year my local garden centre was knocking out 12" stainless spheres called 'gazing globes' for Â£15 each I bought two. I have procured 2" diameter bars of the following: nylon, PTFE for rollers. I also found a 2" diameter glass rolling-pin that I can cut down and fit to a suitable roller.
The question is: If I build two units, one positive top, the other negative top, can I expect to get double length sparks between the two spheres? would I couple the two bottom combs, or earth them?
My thinking is PTFE / glass on one unit; and vice-versa for the other. (If the glass doesn't work, the nylon goes in).
I have bought 2" natural rubber webbing from a posh hand-made furniture builder.
4" PVC sewer-pipe has been bought.
Does anyone know/recommend current doubling?
Looking forward to lots of loud blue flashes!
Nickel

Proud Mary

Mon Feb 06 2012, 08:27PM

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

Hi Nickel,

Antonio Carlos M. de Queiroz gives a well explained and illustrated account of practical dual VDG construction here:

Antonio

Tue Feb 07 2012, 04:08PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

In my double VDG I am using Nylon and Teflon rollers. Sparks between the terminals are really longer than sparks between one terminal and the other grounded. But my terminals are not large, and the sparks are faint.
With frictional charging, the bottom combs shall be grounded. If the machines are really symmetrical, just interconnecting them works too.
Regular glass is a poor, practically useless, insulator. Pyrex glass is much better. Nylon is also not very good. To see what kind or roller material works, rub it against your belt material and see if it gets charged, attracting hairs.
A current doubler is an insulated conductive roller that is connected to a comb taking charge from the belt where it first touches the roller, to charge the roller, and with another comb where the belt leaves the roller going to the terminal, if the doubler is inside it, or to ground, if the doubler is in the lower assembly. The comb charging the roller can be omitted with little difference, as some charge always leaks from the belt.
(That site is mine.)

Nickel

Tue Feb 07 2012, 05:23PM

Registered Member #4367 Joined: Thu Jan 26 2012, 07:54PM
Location: NE Hampshire UK
Posts: 16

Hi Proud Mary,
Hi Antonio,
Thanks for your prompt replies. More questions! (I cannot stop in my quest for knowledge)
Is there a specific type of glass that is a problem? I recall the glass rods in experiments at school seemed to work.
It is possible to obtain borosilicate glass tube in the form of hurricane lamp wind-shields quite cheaply.
The nylon I have is ordinary nylon 6. I would assume that this is an extrusion. To make the rollers I would turn the surface of the bar to expose fresh uncontaminated material. Would this still be useless? It does show quite high on the triboelectric series.
If the rollers are made from solid material with a conductive shaft rather than a conductive roller with a material surface-layer, how will this affect the operation of the unit?
If it is not a silly question, could the bottom rollers share a common spindle, thus giving a V-shape between the columns? The gap between the top spheres would be adjusted by angular rotation about the spindle axis.
I plan to use a belt-inside-column arrangement. Where the belt hole is in the top sphere, I was thinking of bending a circle out of 15 or 22mm dia. copper tube to fit the diameter of the hole. This would be slit around its periphery and fitted to the inside of the hole, to increase the edge radius and thus reduce corona losses. Is it worth a try, or has it been done and found ineffective?
Antonio: I like your site; it is helpful, informative and above all, understandable.

Antonio

Tue Feb 07 2012, 11:26PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The only modern glass that I have seen with good surface resistivity is Pyrex, or borosilicate glass. Other glasses may work reasonably if waxed or varnished. Here I can find Nylon rods used for mechanical purposes. I don't know what type of Nylon. As insulators they are as bad as regular glass. None can be electrized by friction, that is a simple test for high insulation.
The surface of the rollers is what matters. It may even be good to have a conductive substrate under an insulating top layer, because this would keep the charges in the rollers more firmly, as they would be attracted by the conductive substrate.
The "V" solution may be difficult with heavy terminals, that would have to be firmly fixed to the support columns. Two motors are not a great problem.
The ring around the hole improves the insulation, but the best solution is to bend inwards the metal around the hole, if possible. A plastic hose would work too.
Thank you for the comment, Nickel.

Proud Mary

Wed Feb 08 2012, 12:18AM

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

Nickel wrote ...

Antonio: I like your site; it is helpful, informative and above all, understandable.

Antonio's site is a unique resource for people wanting to understand and construct electrostatic machines, and I especially enjoy the historical detail, which helps us understand their evolution and development.

Tetris

Wed Feb 08 2012, 03:58PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Unless you have an 8ft tall one, I doubt you'd get loud blue flashes. I've seen them produce long, white, quiet flashes. Better get a girl to test it though, if your hair is too short. Make sure her hair isn't like mine (I naturally have VDG hair)

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