Re: i have made a van de graaff generator and it only makes small sparks.

Hi John,

I must admit, I have never built a Van de Graff myself, so I can't be very authoritative when debugging yours. Good for you for pulling it off - sounds like a big one! Your description of your machine is pretty good, though, so I have a few ideas for you -

1. It's likely to be a problem with your belt, which you say is round in cross section. All of the charge generated on your belt happens at the surface where the rubber contacts the nylon. Your tube-shaped "belt" may have a very small contact patch. Also, it's important that the brush be very close to the roller - this is easier to do with a flat belt than with a round tube.
2. Where are your 2cm sparks coming from, and going to? If you're bringing a grounded wand near the dome to attract sparks, you may be doing it too fast or too often: Building up charge (and voltage) takes time. Put the wand more than 2cm from the dome, and wait. Also, the wand should have a smooth round surface like the dome - otherwise it'll generate coronas before sparking, and waste some charge.
3. To search for corona discharges (St. Elmo's Fire, a continuous slow loss of charge into the air) run the machine in a complete darkness - look for a faint blue glow at discharge points.
4. In addition to humidity, cleanliness is very important. Wipe down your belt and rollers with rubbing alcohol, and don't touch them with your fingers.
5. PVC is a very common material for the support, so that is probably fine as long as it is clean. PVC does hold static charge easily - you can rub a piece with silk or wool to charge it up - and perhaps this is what you're noticing about your support.

-Ben