Van de grraff problem....sixth time i guess?

Question

OK...i have glass as my upper roller 2 pins as the brushes PVC belt and a 6 v motor...its shaft as the lower roller to gain charge.....but its not working good....on the advice of a gentleman around here i used a sphere(2 inverted steel bowls brought together by tape) and connected to the lower roller....its gaining a maximum charge of .11V that too in the beginning.....instead of the charge increasing its *DECREASING*....and i'm really frustrated bout this.....it lowers to .06 volts....thats it....got any advice whats goin on??

Answer 1

How are you measuring this voltage? You can't use a multimeter to measure the voltage of the Van de Graff generator like you would measure the voltage of a battery. That's because meters measure voltage by allowing current to flow through them. A battery has a much higher current production than a small Van de Graff generator. So if you connect a voltmeter to the globe on the Van de Graff, all you'll do is leak the charge off the globe and measure an insignificant voltage. It would be equivalent to connecting the globe to ground.

Instead, first demonstrate whether the Van de Graff is working. It should build up enough charge within a few seconds of operation so that you produce a significant spark when you bring a grounded blunt object near the globe. You might even get spontaneous sparks from the globe.

If you are not seeing this kind of behavior, the first thing to do is to check that you have isolated the globe from ground. This is sometimes very difficult. I don't recall if you said how it was being held up, but the way you do it has to be extremely high resistance. Some people use a PVC tube. The tube has to be perfectly clean of oils.

You say that you connected the bowls to the lower roller. Was that a typo? You don't want the globe connected to the lower roller.

Look at those web sites that I listed in my other response and see how your Van de Graff compares to them.

Answer 2

Are you grounding it where you need to? You need to pull the charge from somewhere and it's going to take forever if you're just pulling it from the air. Decreasing charge sounds like you don't have the bowls well insulated from the rest.