Positive or negative discharge?

In a VdG generator is that the dome will always be +ve because electrons are pulled off the belt at the bottom and lead to Earth thus leving the belt +ve. As the +ve belt reaches the dome at the top, it draws electrons from the dome leaving that +ve and the belt less so.
The points on the star help to concentrate the charge at the tips (charge density proportional to 1/ r**2 IIRC) and also direct the electrostatic attaction. Having the tips turned as they are results in the electrostatic attraction of the charge (with the surrounding air) being converted into a torque which rotates the cross. If they were purely radial, the attraction would be also and no rotation would take place - maybe a demmo to be given.
The initial reversal of the direction of rotation is probably due to the air immediately around the dome becoming somewhat positively charged together with the dome (like charges repel), but after some time the build up of +ve charge on the dome dominates and air is a reasonable insulator so the leakage is limited.
Richard

I don't know the answer to the OP's question (which probably has something to do with ions, and maybe transients), but a Van der Graff generator can produce both positive and negative charges on the sphere.
Positive ones tend to work better (because the points inside the sphere can spray electrons onto the belt more efficiently than they can collect them from the belt), but both polarities are possible.
I was looking for a reference for this, and looked at the Wikipedia page - but unfortunately even their description of how a Van der Graaf generator is constructed is wrong. Mind you, I found a few other pages which are wrong too..
Anyway:
-- Peter Fairbrother

Peter, you are quite right, if you start applying voltages to the belt. My recollection was with a machine that simply 'wiped' electrons off the belt at the bottom. However, looking at the references you quote it is clear that even just rubbing the surface is not guaranteed to result in removing electrons, depending on the material used, interesting I didn't know that. I have only ever heard of removing electrons, but it's not really my field (pun intended). I would however suggest that irespective of the polarity, the mechanism by which the 'starwheel' rotates is valid. Even if the surrounding air is ionised to some extent it will be the opposite polarity to the tips of the rotor hence the attraction and rotation.
Richard

I've only ever seen a star wheel demonstrated using a piezoelectric source and that was by an uncle that worked for UMIST, it rotated anti-clockwise according to your diagram as the charge leaked off the pins at the end and drove it round. It was impressive and I was about 13 at the time and it was mentioned IIRC that this was the basis on which ion engines worked.

This describes differences between positive and negative corona discharges: jsw