I recently purchased a Van De Graaff generator from Sci-Supply (www.sci-supply.com) and am wondering if it is possible to determine which charges are postitive, and which are negative. The Van De Graaff ground wire in the bottom is connected to the ground wire of the electrical outlet. This would seem to indicate that the lower terminal is negative, and the main sphere/dome is positive. However, if the dome were giving off substantially negative charges, the ground would still work because it is essentially neutral. By the way, the Van De Graaff "discharger" is attached to the blue base on the bottom with a small terminal. This terminal is attached to the ground wire of the electrical supply.

Does anyone know how to quickly tell the difference between positive and negative electrostatic charges in a Van De Graaff generator?