If a metal sphere is charged the self-repulsion of charges will result in all the charge residing on the surface of the sphere, which then behaves as a hollow spherical charge distribution with zero electric field within.In the van de Graaf generator, a rubber belt rubs against a felt pad and carries the charge created up to a metal dome. Since the dome is insulated from the earth, charge builds up on the dome and the potential difference between the earth and the dome increases until the dome discharges and a spark is seen.