The Van de Graaff generator is an electrostatic generator that can reach 5 megavolts. It uses a belt, and two pulleys for the most part. When the belt gets to the lower pulley, it creates a negative charge, and when the belt rolls up to the top pulley, some electrons from the bottom transfer to the top. As it rolls back around, it changes to a positive charge. So one side is positive, and the other is negative.
The sphere at the top shields the upper pulley. The sphere keeps accumulating a negative charge until the rate that the charge that's being lost equals the charging current. The outside of the sphere has a positive charge. When the pie plates are placed on top of the sphere, they get a negative charge, so it repels and flies off the top.
