A Van De Graff generator is an electrical device that produces high voltage due to build up of static electricity. It looks like 2 Aluminum balls on a pedestal. Inside there is a belt which can be made out of any non conducting element. The amount of charge created depends on what materials are involved and how much of them are connected. The problem is that you cannot use any material that doesn’t conduct because; like in the case of lightning any material can conduct electricity if there is a high enough voltage (in the case of lightning gases in the air that normally are good insulators conduct electricity because they are stripped of their electrons and become positively ionized). The Van De Graff generator is useful because it is an easy way of producing static electricity and helps scientists perform experiments with static electricity.  

When the machine is turned on the lower roller starts spinning the element on the belt at a very high speed. Since the belt is made of rubber and the lower roller is covered in silicon tape, the lower roller begins to build a negative charge and the belt builds a positive charge. As the process of charging continues the electrons in the metal dome are stripped by the ball to make it neutral again therefore making the dome positively charged. As this goes on it is theoretically possible for the dome to continue charging forever but the air’s impurities make it difficult.