Principle of operation
Some insulating materials when separated from the surface of others, leave those surfaces electrically charged. In the Van de Graaff Kit the belt is made from rubber and the rollers are made from Nylon (top) and Silicon(Bottom)
The rubber belt runs continuously over these two rollers and this produces a charge when the belt and rollers separate. As the two rollers are made from different materials the top roller produces a net positive charge, and the bottom roller produces a net negative charge.
The Charges are picked up from the moving belt by “combs” situated above and below to the rollers. Actual contact between the combs and the belt is not essential because of the high potential differences. The Combs are simply athe sharp edge of the aluminium foil.
The lower comb is at earth potential and is a drain for negative charge, leaving the belt with positive charges that are carried up to the top comb.
The top comb is connected to a collecting sphere which, will collect and store the charge on its outer surface.
So long as the belt continues to move, the process continues, the drive motor supplying the power to overcome the electrical repulsion between the charges collected on the sphere and those arriving on the belt.