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Van der Graaff designed this current generator in order to create very high potential differences and to accelerate charged particles
The Van der Graaff consists of a motor, connected through a belt with the device itself
Some metal wires lie very close to a belt, at the bottom of the structure
Other metal wires are placed very close to the belt at the top.
On top of the whole structure, a hollow conductor is placed, connected by a conductor to the top metal wires
Due to the friction, the upper pulley and the belt are loaded with the same charge but of different signs
The field is more intense in the case of the pulley because the surface in which the charge is distributed is much smaller
Therefore, if metal wires are placed close to the belt surface, a strong field is established between the wires and the surface
The air between them is ionized, becoming conductive and allowing passage of positive charges from the metal wires toward the pulley
The belt moves and the charges are transported downward
Since the wires are attached to the top hollow conductor, positive charges are being extracted from it, so that its surface is charged negatively
So if we put some little polystyrene balls, which are negatively charged, they are repelled by the can
If we touch the bottom wires, we can charge our finger with a positive charge. Then, we attract the little polystyrene balls
If we put a finger close to the conductor, due to the potential difference, a ray appears