Van de Graff generator is a device used for building up high potential differences of the order of a few million volts.
Such high potential differences are used to accelerate charged particles such as electrons, protons, ions, etc.
Principle − It is based on the principle that charge given to a hollow conductor is transferred to outer surface and is distributed uniformly over it.
It consists of a large spherical conducting shell (S) supported over the insulating pillars. A long narrow belt of insulating material is wound around two pulleys P1 and P2. B1 and B2 are two sharply pointed metal combs. B1 is called the spray comb and B2 is called the collecting comb.
Working − The spray comb is given a positive potential by high tension source. The positive charge gets sprayed on the belt.
As the belt moves and reaches the sphere, a negative charge is induced on the sharp ends of collecting comb B2 and an equal positive charge is induced on the farther end of B2.
This positive charge shifts immediately to the outer surface of S. Due to discharging action of sharp points of B2, the positive charge on the belt is neutralized. The uncharged belt returns down and collects the positive charge from B1, which in turn is collected by B2. This is repeated. Thus, the positive charge on S goes on accumulating. In this way, voltage differences of as much as 6 or 8 million volts (with respect to the ground) can be built up.