Vande graff generator:
In a Van de Graff generator a hollow metallic sphere is mounted on an insulating stand. The pulley at the center of the sphere is connected to the pulley at the bottom of the machine by an insulating belt of silk.
This pulley is continuously driven by a motor. Two comb shaped conductors are mounted near the pulleys. Both the combs touch the belt. The lower comb is maintained at a positive higher potential of the order of 10 4 volts by a power supply. The upper comb is connected to the metallic sphere.
The high electric field near the lower comb ionizes the air and makes it conducting. The negative charges in the air move towards the comb and positive charges stick to the belt. As the belt moves the positive charge is carried upwards. The charge on the belt near the upper comb again ionizes the air. Thus the positive charge on the belt is now transferred to the metallic sphere. (This is the working of the metallic comb)
The machine continuously transfers positive charge from the power supply to the metallic sphere. The potential of the sphere keeps on increasing. If the electric field just outside the sphere is 3 Х 10 6 Volt per meter, dielectric break down of air takes place.
The main limiting factor of this high potential is the radius of the sphere. The potential can be increased further by enclosing the sphere in a highly evacuated chamber.