Van de Graff Generator
In 1929 Robert J. Van de Graff designed an electrostatic machine which produces large
electrostatic potential difference of the order of the order of 107 V
The working of Van De Graff Generator is based on the principle of electrostatic
induction and action of points.
A hollow metallic sphere A is mounted on insulating pillars as shown in fig. A pulley B
is mounted at the centre of the sphere and another pulley C is mounted near the bottom. A belt
made of silk moves over the pulleys. The pulley C is driven continuously by an electric motor.
Two comb-shaped conductors D and E having number of needles, are mounted near the pulleys.
The comb D is maintained at a positive of the order of 104 is connected to the inner side of the
hollow metal sphere.
Because of the high electric field near the comb D, the air gets ionised due to action of points,
the negative charges in air move towards the needles and positive charges are repelled on towards the
belt, moves up and reaches near the comb E.
As a result of electrostatic induction, the comb E acquires negative charge and the sphere
acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere.
The high electric field at the comb E ionize the air. Hence, negative charges are repelled to the belt,
neutralises the positive charge on the belt passes over the pulley. Hence the descending belt will
be left uncharged.
Thus the machine , continuously transfer the positive charge to the sphere. As a result, the
potential of the sphere keeps increasing till it attains a limiting value(maximum). After this stage
on more charge can be placed on the sphere, it stars leaking to the surrounding due to ionisation
of the air
The leakage of charge from the sphere can be reduced by enclosing it in a gas filled steel
chamber at very high pressure
The high voltage produced in this generator can be used to accelerate positive ions ( protons,
deuterons) for the purpose of nuclear disintegration