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Van de Graaff generator

A Van de Graaff generator is a machine which uses a moving belt to accumulate very high charges on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can be up to 5 megavolts; applications for these exist with high voltage X-ray tubes and atom splitting experiments.

A simple Van de Graaff generator consists of a belt of silk running over two pulleys, one of which is surrounded by a hollow metal sphere. Two electrodes, E1 and E2, in the form of sharply pointed cones are positioned respectively near to the bottom of the pulley and inside the sphere. E2 is connected to the sphere, and E1 is made 10,000 volts positive with respect to earth. This high voltage ionises the air at that point, repelling positive charges onto the belt and they are carried up inside the sphere. This positive charge induces a negative charge to the electrode E2 and a positive charge to the sphere (to which E2 is connected). The high potential difference ionises the air inside the sphere and negative charges are repelled on to the belt, discharging it. As the belt continues to move round the sphere charges further positive, until the rate of leakage equals the rate at which charge is induced.

The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de Graaff. A common misspelling of the name is Van der Graaf (with an R and a single F). See also Van der Graaf Generator (band).

A development of the Van de Graaff generator is the tandem accelerator in which heavy negatively charged ions are accelerated through one potential difference before being stripped of more electrons and accelerated again.



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