<P> The voltage produced by an open - air Van de Graaff machine is limited by arcing and corona discharge to about 5 megavolts . Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials of as much as about 25 megavolts . </P> <P> A simple Van de Graaff generator consists of a belt of rubber (or a similar flexible dielectric material) moving over two rollers of differing material, one of which is surrounded by a hollow metal sphere . Two electrodes, (2) and (7), in the form of comb - shaped rows of sharp metal points, are positioned near the bottom of the lower roller and inside the sphere, over the upper roller . Comb (2) is connected to the sphere, and comb (7) to ground . The method of charging is based on the triboelectric effect, such that simple contact of dissimilar materials causes the transfer of some electrons from one material to the other . For example (see the diagram), the rubber of the belt will become negatively charged while the acrylic glass of the upper roller will become positively charged . The belt carries away negative charge on its inner surface while the upper roller accumulates positive charge . Next, the strong electric field surrounding the positive upper roller (3) induces a very high electric field near the points of the nearby comb (2). At the points, the field becomes strong enough to ionize air molecules, and the electrons are attracted to the outside of the belt while positive ions go to the comb . At the comb (2) they are neutralized by electrons that were on the comb, thus leaving the comb and the attached outer shell (1) with fewer net electrons . By the principle illustrated in the Faraday ice pail experiment, i.e. by Gauss's law, the excess positive charge is accumulated on the outer surface of the outer shell (1), leaving no field inside the shell . Electrostatic induction by this method continues, building up very large amounts of charge on the shell . </P> <P> In the example, the lower roller (6) is metal, which picks negative charge off the inner surface of the belt . The lower comb (7) develops a high electric field at its points that also becomes large enough to ionize air molecules . In this case the electrons are attracted to the comb and positive air ions neutralize negative charge on the outer surface of the belt, or become attached to the belt . The exact balance of charges on the up - going versus down - going sides of the belt will depend on the combination of the materials used . In the example, the upward - moving belt must be more positive than the downward - moving belt . As the belt continues to move, a constant "charging current" travels via the belt, and the sphere continues to accumulate positive charge until the rate that charge is being lost (through leakage and corona discharges) equals the charging current . The larger the sphere and the farther it is from ground, the higher will be its peak potential . In the example, the wand with metal sphere (8) is connected to ground, as is the lower comb (7); electrons are drawn up from ground due to the attraction by the positive sphere, and when the electric field is great enough (see below) the air breaks in the form of an electrical discharge spark (9). Since the material of the belt and rollers can be selected, the accumulated charge on the hollow metal sphere can either be made positive (electron deficient) or negative (excess electrons). </P> <P> The friction type of generator described above is easier to build for science fair or homemade projects, since it does not require a high - voltage source . Greater potentials can be obtained with alternative designs (not discussed here) for which high voltage sources are used at the upper and / or lower positions of the belt to transfer charge more efficiently onto and off the belt . </P>

Why such kind of charge is produced at the bottom of the van de graff