 Most modern industrial machines are enclosed in a pressurized tank of insulating gas; these can achieve potentials of as much as about 25 megavolts.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.