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A large spherical conducting shell( of few metre radius) is supported at a height several meters above the ground on an insulating column.
A long narrow endless belt insulating material, like rubber or silk, is wound around two pulleys – one at ground level, one at the center of the shell.
This belt is kept continuously moving by a motor driving the lower pulley.
It continuously carries positive charge, sprayed on to it by a brush at ground level, to the top.
There it transfers its positive charge to another conducting brush connected to the large shell.
Thus positive charge is transferred to the shell, where it spreads out uniformly on the outer surface and a voltage difference of as much as 6 or 8 million volts( with respect to ground) can be built up.