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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 .