PURPOSE: To demonstrate how lightning rods really work.

DESCRIPTION: A ground sphere is positioned adjacent to the Van de Graaff generator so that the ground sphere is charged by induction and creates large sparks. While this system is working, a grounded point is aimed at the Van de Graaff dome from a distance of several times the distance between the dome and the ground sphere. The grounded point discharges the dome at a much lower potential, preventing buildup of charge on the ground sphere and the concomitant spark discharge.

An apparently controversial theory of how a lightning rod works uses this experiment as a model. Being a point, the lightning rod aimed upward into the sky discharges the clouds in the area above it before their potential builds up to the level at which a large lightning stroke could occur.


REFERENCES: (PIRA 5B30.30) See Demonstration Reference File for further information about the theory of lightning and lightning rods.

EQUIPMENT: Van de Graaff generator with ground sphere, ground wire with point.


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