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FunctionContact between two different substances facilitates an electrostatic charge. Because the surfaces are usually rough, there is less contact, and therefore less charge transfer, than if the surfaces were completely smooth. Rubbing the two substances together facilitates a greater buildup of charge. Nonconductors are better at holding the charge.
The glass disk is turned with the crank and rubs against the sheepskin pieces at the top and the bottom of the bearing. The charge builds up on the glass. The U-shaped combs straddling the glass on either side collect the charge, which is conducted to the two tubes. The glass rods at the bottom serve as insulators to keep too much charge from "leaking" to the ground.
Related WorksGerard L'Estrange Turner, Nineteenth Century Scientific Instruments (London: Sotheby's Publications; Berkeley: University of California Press, 1983), 189.