To demonstrate an electrostatic rotator.
Van de Graaff generator with Franklin's wheel.
The Franklin's wheel is placed on the Van de Graaff dome or held by a person in contact with the Van de Graaff dome. When the Van de Graaff is turned on, the device rotates with the points in the direction opposite the rotation. Many people believe erroneously that this doodad works sort of like a rocket: the electrons leave the pointed ends, and the reaction force on the rotator causes it to rotate in the direction opposite that taken by the electrons. Calculation of the momentum of the electrons quickly dispels this theory. In reality, when the electrons leave the points they become attached to air molecules adjacent to the point. The electrostatic force between the negatively charged tips and the negatively charged air adjacent to the tips pushes the rotator.