a pendulum consisting of a heavy weight on the end of a long wire hanging from a fixed point, of the kind invented by Jean Foucault to show that the earth is rotating: the pattern of the pendulum's swinging appears to be rotating to an observer
See Foucault pendulum in American Heritage Dictionary 4
noun
A simple pendulum suspended from a long wire and set into motion along a meridian. The plane of motion appears to turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, demonstrating the axial rotation of the earth.