Foucault pendulum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Foucault's pendulum

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun pendulum with a long wire; can swing in any direction; the change in the swing plane demonstrates the earth's rotation