Purpose:Aid in describing the Cavendish experiment.
Description:The black (steel) balls are temporarily rigidly fixed in space. A steel barbell is mounted to a stiff wire on the axis of the barbell with a small mirror attached to the wire just above the barbell. Light reflects off the mirror, indicating the oscillations of the barbell. This models how, in the full experiment, the equilibrium position for oscillations of the barbell would show the displacement corresponding to the gravitational force between the two pairs of balls, and is observed by noting the movement of the light reflected off the mirror.