Note:  Do not rely on this information. It is very old.

Foucault Pendulum

Foucault Pendulum, an instrument designed to demonstrate the rotation of the earth about its axis. A long fine wire supports a massive metal sphere or pendulum-bob, which is set swinging in any one vertical plane. It tends to stay in that plane, and the rotation of the earth will, by leaving it behind, cause the pendulum to appear to rotate its plane of swinging. The wire should be at least twenty feet long; the first swing should be produced by pulling the bob away from the vertical with a piece of thread, and then burning the thread; and the bob should be heavy enough to carry the motion on for an hour or more. These precautions being taken, the apparent change of plane of oscillation will be rendered evident.