The Foucault Pendulum.
What is it?
A pendulum that is free swinging, in any direction i.e. unlike a clock pendulum that is normally fixed to swing in line with the case.
What does it do?
It just swings!.....However, as it swings, because its swing axis is not constrained, the Earth rotates underneath it! To an observer (Us!), it appears that the axis of the swing is rotating with respect to the ground. Clearly; if it is on the pole; this takes 24 hours, at the equator, it takes forever! At the latitude of Liverpool University, it takes a little over 27 hours.
What is so special about it?
Well....it looks good! Also, it demonstrates a scientific fact...the rotation of the earth. Unfortunately, the freedom of swing brings with it a problem......how to keep it going. Normal mechanisms are useless, because the rotational forces are very weak. Any restriction will prevent the axis of the swing moving. so, a system of replacing the lost energy has to be found, that does not have any effect on the position of the suspension cable.
Well.....how is it done?
A magnet! More specifically, an electromagnet. A special circular magnet was made, with a circular slot, running horizontaly around the cable. The cable had a special 'former' slid over it, which has a disc, that fits neatly into the slot. (see pictures below). When the magnet is energised, it pulls the disc into the slot, and thus pulls the cable.
A system of detection and timing was designed, that 'switched on' the magnet, a short time after the cable has passed through the centre point. Thus the 'pull' will cause the disc to move to the nearest point of the magnet, which is a radius. So, there is no 'sideways' pull, to affect the rotation of the swing.
The detection of the centre of the swing is by means of two beams of infra red light, crossing at the centre. The control system detects the cable passing through the beams, to 'mark' the centre point.
View of the 'head' of the structure, during commissioning, showing the slotted magnet, and the 'control box'. Wires to the sensors can be seen running down to the sensor tubes under the magnet core. The suspension cable can just be seen as it runs at an angle down through the magnet
View of the swinging 'bob' from the floor above.
The swing takes 5 seconds to complete; the highly polished sphere, with brass finials, and the ornate compass rose, are lit up to good effect.
The pendulum passes through two floors of the building, in the main staircase.
Two views of the pendulum in operation are available on "Youtube"....links below...