Foucault pendulum in Motala, Sweden

Funktion of a Foucault pendulum  

The completed pendulum

Foucault pendulum as a watch?

History of the Foucault pendulum-short version

Links to other pendulums


Construction of our pendulum in pictures

Construction of a prototype

Why build a pendulum?


A Foucault pendulum is a classical, beautiful experiment in physics, showing that Earth rotates around  its axis. When the experiment was performed for the first time by Léon Foucault in 1851 in Paris it was the first ocular proof for the rotation of Earth where one with own eyes could  see that earth really rotates around its axis.   Of course this was known at this time but an obvious proof was still missing.

The pendulum in Motala is made up of a  steel bob suspended in a wire (See the picture to the right). The bob must be suspended in a perfectly symmetric way, able to swing freely in all directions if the experiment is going be successful.   Once the pendulum is set in movement, after a while you can notice that the plane of the pendulum´s swing has changed direction clockwise. The plane of swing keep changing its direction and is making a full circle in 32 hours in Motala. Thus the velocity of the rotation is about 13 degrees an hour. 

But this is an optical illusion. In reality it is not the plane of the pendulum´s swing that changes - the pendulum swings in the same direction all the time relative to distant masses of universe*). Instead it is Earth that moves under the pendulum. But for a spectator on Earth it looks like the plane of the pendulum´s swing is changing.

Use the link above "Function of a Foucault pendulum" to see an animation of the function. 

In 2007 Carlsund utbildningscentrum celebrated  their 10-year jubilee. The upper secondary school  then on 11 May could  inaugurate a Foucault pendulum that had been made on the school without external sponsors. To buy a complete pendulum had been far to expensive ( if possible  to find a seller). The pendulum has an electro-magnetic drive that gives the bob a little kick every time it has passed a turning-point. In this way the pendulum keeps swinging all the time to compensate for the air resistance.

*) Strictly, this is true only at the North and South Pole. At other latitudes the motion is more complicated.




About every twelve minute a new peg is

falling ....


The Foucault pendulum is hanging  by the entrance of Carlsund.



Tecnical data

Length of the pendulum

6 metres

Diameter of the wire

1,5 mm

Mass of the bob

ca 13 kg

Diameter of the bob

15 cm

Material of the bob

steel, with 3 μm gold

Diameter of the ground plate

120 cm

Number of windings of the coil

About 1300

The coil´s exterior diameter

220 mm

Length of copper wire

ca 600 metres


ca 40 cm


4,9 seconds

Magnetic field in the centre of the coil by  0,7 A

19 mT

In Motala, situated in latitude 58° 33´ north, the time for the plane of oscillation to rotate 360° is about 28 hours. The plane rotates  307° every 24 hours. The velocity of the rotation is given by 15° sin λ, where λ is the latitude.


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