An instrument which demonstrates a rotation of the Earth.
It is named after a French physicist J. B. L. Foucault who was the first to make that experiment in 1851. He took into consideration a fact known in physics that that a plane of the pendulum swings does not change in space. Than if the Earth did not move, the pendulum would move in a plane which would not change its position in relation to the surface of the Earth.
The experiment stated that a plane of the swings was slowly rotating from the east to west in relation to the surface of the Earth, consequently our planet must rotate.

The pendulum loosely suspended over the earth pole continously changes a plane of the swings, showing in this way that the Earth revolves. In lower latitudes a change of the direction of the swings comes slower, on the very equator, however, it does not occur at all.

If Foucault pendulum was placed on the pole, the plane of its swings would complete a full rotation in 24h (23h 56m) i.e. in time that the Earth needs to make a full rotation on its axis. The time T of the full rotation of the plane of the pendulum swings at the latitude q can be calculated after a formula:
T = 24h/sin q
Thus it is evident that if the pendulum was placed not on the pole but somwhere in middle latitudes, the time needed to complete a full rotation of the pendulum swings would be longer. However, on the equator we would not observe a rotation  of the plane swings in relation to the Earth.
In Frombork the time of the full rotation of the plane of the pendulum swings in Foucault’s experiment is about 29h 5m.
The lenght of the steel rope is 28 m, the ball weighs 46,5 kg.

The experiment, made in 1852 by Foucault at Pantheon, in Paris.
Contemporary picture.