If you suspend an iron bob in a wire and start swinging it after a while you will see that the bob swings in another direction than you started it. The plane of swinging has shifted a bit counter clockwise. How can this be?
Actually, it is not the Foucault pendulum that has changed its direction. It will swing all the time in the same direction relative to distant stars. Instead, it is the building that is shifting underneath the Foucault Pendulum! And as the pendulum is attached to the building it will rotate with Earth.
Suppose the pendulum is suspended direct over the North Pole and that you start swinging it in some direction. The pendulum then moves there and back in the same direction as you started it. There are no forces that can make the plane of swinging alter its direction. The direction is all the time the same.
But since Earth moves underneath the pendulum it looks as if the plane of the swinging of the pendulum is changing.
But how did Foucault get his idea to make this experiment? Well, he had noticed the following:
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put a long metal rod in a lathe without starting the lathe - then set the rod in motion up and down. The plane of swinging of the rod is now vertical (the lathe is still not in motion)
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now start the lathe. You can notice that the plane of swinging is not changing, though the attachment to the lathe is moving!. No matter how fast the lathe is rotating - the plane of swinging does not care if its attachment rotates or not.
Foucault realized that this was on comparison with the pendulum and Earth rotation.
This experiment was performed by Foucault for the first time in 1851 in the Meridian room in Pantheon in Paris. He had got instructions from the president of France at this time, Louis-Napoleon Bonaparte, to make an imposing experiment to be shown at the Paris Exhibition. Therefore he created a pendulum with a length of 67 metres and with a cannon ball weighing 28 kg.
The experiment was a formidable success. Day by day Foucault was standing making his experiment in Pantheon explaining the function for the spectators. The news of his experiment was spread rapidly over the world, pendulums were built around the world and a veritable pendulum mania was created.
Today you can see Foucault pendulums for example in museums and universities around the world, the UN-house in New York has one, donated by the Netherlands in 1955.
Nowadays it is common to incorporate an electromagnetic to keep the bob swinging.