Foucault pendulum

The Foucault pendulum, or Foucault's pendulum (named after the French physicist Léon Foucault) was pendulum created as an experiment to show the rotation of the Earth. 
The experiment.
On January 3, 1851, Foucault did the following experiment: He mounted a 2 metre long pendulum, and let it swing, in the basement of his house. On the floor, he marked the axis, along which the pendulum was swinging. After some time, he noticed, that the pendulum no longer swung along the axis he had marked on the floor, but on an different axis. When he tried to explain this, he considered, what could have caused the axis to change. He considered gravity. Gravity could not have caused the axis to change, as it only acts vertically. There were no other forces acting on the pendulum. So: according to Newton's laws, the pendulum did not change the axis it was swinging in. So it was the floor (or the earth) that changed this axis, because it rotated. 
Vincenzo Viviani (1662-1703) had made similar observations, but he didn't attribute them to the rotation of the earth. 
On the 3rd of February 1851, he repeated the experiment, with a 12m pendulum at the Paris observatory. On 26th of March he presented the experiment to the public: At the Panthéon, he used a 67m long pendulum, with a weight of 28kg. At the bottom of the pendulum, there was a tip that left a trace in a sandbox. This was an easy way to show the earth's rotation to common people. The metal ball of this pendulum still exists: Until 1946, it was stored at the Conservatoire national des arts et métiers, then it was returned to the Panthéon.
Caspar Garthe repeated the experiment at Cologne Cathedral, Friedrich Magnus Schwerd did in Speyer Cathedral. Their results were deceptive, and not nearly as good as those obtained in Paris. In 1879, Heike Kamerlingh Onnes wrote his thesis on the subject, doing extra calculations on the exact magnitude of the effect. In his thesis, he also pointing out the problems the people had encountered in Cologne and Speyer.
An exact copy of the original has been operating in the Panthéon since 1995. It takes 31h 50min to do a full rotation (of 360 degrees). 
Because a Foucault pendulum is an easy way to show the rotation of the earth, such devices can be found in many science museums around the world today.