This short Foucault pendulum has its elliptical distortion
by another of Foucault's discoveries, the magnetic brake
based on eddy currents. A magnet moving over a conducting, non-magnetic metal creates a so-called eddy current which in
turns tends to slow down the magnet. On this pendulum it's the brass ring under the bob which acts as a brake. The rings also
controls the amplitude of the swing, preventing the bob to bump against the glass tube.
English translation by Jean-Marc