Leon
Foucault pendulum indicating the time.
This pendulum has its oscillation period set to one second. This phenomenon is used to turn the seconds hand, which drives the minutes and hours hands. The use of a Foucault pendulum as a clock nevertheless imposes a few constraints
:1) to set the period precisely,
2) to compensate for variations in the length of the pendulum due to temperature differences,
3) to stabilise the amplitude in order to respect the isochronism.
The first Foucault pendulum/clock I created (Pestoline) used a perfectible system: the pendulum wire slid into a hole and could be adjusted upstream, in the same way as a guitar string is tuned. But shortening or lengthening it also changed the position of the magnet in relation to the coil, thus varying the strength of the impulse. Finally, fine-tuning the duration of the amplitude was a miracle. In this model, the wire does not move: it is the hole that can be raised or lowered. This system even allows the period of the balance to be adjusted while it is moving without disturbing its course, an option that is never found on conventional clocks. Temperature compensation is provided by a bimetal located above the assembly. The problems inherent to the machining of the Charron ring were solved by the use of a ruby: the circle is precise and there is no wear.
But why on earth two separate dials? Because the one on the left indicates the time given by the pendulum, and the one on the right is only a quartz clock serving as a reference. This finally gives a clock that can satisfy everyone: people in a hurry will always read the more accurate right-hand dial. Poets will enjoy the one on the left and scientists will read the average of the two. If, for example, the second hand of a pendulum is delayed for a moment, it will then catch up a quarter of a revolution later. This is due to the exentration of the Charron ring, but the average of these fluctuations becomes accurate over one revolution. This pendulum is therefore not set like a normal clock because any eccentricity of the Charron ring causes an acceleration of the passage of the pendulum which will invariably be followed by a slowing down of the same amplitude 1/4 revolution later. One must therefore set the average time of half a revolution, varying according to the latitude where the clock is located: 16 hours 34 minutes and 14 seconds in my workshop in Sion.

This pendulum is an enigma to me because it has worked very well in all the places I have exhibited it, and this without any adjustment other than the level. A mystery.
Time-lapse of the rotations of 5 Foucault pendulums during a few days. The Léon pendulum is in the centre.
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