Wherever you are in the
world this clock will
tell you the time, the latitude, the hemisphere, and will demonstrate
the Earth's rotation.
A Foucault pendulum is quite simple: a bob oscillates and the
rotation causes its oscillation plane to slowly turn with
reference to its environment. The plane rotates in 24 hours clockwise at the North pole and anti-clockwise at the South pole.
For a hypothetical observer looking from a star the oscillation plane stays fixed, it's the Earth which turns.
The pendulum's movement is sustained by an electromagnet that
the bob's movement, sending an impulse at the
appropriate time. The time reference is therefore given by the pendulum's period which only depends on its length.
(less than 1 meter) Foucault pendulums
are extremely rare because their design entails unsolvable problems.
pendulums are much easier to design.
The first issue is the suspension. If it is less than perfect the pendulum will come to oscillate in the "easiest" plane and will stay
there. This means that the steel wire must be free of internal tensions and must be a perfect cylinder. The wire attachment itself
must be perfect: your usual 4-pronged precision pin vise won't do the job unless polished to perfection.
The second issue is the elliptical precession. On long pendulums it is too small to be a factor in the design. On the other hand
for pendulums shorter than 2 meters it prevails over the Foucault effect itself. That's why it must be cancelled or at least
minimized. This is done by placing a Charron ring at one third of the pendulum's length starting from the top. The ring's inside
surface must be perfectly polished. A short pendulum simply cannot work without some device to minimize elliptical
All these issues compounded make a short Foucault pendulum an outstanding achievement. The time spent tooling and
assembling is less than 10 percent of the total time. All the rest is spent on tuning. And for each new adjustment you'll have to
wait at least 16 hours to see any change...
This pendulum has a 1-second beat which I use to drive the
which in turn drives the minutes and the hours hands.
Using a Foucault pendulum as a clock introduces additional constraints: 1) the period must be finely adjustable; 2) the
pendulum's length must be temperature-compensated; 3) the amplitude must be constant to ensure isochronism.
My first Foucault pendulum (Pestoline) used
a clever but perfectible system: the pendulum's wire was sliding inside
a hole and
its length could be adjusted above the hole, the same way you tune a guitar string. However adjusting the wire length would
also change the distance between the bob and the electromagnet and therefore the magnetic force on the bob. Under these
conditions fine adjustment of the pendulum's period became a real challenge. For this model the wire does not move, it's the
hole that slides up or down for adjustment. With this system you can even fine tune the pendulum's period while it oscillates,
without any impact on its course, which is not possible on classical clocks. Temperature compensation is done by a bimetallic
device at the top of the suspension. The problems raised by the tooling of the Charron ring were solved by using a jewel: the
inside circle is perfect and wear is nil.
Why do we have two different dials? The one on the left shows
as given by the pendulum, the other one is a quartz
clock which is used as a reference. What we have here is a clock that finally suits everybody's taste: time-obsessed people will
always look at the right, more accurate, dial; poets will enjoy the left one for isn't it true that what we treasure most in people as
in objects is their quirks? And scientific people will carefully consider the average of the two indications. If, for example, the
seconds hand of the pendulum is late it will catch up ¼ of a revolution later. This is due to the Charron ring eccentricity and
these fluctuations average themselves out. That's why you do not adjust this clock in the usual manner: if an eccentricity in the
ring causes the pendulum to accelerate it will then slow down by the same amount ¼ of a revolution later. What must be
adjusted then is the average period over a half-revolution. The length of the half-revolution depends on the latitude of the
location. In my workshop it is 16 hours 34 minutes and 14 seconds.
First tests show an offset of less than one second per day, easily corrected by adjusting the pendulum's length. If you want to check the test data click here (readable only with the MicroSet sofware from Bryan Mumford).
A short Foucault pendulum that operates correctly is just
The pendulum installation must be done once and for all.
It will not be possible to move it around and have it operate in different places because the adjustments and levelling are very
critical and must not be tampered with once the pendulum is operating correctly. When everything goes well the adjustments
can take up to 2 weeks at set-up time.
Please email or call me if you want to order. The minimum
delay is three months. Please specify the latitude at which the
is to be used, to allow customisation of the hour ring. A full custom
based on your own specifications is also possible (glass protection,
base or accessories, different design, different materials).
English translation by Jean-Marc Julia