JF Bell, The late-twentieth century resolution of a mid-nineteenth century
dilemme generated by the eighteenth-century experiments of Ernst Chladni on
the dynamics of rods, Arch Hist Exact Sciences 43 (1991) 251-73,
which makes a passing remark on Foucault in note 11, apropos the
observation that "History abounds with examples of unwarrented rejection
of valid experiments":
"Foucault presented the result of his experiment to the French Academy on
February 3, 1851 and demonstrated the experiment to the general public in
the Pantheon in May 1851. Foulcaut's experiment which demonstrated the
rotation of the earth aroused his contempories to publish over 60 papers
that same year. There were debates among theorists who adopted opposing
analytic approaches. There was discord among the experimentists, some of
whom, not appreciating the demanding requirements of Foucault's experiment
on the pendulum, obtained conflicting results. Thus was generated a heated
controversy that dominated the remaining 17 years of Foucault's life of
only 47 years. Before publishing his first data on the pendudlum, Foucault
had determined with great precision the influence of errors introduced by
the use of wires that were elastically anisotropic, imperfectly circular,
or improperly mounted at their point of attachement. He had painstakingly
ascertained the proper conditions by the ingenious use of an experiment ...
"
He gives a reference to
P Acloque, Oscillations et Stabilite selon Foucault, Paris: CNRS, 1981, and
his review of it Isis 73 (1982) 268.
There is a Foucault pendulum at the London Science Museum, which they
restart every so often. (Every hour or two? It is a long time since I have
been there.) A friend asked why they didn't let it run longer, and they
said something like that the disturbance caused by the public caused it to
loose accuracy after a longer period. Most people know something, but not
too much, about the pendulum; see, for example, Umberto Ecco's novel with
that title, which contains two or three egregious errors in its first
pages. You can illustrate this lack of appreciation of the subleties of the
experiment easily: take a piece of string and a weight, a bunch of keys,
for example, and make as careful a setup as you want. With a very little
practise, you can make it (and therefore the earth?) precess in either
direction, for no clearly visible reason!
David Fowler
At 3:59 pm +1000 27/1/99, Michael Deakin wrote:
>John Dawson wrote:
>
>>
>> Although that is rather an old-fashioned science museum, it is one of the
>> few places I've been that has a Foucault pendulum marked to show where the
>> bob will be at a given hour -- the point being that it does *not* go
>> through a full circle in 24 hours, as many suppose. (It would only do so at
>> the poles.) I confess that I myself was unaware of the latitude dependence
>> until I visited the CNAM. The precise equation involved is a simple example
>> of a natural context in which the cosecant function arises.
>>
>
>Some years ago, a sequence of Foucault pendulums was built here at
>Monash by the late Carl Moppert of this department and (now Emeritus)
>Professor Bill Bonwick of Electrical Engeineering. The latest of
>this series still functions in the building which houses my office
>and it takes up the whole of an otherwise unused liftwell.
>
>Bonwick devised a unique drive that can only accelerate the pendulum
>(and by just enough) in the direction of its motion, thius avoiding
>the problem of "running down".
>
>A more serious problem with all Foucault pendulums is that of
>"ellipsing". For a pendulum to swing in a plane is an unstable mode
>of oscillation. The full solution is illustrated on the cover of the
>Dover edition of Routh's "Advanced dynamics of Rigid Bodies" and it
>consists of an elliptical motion with the ends of the ellipse
>rotating at a steady rate. This "ellipsing" must be suppressed as it
>is a much larger effect than the Foucault effect, which is hard to
>deteect if ellipsing is taking place.
>
>Most Foucault pendulums use a device known as a Charron ring to this
>end, but the theory of this is not entirely agreed and the results
>not wonderfully good. In the American Journal of Physics of (some
>10?) years ago there is a lengthy discussion on the matter.
>
>Moppert & Bonwick did not use a Charron ring, but opted for a sponge
>rubber sleeve at the maximum amplitude of the swing. Later this was
>replaced by further electrical controls.
>
>The results are still in considerable error, but are the best ever
>achieved. Moppert conducted an extensive correspondence with the
>curators of all known Foucault pendulums at the time and many
>curators quite openly admitted to "cheating", by advancing or
>retarding the pendulum in the hours that the public had no access.
>
>A smaller version of the Moppert-Bonwick pendulum hangs in the McCoy
>(Geology) Building at the University of Melbourne. Monash has
>another Foucault Pendulum on display in its Physics department, but
>this is a smaller and conventional affair with a Charron ring.
>
>For more on the theory, see Moppert's article in Q J R Ast Soc 21
>(1980), pp 108-118.
>
>Mike Deakin