What has not (yet)
been understood
Latest Change 2023-12-24
In brief:
Some aspects of the Foucault Pendulum which I have not(yet)
understood, for which I have not(yet) found a satisfying explanation,
for which different sources in the literature give different or sometimes
conflicting opinions.
Also, in my data processing I have room for improvement on the subject of fitting a dataset to an ellipse.
I present the problems in the form of study tasks.
The most important theoretical studies I've seen are:
Kamerlingh-Onnes, His thesis (1879, in
Dutch. I do not know about an English translation, there seems to exist a German version)
K.O. gives a thourough mathematical description of the behaviour of a point-mass in a rotating coördinate system.
Schumacher, see below.
The biggest problem with Foucault pendulums is the tendency to develop an elliptical path which disturbs the Foucault Precession (FP).
This tendency has nothing to do with the earth's rotation, it
would also happen on a stationary planet or on the eart's equator where
the FP is absent.
Many publications about ellipse suppression methods exist, but
few, if any, report convincing success. With the exception of Charron,
who's principle is almost universally used in permanently running
pendulums.
Study task:
Explain / derive why an elliptical path develops even in a pendulum on a stationary planet.
Explain / derive why the ellipse problem is more serious in short than in long pendulums.
Or does it only depend on the ratio of length and amplitude?.
Does the Q, the (inverse) amount of energy loss per period, has anything to do with the ellipse forming?
The answers might also be given in the form of a numerical simulation using e.g. finite element methods.
Schumacher
derives that when a repelling impulse is given at a certain moment in
the pendulum's period that the precession of the ellipse (not the ellipse
itself) will be perfectly suppressed.
Study task:
- Check the derivation of Schumacher, make a description of all
steps-in-between, such that a less matematically trained person also
can follow it.
- Check that the neglections which S. applies are justifyable and
acceptable.
- Predict what happens if the impulse is to early or to late, or to
strong or to weak, in terms of the FP regularity and total time.
- In my spreadsheet
is a section for the Schumacher calculation where manual iteration
should be done. Try to automate that iteration process, or better, make
it superfluous by making a direct derivation for the correct momernt.
Note: My experiments so far tend to confirm the Schumacher Criterium.
Obeying the condition I found that the Foucault Precession was reasonably
regular although there was a substantial ellipse which periodically
changes direction.
Alternating
ellipse:
Kamerlingh-Onnes has mathematically derived the periodical change in
ellipse direction, but my knowledge of math is insufficient to follow his
derivation.
Study task:
- Explain why the ellipse periodically changes direction using the simplest math.
- Explain why the changes occur when the pendulum swings East-West or North-South.
- Explain / derive if this also would happen on a non-rotating planet.
(solved) - Explain why the amplitude of the short axis varies more like a
squarewave than like a sinewave. That is, when it changes it does so
quite rapidly, within a few minutes. Is this real or is it an artefact from
the way the short amplitude is determined in my software?
Below is a screenshot of the analisys program.
Yellow staircase: time of the day in hours
Cyan: angle of pendulum swing plane, ranging from +180 to - 180 deg. 180 and 0 deg are E-W, +90 and -90 are N-S
White: amplitude of short axis.
Green: Peak amplitude in center detection coil.
The amplitude of the Center coil signal clearly shows maxima when the
ellipse changes direction, that is when the bob goes over the center of
the system, but no trace of the suddenness of the changes.
2024-01-24: This problem has been solved. The cause was not in the algorithm processing the data, but in the timing of the measured data.
Consider the following: Pendulum period T = 4.16 sec, pendulum amplitude
r = 230 mm. So the velocity near the center pass is ϖ.r = 0.347 m/sec.
The information from the Hall sensors is sampled at intervals of ca. 1.5 ms. This corrsponds to a fraction of a mm Bob movement.
The data are sent by the Arduino at a 10 Hz rate, not synchronised with
the center pass. So near the center we have information on a 3.47 cm
grid.
The info about a CenterPass is sent in the first message after the real pass, in the old situation together with the most recent position info
from the Hall sensors. This means when the message with the CenterPass
is sent, the Bob may be between 0 and 3.47 cm away from the center,
which gives a systematic error in the calculation of the short axis.
The improvement is a small change in the Arduino Firmware, we now freeze
the position info at the moment of the center pass, until the message
has been sent. The squarewave shape has disappeared now.
Fitting a dataset to an ellipse.
In my dataprocessing I use a rather rude method to derive the ellips
parameters (long axis, short axis, angle of long axis and direction) from a set of
co-ordinate pairs. See the description here.
There may be a beter way. Something like the least squares fitting of a straigth line
Study task:
Produce an algorithm, preferably written in C or Pascal that:
Given a array of co-ordinate pairs (x, y), laying on the perimeter of an ellipse with some small measurement errors,
calculates the long axis, the short axis, the angle of the long axis
w.r.t. the
horizontal and the direction in which the elliptical path goes.
The long axis of the ellipse can have any direction 0 ..
pi. Angles > pi are not distinguishable from angles < pi. The
short axis will never be as long as the long axis
(circle). The data in the array always start near the center,
immediately after a centerpass has been detected, and the data pairs are
in the order of the Bob's path.
Crane recommends
both attracting and repelling drive with the argument that it would
eliminate effects of certain asymmetries in the setup. However he
hardly explains which asymmetries these are.
Note that Schumacher
has some comment on this.
Study task:
- Give a mathematical derivation that the story about the center magnet is
correct/incorrect.
- Indicate what happens when the magnet is to weak/far off
or to strong/close by
-
Indicate/derive which asymetries can be suppressed by the combination
of attracting and repelling drive and indicate what happens if the
attracting and repelling impulses are not equal.
Permanent
magnet in the center:
Crane recommends a
repelling magnet in the center to eliminate the precession of the elipse,
Intuitively it feels that an attracting magnet in the center will reduce
the amount of ellipse. After all, when the Bob passes straight over it
will not have a net effect, because the effect of the atracting force
will be cancelled by that of the repelling force after the pass.
However, if there is an ellipse, there will always be an
attracting force towards the center.
The contra is: If it is so simple, why is it not done always?
Study task:
- Explain whether or not a permanent magnet in the center of the base
unit will reduce the ellipticity. The Bob does have a permanent magnet
in the lowest central position.
- Explain what will happen when the magnet is to strong or to weak or is not very precise in the center.
Szostak recommends
a repelling drive to counteract the forming of an ellipse. T.m.h.o. his
figure 3 indicates just the opposite (Or I do not understand it)
Study task:
- Explain that Sz is right / wrong with his argument in favour of
repelling drive.
Make the pendulum swing time independent of the amplitude.
It is known that the period of a pendulum depends slightly on the amplitude, even for very small amplitudes.
With my backgound as electronic engineer i am familiar with the figures
of Lissajous, a.o. an ellipse, if you plot two sine shaped signals of
the same frequency on an
X-Y oscilloscope. The shape and orientation of the ellipse depend on the
amplitude and
phase of the two signals. If they have exacly the same
frequency the ellipse will be stable. If the frequency of the signals differs
slightly the ellipse will change shape and orientation in a
regular way.
My feeling is that the precession of the Foucault Pendulum's ellipse is
connected to the difference in swingtime between the long and short
axis' of the ellipse. The Lissajous story tells me that the ellipse
should be stable when the swing times are exactly equal
Christian Huygens made the period of his pendulum clocks nearly
independent of the amplitude by clamping the top of the "wire" between
two cycloidally shaped brackets. The cycloïd is known to have this
property for a pendulum with a specific length.
A picture from Huygens. Upper right the cycloidal brackets.
This picture demonstrates that a pendulum with the wire locked between two cycloids has a period independent of the amplitude.
The length of the pendulum must be such that the bob touches the cycloid exactly in the middle.
The question is: If we make the swing time of our pendulum independent of the amplitude, will the ellipse precession disapear?
The Huiygens trick can be applied using a properly shaped tube around the pivoting point (kind of trumpet shape).
The contra is that the formula for the ellipse precession tells me that
the precession increases with a larger short-axis. This is contray to
the thought where the difference between the short- and long axis'
swingtimes is responsible for the precession. In that case we should see
less precession when the swingtimes are closer to each other.
Study task:
Pull me out of this swamp.
Pivotting point not well defined.
In my setup there is no well defined pivotting point; the pendulum wire
with a finite thickness is mounted in a minature type of drill clamp which is fixed to the
structure of my house. The swinging of the pendulum bends the wire
slightly making the exact pivotting point go slightly up and down during
the swing.
Also bending the wire produces some sideways force on the wire.
Study task:
Given the parameters of the pendulum make an estimate of the way the
effective pivotting point moves during the swing, and what the effects
might be.
The parameters are: L= 4202 mm, a= 230mm, Bob weight = 5.858 kg, wire thickness 1 mm round, elasticity modulus 210 N/mm².
The wire is operated in the range of forces where pure elastic stretching occurs.
Asymmetries in the system:
At many locations an asymmetric situation will exist. Maybe small but
certainly not zero. Some known or very likely abberations are:
- The magnet in the bob (or its field) does not sit perfectly in line
with the suspension wire and the center-of-mass. This can be seen when
the bob is rotating along its own axis. Such a rotation generally occurs
when te bob is launched by hand. We can then see that the center-pass
time varies slightly with a period of some tens of seconds. Eventually this
rotation dies out.
- The drive coil is not perfectly centered w.r.t. the centerpas
detection coil. The centerpass coil is rather well centered during the
center-adjust procedure, but the drive coil will have some deviation.
Also these coils are "wild" wound which may lead to a non-coaxial
magnetic field.
- The Rim coil might not be perfectly symmetric around the center- and drive coils.
- The plane in wich the coils lay might not be exacly horizontal.
- The bob has a rather strong magnet and will be influenced by the
earth's magnetic field. Also the magnet in the top mount experiences
this field.
- Some 30 cm below the coil plane are iron rods as reinforcement in the concrete floor of my living.
Study task:
Give estimations for the effect of these asymmetries.