Theory,
Spreadsheet and
References
Latest change 2024-04-08
In
brief:
What I found on the internet and a few formulas I derived myself.
References to the spreadsheet I developed during my search.
Download the spreadsheet
in Zip format. It can be opened in Excel,
OpenOffice, LibreOffice, etc.
The fields wit formulas are protected against accidental change.
The
password to unprotect them is "Protect".
The lines with green text give some
differences or ratios between
results which should differ only slightly. If you see large
numbers
there then most likely there is something wrong with the formulas.
Subjects for further study
Contents:
Terminology
In the order of the spreadsheet:
Constants
Pendulum Swing Times
Foucault Precession
Intrinsic
Precession
Quality factor of the
pendulum
Drive
timing according to the
Schumacher equation
Sensitivity
of the Floor Unit adjustment
Energy in the system
Forces on the cable
Forces on the
mounting
point
Thermal
expansion of
the cable
Passage
times for a certain distance
Height of
tip
of bob at some distance from the center
Height
from
top for a given excursion of the cable
Dimensions
and weight of a cylindrical Bob
Dimensions
and parameters of the Coils
References
Articles and
sites
about theory and practice of several pendulums
Terminology.
In
the literature about Foucault Pendulums often the following naming
is
used for the several parameters. I'll stick to this terminology as
good
as possible.
g for the
accelleration
of
gravity on earth [9.81 m/s2] (can locally differ up to
1%,
also dependent on height)
φ (fi) for the northern or
southern latitude of the location of the pendulum. Southern
latitude is
sometimes given as negative.[degrees or radians]
L for the length of
the
pendulum, from the deflection point at the top to the center of
mass of
the bob. [m]
M for the mass of
the
bob.[kg] The mass of the cable is mostly neglected.
T for the period of
the
pendulum. [sec]
ω (omega) for the
actual
frequency of the pendulum. [rad/sec]
ω0 (omega-null) for the frequency at very
small
amplitude [rad/sec]
a for the amplitude
in
the
desired direction, the longest axis of the ellipse. [m]
b for the amplitude
of
the
shortest axis of the ellipse. [m]
ΩF
(Omega-F) for the Foucault precession. [rad/sec]
ΩI
(Omega-I)
for the intrinsic precession due to the elliptical path.
[rad/sec]
ΩA (Omega-A)
for the
siderical rotation of the earth.[rad/sec] 1 siderical
rotation
is approximately 23:56:4.14 [hh:mm:ss.ss]
Θ (Theta) for the
angle
at maximal excursion.[rad]
Q for the quality
factor
of the
pendulum [dimensionless]
E the energy in the
system
systeem
[Joule] (alternates beteween potential energy and kinetic energy)
In the order of the spreadsheet
Constants
The gravitational constant can be slightly different at your
location.
For my location (vicinity of Arnhem, NL) I found 9.8123. source: https://upload.wikimedia.org/wikipedia/commons/c/ce/Valversnelling_in_Nederland.svg
The siderical day is defined as the rotation of the earth with
respect
to the far "fixed" stars. It is slightly shorter than the well
known
24-hours day, because that one is related to the earth's rotation
w.r.t.
the sun. The earth also rotates around the sun in one year and so
the
difference is about 1/365 of a day.
Pendulum
swing
times
The exact period of a pendulum is hard to calculate because it
also
depends on the amplitude. Most formulas are approximations..
The most well known is:
T = 2 π √ (L/g)
[sec]
(1)
The theoretically exact solution with a lot of terms having
a
regular
pattern:
(2)
T = 2 π √ (L/g) * [ 1+ (1/2)2 sin2(Θ0/2) + ((1*3) / (2*4))2 sin4(Θ0/2 ) + ((1*3*5) /
(2*4*6))2sin6(Θ0/2 ) + ((1*3*5*7) / (2*4*6*8))2 sin8(Θ0/2 ) + ...] [sec]
where Θ0 =
arcsin
(a/L)
Below is a piece of FreePascal code to calculate this formula.
Supposedly exact but converges with fewer terms which
follow a
(for
me) unknown
pattern:
(3)
T = 2 π √ (L/g) * [ 1+
1/16 Θ02
+ 11 / 3072 Θ04 + 173 / 737280 Θ06
+ 22931 / 1321205760 Θ08
+ ...]
[sec]
where Θ0 =
arcsin
(a/L)
An approximation which gives reasonal results without an infinite
series of terms:
(4)
T = 2 π √ (L/g) *[cos(θ/2)]^-{0.5*[cos(θ/2)]0.125}
The formula derived by Lima and
Arun:
T = -2 π √ (L/g) * [ ln(a)
/ (1-a) ]
[sec]
where a = cos (Θ0/
2)
(5)
For the time being I use formula (2) , expanded over 3
terms.
The Foucault Precession is
ΩF
= ΩA / sin φ
/ [1 - (3/8 * a2 / L2)]
[rad/sec] For NL at 52° that gives about 31 hours for 1
rotation. (6)
For small excursions the part between [
] can be omitted, the difference is generally a few
minutes. (7)
In my calculations I use the complete form.
Intrinsic
Precession of the Ellipse
Is given by
Ωe
= 3/8 * ϖ0 * ab / L2
[rad/sec]
(8)
where ϖ0 = 2 * π / T [rad/sec]
The
Qualityfactor
The Quality factor of a resonator (also e.g. a tuning circuit in a
radio
receiver) is the ratio between the energy in the system and the
energy
loss per period.
With a Foucault pendulum it is nearly impossible to calculate the
Q in
advance, because the losses, mainly by air friction, are difficult
to
calculate.
There are however some methods to measure the Q of a practical
pendulum.
1/ Measure the amplitude of the pendulum, then stop driving it and
count the number of periods before the amplitude has reached half
of
its
original value. Multiply that number by 4.53 and you have the Q.
2/ The same but count until the amplitude is 37% of the original
value.
Multiply by pi and you have the Q.
3/ Use both methods and take the average.
I used only method 2.
Q = π * τ / T
[-]
(9)
where τ is the time when the pendulum has reached 37% from the
begin
amplitude after stopping the drive.
Q is also defined as
Q = 2 π E / Eloss
[-]
(10)
where Eloss is the energy loss per period, so
Eloss = 2 π E / Q
[J]
(11)
and this is the energy which has to be added during each period to
keep
the pendulum in motion.
So per half periode half of it.
Drive
timing
according to the
Schumacher equation
Schumacher
(download) describes in his article that if the drive pulse is given on a
very
special moment during the swing, the precession of the ellipse
(not the
ellipse itself) is perfectly suppressed. It turns out that his
equation
[19 in the article] cannot be solved analytically, the only way is to find
an optimum by iteration (trial and error)
Change "Drive Position" such that the "Ratio Left / Right"
becomes 1.0 as good as possible. "Drive Position" in meter or in
samples
after the zero crossing then gives the optimal drive moment.
Note that Schumacher's formula is quite sensitive for the Q
and for the amplitude of the pendulum. You have to know the Q and
keep
the amplitude under control.
As the Q is mainly determined by the air friction it will be sensitive
to the viscosity of the air, which is dependent on air pressure,
humidity and temperature.
Sensitivity
of the Floor Unit adjustment
Based on 0.1 mm displacement per rotation of the knob.
Energy in the system
The energy in the system can be calculated in two ways. From the
difference in height that the bob passes between the zero crossing
and
the maximal amplitude, and from the velocity of the bob at the
zero
crossing.
The kinetic energy at the
lowest point,
that is at the highest velocity, is
Ev = 1/2 M (2 π / T)2
[J]
(12)
This will give a slightly incorrect result because it assumes the
motion of the pendulum to be perfectly harmonic (sine shaped) and
that
is not the case, there are odd harmonics in the actual motion.
The height reached at the maximal excursion is:
h = L - √
(L2 - a2)
[m]
(13)
and so the potential
energy there is:
Ep = g * M * h
[J]
(14)
There must be: Ev =
Ep =
E
(15)
Forces on the cable
The maximal force appears at the center passage, there the
centripetal
force
adds to the weight of the bob.
Fmax = M * g + M * v2 /
L
[N]
(16)
where v the velocity at center passage, approximated
by 2 * π * a / T
The minimal force appears at the end of the swing
Fmin = M * g * cos(Θ0)
[N]
(17)
where Θ0 the angle at maximum deviation:
arcsin
(L / a)
Forces on the
mounting point
It may be interrresting to have a look at these figures to judge
the
stability of your mounting point. The slightest movement here will
degrade the performance of your pendulum.
Thermal
expansion of the
cable
This can be important with long pendulums because it may change
the
height of the bob above te floorunit. This will influence the
effect of the drive coil and the amplitude of the sensed signals.
However, the thermal expansion of the building itself will have an
influence too, but is rarely known.
Passage
times
for a certain distance
This is to calculate passing a certain distance in sample-ticks
after the
center passage.
I assumed here that the bob is making a perfect hamonic
(sine-shaped)
movement. That is not the case, but the deviation is a few
promilles.
Height
of the tip of
the bob at some distance from center
The height ht of the bob-tip at a distance d from the center is:
ht = Lt *(1-cos(Θ)) where Θ = asin(dt/Lt) and Lt the
length
from the top to the tip of the bob.
Height
from top
for a given excursion of the cable
This was to calculate the height at which the magnet for the Hall
Sensors had to be
placed such that it makes a certain excursion.
Dimensions
and weight of a cylindrical Bob
Design your own Bob.
Dimensions
and parameters of the Coils
Dimensions are according to this diagram:
Number of wires parallel.
For the Drive Coil for the Chapel Pendulum I had available only to
thin
or to thick wire, and many bobines of the thin wire. So I decided
to
wind it with multiple wires, each to thin. This requires some
extra
calculation of what the final resistance would become.
Diameter of wire
I have ignored the thickness of the insulation layer.
Fill Factor.
Here it is the ratio between the total cross section of the copper
wires and the available space as defined by the coil former
dimensions.
If all the space is perfectly filled with round copper wire, each
side-aside you may expect
a fill factor of π / 4 = 0.78.
In practice you will never realize that.
If the outcome is much smaller there
is room for more windings. If it is larger the windings will
overflow
the available space.
Passage times of the Bob
are given in Sample ticks after the zero passage.
You will need these values to estimate the "expect" and "missed"
times in the BobControl settings.
References:
(1) can be found in any textbook about Foucault pendulums.
(2) en (3) https://en.wikipedia.org/wiki/Pendulum_(mechanics)#Arbitrary-amplitude_period
Here a FreePascal function to calculate the exact swing time.
function
ExactSwingTime (Length, Amplitude, Gravity: extended): extended;
//
we use the formula given in:
//
https://en.wikipedia.org/wiki/Pendulum_(mechanics)#Arbitrary-amplitude_period
//
as the Legendre polynomial solution
var Theta,
SinHalfTheta, Term0, Term1, Term2, Term3, Term4, Term5: extended;
begin
Theta:=
arcsin
(Amplitude / Length); // angle at maximum position
SinHalfTheta:=
sin (Theta / 2);
Term0:=
2 * pi * sqrt (Length / Gravity); // the classical approximation
Term1:=
sqr (1 / 2) * power (SinHalfTheta, 2);
Term2:=
sqr (3 / 8) * power (SinHalfTheta, 4);
Term3:=
sqr (15 / 48) * power (SinHalfTheta, 6);
Term4:=
sqr (105 / 384) * power (SinHalfTheta, 8);
Term5:=
sqr (935 / 3840) * power (SinHalfTheta, 10);
ExactSwingTime:=
Term0 * (1 + Term1 + Term2 + Term3 + Term4 + Term5);
//
show the contribution of each term
writeln
(format ('%20.18f, %20.18f, %20.18f, %20.18f, %20.18f, %20.18f',
[Term0,
Term1, Term2, Term3, Term4, Term5]));
end;
(4) was found in a link which is now dead
(5) was found in Lima
and Arun (download)
(6), (7) without the term between [ ] found in many publications
about
the Foucault pendulum.
The term between [ ] was found in Haringx
and
Suchtelen (download)
(8) -up are commonly known in kinematics and mechanics.
Articles
and sites
about theory and practice of the Foucault pendulum
The Dutch physicist Heike Kamerlingh Onnes (yes, the one from the
liquid Helium and the superconductivity) had his doctoral thesis
in
1879 about the Foucault pendulum.
His thesis (download) (in
Dutch. I do not know about an English translation, there seems to exist a German version) contains
besides
comprehensive math about the ellipse problem, a description of his
rather short pendulum, which had some very smart details.
The title of his thesis is (translated) "New evidence for the axial rotation of the Earth".
Source: www.lorentz.leidenuniv.nl/history/proefschriften/sources/Kamerlingh_Onnes_1879.pdf.
Schulz-DuBois
“Foucault Pendulum Experiment by Kamerlingh Onnes and degenerated
Perturbation Theory” contains a hommage to Kamerlingh-Onnes.
Cartmell et al
describe problems you may encounter when you try to construct an
extreme precise pendulum to prove a certain relativistic effect.
Crane
(download) eliminates
the precession of the ellipse (not the ellipse itself) with a
repelling
magnet in the center. The exitation of the bob is both attracting
and
repelling.
Salva et al
(download) describe a short pendulum with eddy current damping, and a
tracking
mechanism with Hall sensors.
Salva mentions E.O. Schulz-DuBois, Am. J. Phys. 88, 173 (1969) as
a
reference to the work of Kamerlingh Onnes. I was not able to
verify
this.
Lima and
Arun (download)
derive an accurate formula for the period of a pendulum at larger
amplitudes.
Longden
(download) describes some constructions for mounting a pendulum such that
ellipse
forming is limited. I suppose he was not aware that ellipse
forming is
fundamental, even with the most perfect mounting.
Roland
Szostak
(download) builds a simple pendulum for schools. He indicates that repelling
excitation is to be preferred over attracting, but argues that
with
only a simple drawing which t.m.h.o. shows the opposite.
Schumacher
(download) derives that when a repelling impulse is given at the right
moment, the
precession due to the ellipse can be eliminated completely.
The distance d at which the impulse should be given depends only
on the
Length, the Amplitude and the Q-factor of the pendulum. It turns
out
that the higher the Q-factor is, the earlier the drive pulse
should be
given.
My experiments up to now tend to confirm this. In all pendulums I
operated up to now I've seen alternating ellipses, changing
direction
around E-W and around N-S. When the impulse is
given to early the ellipse precession is overcompensated, that is,
during a CCW ellipse the precession goes CW and during a CW
ellipse the
precession goes CCW. When the kick is given to late it is just the
opposite.
Pippard
(download) treats
a driving method where the top mounting point is lifted
periodically,
also known as piston drive.
He also directs to his book "The physics of vibration" (download)
Haringx
en Suchtelen (download) tell about the pendulum in the UN building in
New
York, a present from NL in 1955. Interresting is how the principle of the Charron ring is implemented here.
Giacomo
Torzo
(download) describes the pendulum in Padova (it). There is also a nice youtube
video and an interview with Giacomo (Italian spoken).
Walter
Lewin
(video) demonstrates in his last college that the period of a pendulum is
dependent on the
amplitude, but not on the mass of the bob. He himself takes the
role of
bob and he can draw beautiful dotted lines on the blackboard.
There are
some other physics experiments too.
Professor Kielkopf describes a lot of details of the pendulum in
the University
of
Louisville, (download) KY, USA
Witzel
(link) describes a short pendulum with piston drive and an attracting
magnet
to counteract ellipse forming.
A manufacturer
of
Foucault pendulums (link) has several manuals on the site. These
reveil
quite some information about the construction and operation.