Tips for a
successfull
D.I.Y. Pendulum
Latest change
2025-07-22
Required skills:
Patience:
Experiments with a Foucault Pendulum generally last days or weeks.
The
good part of the message is that you can do something else in the
mean
time, but only if you have the means to precisely log the
behaviour of
your pendulum. My approach with electronics and software allows
for
this.
Metal working:
Many parts needed to construct a working pendulum are not
available in
regular shops. You simply need to make them yourself. I regard
(access
to) a lathe and a stable column-type drilling machine as a
minimum. A
milling machine makes several jobs to a pleasure.
Wood working:
People who are used to d.i.y. work in the home often have enough
tools and experience for the woodworking required. Manufacturing
the
coil members to the required precision may ask for some
improvisation.
Some parts may lend themselves to maker processes like 3D printing, laser cutting etc.
Electronics:
You don't need to be a professional for this, but basic
understanding
of working with operational amplifiers, Ohms Law, Voltage and
Current
levels, etc. is required. Basic experience with (and access to) an
oscilloscope is as good as mandatory.
Software:
It is almost inevitable to (be able to) modify the Arduino
firmware and
the PC software I present here to your particular needs. Or completely design your own. So if you
don't know already, learn programming, or find a friend who knows
to. I use (Arduino) C, C++ and Free Pascal.
Study the theory as good as you
can. I am certainly not a skilled mathematician or physicist, I am
a retired
electronic engineer and yes that means a certain math background,
but
most of the math I've seen in the articles I mention go beyond my
skills. Not withstanding that I could mostly "read between the formulas"
and understand the messages there.
Suppress (the effects of)
the elliptical path any
pendulum tends to follow.
I use only the method according to the Schumacher article with rather
good results. That means that the timing of the drive-pulse is
extremely important.
I have no Charron ring or some similar mechanism to reduce the
amount
of ellipse. My experience so far is that a correct timing also
seems to limit
the amount of ellipse.
Use a drive coil of small
diameter.
The Schumacher equation predicts that a bob with a high Q needs to
be
driven very close to the center. So, to make your coil effective,
give
it around the same radius. However, you generally won't know the Q
in
advance and maybe need to redesign your coils after some
experiments.
Use a Center-Detect coil of sufficient diameter such that
it will give enough signal, even when a substantial ellipse has
developed.
Do implement a Rim Coil as
I
did. Give it a radius of around 2/3 of the intended amplitude of
your
pendulum. It will be very helpfull in accurately determining and
controlling the amplitude of the pendulum. The Schumacher method requires a stabilized amplitude.
It must be said: in the literature I've never seen an explicit mechanism for amplitude control.
Do implement a position measuring system.
I regard it as the only way to accurately monitor the behaviour of your
pendulum. Currently I use the system with Hall sensors and a magnet
around the wire. Earlier I mentioned a system
based on cheap optical sensors, but that did not work well. The sensors,
although with analog output had a step-wise tranfer function, to rough
for the purpose.
I have now (july 2025) a system based on AC capacitive coupling. It will soon be available as a kit with electronic circuitry.
Use a simple clamp to fix
the
wire at the top. There exists a variety of bearing constructions
with
balls, cylinders, knife-edges etc. These are all difficult to
construct
with the required precision and / or may introduce anisotropy or
friction
which -even in the slightest amount- cause problems like
developing a
substantial elliptical path or complete absence of Foucault
Precession
because of stalling at some angle.
Fix the wire such that the weight of the bob is taken by some
construction above the clamp (e.g. a winding axle). The clamp then only
cares for the horizontal fixation of the wire. This way there is no
need to tighten the clamp very strong with the risk of cutting into the
wire.
This also eases the height adjustment of the bob.
I use Piano Wire, also
known as
Music Wire and family of Spring Steel. Piano Wire has a tensile
strength around 6 times that of normal iron wire and is
manufactured
with very narrow tolerances. It is made to withstand all tortures
of master
and amateur pianists without going out of tune.
I once used normal iron wire of 1mm diameter (tensile strength 36
kg/mm2)
with a bob of 6 kg and a period of 4.2 seconds. It broke after
around
12 days of operation because of material fatigue.
Stranded steel cable can be a good alternative, but is probably
not
easy to aquire. A disadvantage of stranded cable is that it
lengthens
and rotates when it is loaded, and that it has a low torsion
spring
constant. This may cause your bob to rotate along its axis for
long
time.
Try to use a bob as compact as
possible.
The weight itself is not that important, although mass goes with
the
3rd power of linear dimension and surface (as seen from the side
and
determining air friction) goes with the 2nd power of linear
dimension.
A bob made from high density
material is smaller and has less air friction compared with a
lower
density bob. I use lead, which is quite dense (11kg/dm3) and
modestly
cheap. Besides that you can easily melt it and pour it into your
bob's
shell.
Also the shape of the bob is not critical. A sphere might be the
optimal compromise between mass and air friction, but is hard to
manufacture. A cylinder is much more easy to fabricate. My
pendulum
appears to have a Q over 3000.
Log as many parameters of your
system as you can think of. In case of problems it can tell you a lot
about the (dis)functionng of your system. Take care that you can analyse
the data in an easy and flexible way. I wrote special software for
that.
Put a wind screen around your
pendulum. My experiments have shown that it is very sensitive to even
imperceptable air draught from e.g. an airco or other mechanical
ventilation systems or house heating systems. Use a canldle or so and
check at the location of your pendulum that it has a steady flame. If
the flame flickers or wavers your pendulum may be in trouble.
Do not believe everything I write on this site. Sometimes I am wrong.