Tips for a successfull D.I.Y. Pendulum Latest change 2017-09-10
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
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.
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.
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.
It is almost inevitable to (be able to) modify the Arduino firmware and
the PC software I present here to your particular needs. So if you
don't know already, learn programming, or find a friend who knows to. I use C and 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 lines"
and understand the messages there.
Suppress (the effects) of the elliptical path any pendulum tends to follow. See the Ellipse chapter.
I use only the method according to the Schumacher Equation with very
good results. That means that the timing of the drive-pulse is
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 limits
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.
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 swing.
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 all are difficult to construct
with the required precision and/or may introduce asymmetry 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.
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
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.
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 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