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 this.
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
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
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
I use only the method according to the Schumacher Equation with
very 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 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 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.
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
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.