PSN-L Email List Message

Subject: Re: pendulum suspensions
From: tchannel1@............
Date: Sat, 10 Nov 2007 15:14:10 -0700

Hi Bob,  Thank you for this interesting input. and the wed links.


----- Original Message ----- 
Sent: Saturday, November 10, 2007 2:22 PM
Subject: pendulum suspensions

> Hello,
> This is my first post to the list. I am Bob Holmström, Editor of the
> Horological Science Newsletter (NAWCC Chapter 161).  Pendulum suspensions
> are of great interest to those interested in precision time keeping.
> Perhaps there is some use discussing what clockmakers have learned even
> though a clock pendulum needs to move constantly over a much larger angle
> than the devices discussed here.
> But first, since Dr. Peters suggestion to use a magnet suspended carbide
> ball is what prompted my response, I want to discuss a prior experiment:
> Marcel Bétrisey, a Swiss artist who has created many clocks and Foucault
> pendulums from "found materials", has experimented with a Foucault
> pendulum suspended from a ball attracted to a high strength magnet.  He
> found that the motion was not "homogenious", in that the amplitude of the
> motion and the rate of change of the direction of motion had significent
> variations. Anyone who has attemped to build build an accurate Foucault
> pendulum can attest to the fact that the suspension design is critical and
> even then error correction devices such as Charon rings or Eddy current
> damping are required to get anthing close to accurate motion.
> Unfortunately the error correction devices add there own errors!
> Kammerling Onnes (1890's) used a crossed knife edge suspension without
> error correction for a Foucault pendulum and achieved some of the most
> accurate results ever obtained.  Marcel's best results have been with
> pendulums suspended from guitar wire located by a stack of close fitting
> watch jewels in a collet.  Some of Marcel's work can be seen at
> and  For a real treat on the interaction
> of art, science, and horology see his radiometric pendulums.
> re clock pendulum suspensions:  Most pendulums hang from flexures,
> including very precise clocks such as those constructed by Shortt and
> Fedchenko.  The connections between the flexure and its support and
> pendulum are very important - simple clamping can be problematic - Shortt
> and Fedchenko used spring machined from blocks (i.e. thick ends) with
> great success.  Modern edm techniques make this type of spring relatively
> easy to construct.  Gravity wave detection pendulums use similiar
> suspensions constructed from low loss materials such as indium.
> True knife edge suspensions probably do not exist - contact forces and
> irregular contact geometry produce something less than ideal.  Much better
> results are obtained with edges shaped to a small radius that can be
> obtained by lapping with an appropriate jig or by more modern edm methods.
> One test of a pendulum suspension for a clock is to see if it shows true
> "circular error" (a horological term probably best expressed as period
> error due to the fact that a pendulum is not truely isochronus - i.e. the
> period varies with amplitude).  It is very difficult to construct a
> pendulum that agrees well with theoretical predictions. Plotting period
> versus amplitude squared makes extrapolating the data to zero amplitude
> much simpler than the traditional form of plot and it shows the errors
> quite readily. The idea was suggested by Stroud and Matthews, "Period of a
> rigid pendulum pivoting on flattened knife edges" Physics Education,
> Volume 22 (1987) pages 170 - 173.
> I intend to evaluate Chris Chapmen's crossed roller suspension and
> Meredith Lamb's hard disc id suspension when I return home in a couple of
> weeks.
> The best pendulum suspension results that I have obtained so far are with
> an air bearing suspended pendulum.  The pendulum was suspended from both
> ends of a rod passing through a porus graphite air bearing, first with
> magnets controling side to side drift and then air bearings on the ends
> also.  Some results and photos of the setup are at
>  Perhaps not the most practical - the air
> bearings are not cheap and a continuly running air compressor is a bit
> annoying!)
> Note: John Harrison used a third method to control end to end drift and to
> reduce friction in his H1 chronometer e.g. he used a small fiber from each
> end of the rod to a fixed support - in addition the rod was supported on
> "anti-friction" rollers (imagine segments of two large wheels with the
> shaft resting at the intersection of their radii at each end of the
> shaft.)
> Bob Holmström
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