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Single minded Foucault pendulum

  1. Mar 6, 2014 #1
    I have completed a bottom driven pendulum that seems to have a preference to swing E/W. It is a 10kg bob on a 6M wire. The pivot is a tungsten carbide pin resting on a watch jewel. No matter how I start it it precesses at exactly the correct rate for 32 degrees latitude then stalls when it reaches an east/west swing. Thinking there might be some asymmetry in the drive coil's field I rotated the base but got the same result. I then tried rotating the pivot but no change. I fine tuned the charron ring and even tried without it entirely but it still settles E/W. I have also checked the area for magnets and power cables just in case. I am out of ideas! BTW it is almost exactly true E/W.

    lt is a beautiful thing for the 4 or 5 hours that it works but then becomes more compass than Foucault pendulum.
  2. jcsd
  3. Mar 6, 2014 #2
    What do you mean by bottom driven, exactly?
  4. Mar 6, 2014 #3
    The drive is a coil in the center of the pendulum base with a hall effect proximity sensor in the center. The coil is controlled be a timing circuit. The bob has a mild steel disk in the bottom. As the bob passes over the sensor the coil is depowered and remains off until the bob approaches again. The delay time can be adjusted to give the bob just the right pull to control the swing radius.
  5. Mar 6, 2014 #4
    It would appear that your pendulum has some damping in the NS direction and not in the EW. Is the base on which the whole contraption sits rigid in both directions. A micromovement would not be noticable to you.
    Perhaps the floor is more rigid in one direction than the other due to configuration of the joists, or some other factor. I would test it out in another location. If all else is OK, with the pivot and electronics being OK, that is all I can think of at the moment.

    The only place where there would be soley EW continious movement would be at the equator if the bob had been initially released in the EW sense. The pendulum motion would be planar, and without the elliptic movement there would be no restoring gravitational force perpendicular to the major axis of an ellipse.
  6. Mar 7, 2014 #5
    The floor is a concrete slab and the pivot is bolted to the ridge beam of a cathedral ceiling. Not many places I can get 6 or 7 meters vertical but I will try moving it further out into the room. You also made me think to check that the base is exactly level. I will report back after trying those ideas.
  7. Mar 7, 2014 #6


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    This is a lovely experiment.
    Have you looked around the floor with a magnetic compass for irregularities? Is there a steel frame under the concrete?
    How is the arrangement suspended at the top (how rigidly)? Could you measure the strain (lateral) on the mounting with a gauge?
    (Basically 256bit's comments)
    I can't picture the top mounting; is it like a gimbal? Can it rotate around a vertical axis? The images I ca find on Google are a bit fuzzy.
  8. Mar 7, 2014 #7
    The slab is on grade so nothing more than some reinforcement mesh in the concrete and the pivot is bolted to a pair of large wood beams. The pivot arrangement might be the problem however. I had not considered this before because all of the arrangements I had found constrained pendulum rotation. My arrangement allows about 90 degrees of rotation. Here is a basic question: Would a Foucault bob unconstrained in rotation tend to rotate in synch with the plane of the swing? If so, that may be my problem. As the pendulum reaches the limit of rotation a negative bias may be gettin introduced. If rotation were not allowed the restraining force would be equal in all directions.
  9. Mar 7, 2014 #8
  10. Mar 7, 2014 #9
    That is the basic layout. I originally used a fixed collet at the top but had trouble with wire fatigue so I suspended the collet below a pin and jewel pivot. I will have to find a way to post some pictures.
  11. Mar 8, 2014 #10


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    Not surprising, I suppose when all the bending is right up against the collet.
    My experience of Foucault is limited to the 'big ones' - as in the Pantheon in Paris. A stunning demo with such a massive amplitude swing over the ground. Fewer problems of stress when the wire is so long and the angle is so small, I guess. I'd bet the first people to see it working were convinced there was a trick involved!
  12. Mar 9, 2014 #11


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    Gashmore, welcome to PF!

    Does east-west happen to coincide with the orientation of the ceiling beam? Or 90 degrees to the beam? I admit that I don't know if the pendulum should tend to align itself one way or the other w.r.t. the beam, I am just wondering if some slight motion of that beam could somehow affect the pendulum motion over time.

    People often post them to a free website like photobucket.com, then include the image URL within "image tags" when posting here. The process is explained in this post; search for adding images on that page and follow the instructions, which refer to a figure farther up in the post.

    FYI, clicking the "Go Advanced" button, when composing a post, will bring up a lot of enhanced editing features. There is an "Attach Files" option, which you might find useful as well.

    Good luck on solving this mystery! :smile:
    Last edited: Mar 9, 2014
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