I am having issues building a recreation of Focaults Pendulum (basic, inexpensive, undergrad project). At this point, I am going to purchase aircraft cable or suspension cable unless someone can recommend a slightly flexible rod.

I am going to use a 32oz plumb bob because I can't find a symmetrical spherical bob unless someone has suggestions. Last, if anyone has any recomendations on a frictionless or low friction pivot (360 degrees and pendulus) that would be amazing.


There are two simple options for the bearing

The first is to suspend the bob from a flexible cord passing through a fairly tight grommet so it will maintain a constant point of contact in all directions.

Another option is to get a large ball bearing, ideally plastic or unhardened (or at least not through hardened) steel and then drill and tap it for an attachment point for your pendulum (or weld a nut to it in which case 316 stainless is a good option.

You can then mount this in a plate with a hole of something like 95% of the diameter of the bearing, for this slip-on pipe flanges are convenient. To get the best possible bearing surface fit you can lap the ball into the join with valve grinding paste or similar.

The example below is a slightly more complex version and can be locked made from a 50mm diameter 316 stainless ball bearing and a 1/1/2" bsp pipe flange. These flanges are failry thick but if you chamfer the top edge of the hole it gives you a bit more clearance and it can act as a reservoir for grease.

enter image description here

Edit to address questions in the commenst :

1) For a grommet or collet type support monofilament rather than braided wire might be best as the tension in braided wire might mean it behaves slightly differently in differnt directions.

2) For the ball bearing option it might be easiest to weld a nut to the bearing (or drill and tap for a stud) and thread the end of a round bar to fit. Idally you want the connection to the ball to be fairly rigid or there is little point using the ball at all.

3) There is probably no advantage in going for a very heavy ball as more weight will only increase the friction at the bearing surface having said that a larger ball give syou a larger range of movement. In my example a 50mm ball in a 48mm beveled hole gives around 45 degrees of motion in all directions from vertical. With this approach you are not relying on the felxibility of a 'string' of provide the range of mevement so you have more options in how you attach the bob to the bearing whihc in turn should allow you to use a heavier bob while keeping bearing friction and damping to a minimum.

4) I would guess that the magnetic donut is providing an impulse to the pendulum to compensate for friction losses and keep it swinging indefintely. This requires a coil and a modified ocsillator circuit...but that probably merits a new question.

| improve this answer | |
  • $\begingroup$ This is amazing, detailed, and you didn't say something stupid like well maybe if you balanced a rod on a blade you could have a frictionless pivot... 1. Okay so if I went with the gromet option what kind of wire or chord would I need (im using a 2lbs-3lbs plum bob. 2. If I decide to go with the second unhardened ball bearing again would you use a galvanized cable, rod, or again a gromet with wire? $\endgroup$ – Fixieis Oct 30 '16 at 0:30
  • $\begingroup$ 3 . one last question If I installed a magnetic donut at the top like the focault pendulum in the museum has( I dont have the electronics but I can get the magnet? $\endgroup$ – Fixieis Oct 30 '16 at 0:35
  • $\begingroup$ Can the steel barring be a very heavy steel barring like weighing a lbs? $\endgroup$ – Fixieis Oct 30 '16 at 5:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.