How do I set a pendulum?

METHOD FOR ADJUSTING A FOUCAULT PENDULUM.

It’s almost a case of solving an equation with 6 unknowns: the level of the coil, the centring of the coil, the level of the suspension, the centring of the wire attachment, the anisotropy of the suspension wire and the centring of the Charron ring. Change just one of these parameters and the pendulum will swing without rotating. This method is therefore a logical guide to getting the pendulum to work. But let’s not forget that a working Foucault pendulum is almost a miracle. Setting up a pendulum takes time and a lot of logical thinking. Make a note of every change you make. Never rely on luck or chance, as they will waste a lot of time and teach you nothing. You need to tame a pendulum. Above all, note the position of the oscillation plane rather than the time, because it can tell us a lot more.

1) Time needed for adjustments

This depends very much on the latitude where the pendulum is located. A pendulum in Singapore needs 1,070 hours to make one complete revolution: the time you have to wait each time you make an adjustment… At the latitude of Paris, it takes 30 hours to make one complete revolution. You can divide this time by two, as the pendulum will take the same route again after 15 hours. For rough settings, wait an hour before seeing the results. For final adjustments, wait 15 hours. Allow a minimum of two weeks at Paris latitude.

First operations

1) Set the general level of the pendulum (spirit level)
2) Fix the pendulum firmly against a wall. It is very important that the pendulum is perfectly stable.
3) Check the level on the wire at the height of the Charron ring (stabilised pendulum, using a cone or dynamic pendulum using an oscilloscope).
4) Adjust the level of the electromagnet using a spirit level.
5) Visually adjust the axis of the coil in relation to the magnet.
6) Carefully launch the balance wheel

If all these operations are carried out in order, the pendulum should not work. You will have a better chance of getting a ballpoint pen to balance on its tip the first time..

2) Fine tuning

1) Formalities for electrically adjusting the axes and levels of the coil
IMPORTANT: First disconnect the coil from the oscillating circuit – Allow the pendulum to stabilise until it is completely still – Send a few short one-second pulses using a 9-volt battery, respecting the polarity: the pendulum must be driven by the coil and not attracted– Note the direction of oscillation. – Let the pendulum stabilise again. At this point, we still don’t know whether this movement is due to the coil not being properly centred or whether it is not perfectly level, as both errors cause the same effects.- Note the location, then move the coil a little in the direction of the first oscillation.- Repeat the operation as often as necessary until the pendulum is stable.- Now turn the whole suspension half a turn.- Feed the coil again. The pendulum must remain stable. If not, this means that the electromagnet is not level, that the axis of the pendulum is not perfect or that the magnet does not have a constant magnetic field on its surface. Note that a bent magnet and a tilted coil can make you think that everything is fine, because their effects cancel each other out!
Restart the pendulum. If it still doesn’t work, don’t worry: it’s normal.

3) Locate the problem
Is it at the top (suspension), at the bottom (electromagnet) or in the wire? To find out, simply change one of the parameters slightly. It’s best to start at the top.

4) Symptoms and solutions
– The pendulum always swings in the plane in which it was sent, no matter which way it swings.
This is the worst case scenario.

– The pendulum starts well, makes an ellipse and stops
Rough centring and/or levels of the solenoid or suspension. Review procedure 1.

– Does the pendulum make an ellipse and go in any direction?
That’s better, but there’s still a way to go. The problem is probably at the bottom.

– The pendulum always remains in the same position.
It’s likely that one of the levels is wrong. To confirm this, turn the whole suspension in one direction and wait to see if the direction of the pendulum follows the movement a few hours later. If nothing happens, the problem is at the bottom. Turn the electromagnet a quarter turn and see what happens. By habit, I always turn the suspension about 1/4 hour clockwise.

– The pendulum goes in the right direction, but stops after a few hours.
Things are finally starting to get interesting. From now on, we’ll have to follow an order to find the causes of the problem. First of all, if you don’t want to waste too much time, all changes should be made clockwise for the northern hemisphere and anticlockwise for the southern hemisphere. Always go with the flow. Because if you go against it, it will take much longer to see the effects of your changes.

The problem is probably at the top. To find out, turn the whole suspension a quarter turn after marking its position. The pendulum should follow the movement and stabilise a few hours later a quarter turn further back. If this is the case, the problem is at the top and not at the bottom. Mark the new position and turn the suspension a quarter turn further.

Here, it’s possible that the pendulum will start to work properly. If the wire has internal tension in one direction and the suspension is not perfectly level, it may be that a state of equilibrium is reached and the two forces cancel each other out. In this case, the adjustments made will not be valid for another wire.
If the direction of the pendulum always follows the position of the suspension, whatever it is, then the problem(s) is at the top. The problem may be with the suspension, the wire, the centring of the wire or the attachment of the wire. To find out if it’s the wire, turn it a quarter turn, leaving the suspension fixed. If the pendulum follows the movement, the problem is with the wire or the wire attachment. It may be that the wire attachment is a little off-centre.
To determine whether this is a suspension fault, turn the thread guide only. If the pendulum follows the movement, we know the culprit. In this case, the last sapphire is chipped or badly set. To find out if it’s the wire attachment, turn it while keeping the entire suspension fixed. If nothing happens, the problem is at the bottom. You should be aware that an electromagnet whose level is off by even 0.2 millimetres will cause the pendulum to stall. The first pendulum I made could only work if you slipped a small sheet of paper under the coil at a very precise point. This is to give an idea of the importance of levels.

– The pendulum turns in the right direction, makes complete turns on itself, but does so taking longer than it should. It’s as if it’s lingering along the way. So there’s still a small error counteracting the effect of the earth’s rotation. As it approaches, the pendulum struggles to turn freely, as if it were “walking against the wind”
A pendulum acting in this way is very sensitive, and anything can make it work… or stop. So you need to fine-tune the settings. With the habits acquired during the other days, it has become easier to detect problems.

– The pendulum works well, but makes a small ellipse in a certain position, always the same one. It can even happen that this ellipse becomes larger and the pendulum stops.
The axis of the electromagnet is slightly off. It must be moved perpendicular to the pendulum’s plane of oscillation. This fault is insidious, because the problem is only visible for half an hour out of a complete rotation. And therefore never at the same time..

5) The pendulum is working perfectly.

Don’t touch anything else. It’s hard to get it to work properly, but curiously few things disturb a Foucault pendulum once it’s properly adjusted. You don’t even have to worry about draughts, ground vibrations, magnetism or other phenomena. No precautions are required when swinging the pendulum: just aim for the centre and that’s it. All my pendulums work without problems in an open workshop with draughts and people passing by.

Much has been written about the disturbances that affect the smooth running of a Foucault pendulum. The most common mistake is to believe that the disturbances are external to the pendulum. It is often thought that the earth’s magnetic field, for example, disturbs the Coriolis effect and that ferrous metals should be avoided during construction. I thought so too, until I built a pendulum with an iron balance and it worked perfectly. Never look elsewhere if your pendulum doesn’t work straight away: blame one of the settings instead. And always remember that a short Foucault pendulum that works is something of a miracle.