A Foucault pendulum is one of the most elegant physics demonstrations you can build: a heavy weight swinging on a long cable, slowly rotating its plane of swing as the Earth turns beneath it. The core setup is straightforward, but the details of the pivot, the weight, and the release method make the difference between a pendulum that visibly demonstrates Earth’s rotation and one that just swings in circles. Here’s how to build one that actually works.
How a Foucault Pendulum Works
A freely swinging pendulum maintains its plane of oscillation relative to the stars while the Earth rotates underneath it. At the North or South Pole, the pendulum’s swing plane would appear to complete a full 360-degree rotation in about 24 hours. At lower latitudes, the rotation is slower. The rate at which the swing plane precesses depends on your latitude according to a simple relationship: the pendulum’s rotation rate equals Earth’s rotation rate multiplied by the sine of your latitude.
At 45 degrees latitude, for example, the plane of swing rotates about 10.6 degrees per hour. At 30 degrees latitude, it drops to 7.5 degrees per hour. At the equator, there’s no precession at all. This means your location directly determines how long you’ll need to watch before the rotation becomes obvious, and it influences design choices like cable length and bob weight.
Essential Components
Every Foucault pendulum has four main parts: a suspension point (the pivot), a cable, a heavy bob, and a reference scale on the floor to track the rotation.
- The cable: A thin steel wire or braided cable, as long as your space allows. Longer is better. A pendulum of 5 meters or more gives you a slow, stately swing that’s easy to observe and less prone to problems. Shorter pendulums (1 to 3 meters) can work but demand much more precision in every other component. Piano wire is a popular choice because it’s strong, thin, and resistant to stretching.
- The bob: A compact, heavy, symmetrical mass. A solid metal sphere or cylinder works well. Weight matters because a heavier bob resists air currents and maintains its swing longer. For a home-scale pendulum of a few meters, something in the range of 5 to 15 kilograms is reasonable. Symmetry is critical: if the bob’s center of mass doesn’t line up with the cable, it will introduce wobble.
- The pivot: This is the single most important part of the build. The pendulum must be free to swing in any direction without the pivot nudging it toward a preferred plane. More on this below.
- The floor scale: A circular ring or set of degree markers placed on the floor beneath the bob. This is how you track the precession over time. A printed protractor scale or a ring of small pins works fine.
Choosing the Right Pivot
The pivot is where most Foucault pendulums succeed or fail. Any friction or asymmetry at the suspension point will drag the swing plane in a preferred direction, masking or distorting the Earth’s rotation signal. Research into pendulum design has consistently shown that the support must be perfectly symmetrical.
For a home or classroom build, you have a few practical options. A high-quality spherical ball joint is widely considered the best balance of simplicity and performance. The inherent symmetry of a ball joint means it doesn’t favor any particular swing direction. A double knife-edge pivot (two perpendicular blade edges stacked at right angles) also works but is harder to align perfectly. Gimbals with two axes of rotation are another option, though they introduce more friction surfaces. Fluid bearings and magnetic bearings exist for high-end installations but are impractical for most DIY builds.
The simplest approach for a short pendulum is to suspend the wire from a small universal joint or ball-and-socket fitting mounted rigidly to a ceiling beam or overhead structure. The mounting point itself needs to be completely rigid. Any flex or give in the support structure will absorb energy unevenly and distort the swing.
The Biggest Problem: Elliptical Motion
The most common issue with Foucault pendulums is that the bob drifts from a clean back-and-forth swing into an elliptical (oval-shaped) orbit. This elliptical motion introduces its own precession that has nothing to do with Earth’s rotation, and it can completely overwhelm the signal you’re trying to observe.
Elliptical motion creeps in from several sources: friction in the pivot, slight flex in the support structure, air currents pushing the bob sideways, and imperfect release technique. Even a tiny sideways velocity at launch will start the bob tracing an ellipse instead of a straight line.
The standard fix for short pendulums is a device called a Charron ring. This is a fixed metal ring mounted around the cable at roughly one-tenth of the way down from the pivot. The ring’s inner diameter is just slightly larger than the cable’s natural resting position, so the wire only touches the ring briefly at the extremes of each swing, where sideways drift is greatest. That brief contact nudges the cable back toward a straight-line path. For a 3-meter pendulum, the Charron ring would sit about 30 centimeters below the pivot.
An alternative approach, useful for shorter pendulums, places a lightweight loose ring below the bob instead. A small stem extending beneath the bob pushes this ring sideways by about a millimeter at each swing extreme. This tiny correction, repeated every half-swing, is enough to keep the motion linear without noticeably draining energy from the system. If you’re using a gimbal pivot, adjusting the relative heights of the two rotation axes can also reduce ellipticity at its source.
Releasing the Pendulum Cleanly
How you start the swing matters enormously. Any sideways push during release will immediately introduce elliptical motion. The classic method is to pull the bob to one side with a taut thread tied to a fixed point, let it hang motionless, then burn or cut the thread. This releases the bob with zero lateral velocity. Use a thin cotton thread that burns cleanly and quickly, and make sure the thread pulls the bob in a perfectly horizontal direction.
Pull the bob to an amplitude of roughly 3 to 5 percent of the cable length. For a 5-meter pendulum, that’s a displacement of 15 to 25 centimeters. Too large an amplitude introduces nonlinear effects; too small makes the precession hard to see against the floor scale.
Keeping the Pendulum Swinging
Air resistance and pivot friction will gradually reduce the amplitude of your pendulum. A heavy bob on a long cable in a still room can swing for hours, but shorter, lighter setups may die down in under an hour. Since you need the pendulum to swing long enough to observe a meaningful rotation (at least a few degrees), you may need a way to sustain the motion.
Museum and university installations typically use an electromagnetic drive: a coil embedded in the floor gives the bob a small magnetic push each time it passes overhead. For a DIY build, the simplest version uses a small electromagnet triggered by a sensor that detects the bob passing its lowest point. The push must be perfectly symmetric, aimed straight along the existing swing direction. Any sideways component in the drive will introduce the same elliptical problems described above.
If an active drive is too complex, focus instead on maximizing passive swing time. Use the heaviest bob you can safely hang, minimize air resistance by choosing a compact shape (a sphere is ideal), and seal the room against drafts. Even closing doors and windows makes a noticeable difference.
Sizing for Your Space
The period of a pendulum’s swing depends only on its length. A 1-meter cable gives a swing period of about 2 seconds. A 5-meter cable gives roughly 4.5 seconds. A 10-meter cable gives about 6.3 seconds. Longer pendulums swing more slowly and majestically, and they’re less sensitive to small imperfections in the pivot and release.
For a classroom or home ceiling of about 3 meters, you can build a functional Foucault pendulum, but you’ll need careful attention to the pivot and a Charron ring to manage ellipticity. A stairwell, atrium, or barn with a height of 5 meters or more gives you significantly more margin for error. The famous original Foucault pendulum used a 67-meter cable in the Panthéon in Paris, which made the effect dramatic and unmistakable, but you don’t need anything close to that scale.
At mid-latitudes (around 40 to 50 degrees), expect the swing plane to rotate about 9 to 11 degrees per hour. After two hours, you should see roughly 20 degrees of rotation against your floor scale. That’s easily visible if your reference markers are spaced at 5-degree intervals around a circle beneath the bob.
Building the Floor Scale
Place a circular reference directly beneath the pendulum’s rest position. A ring of evenly spaced pins or pegs works well because you can watch the bob knock them over as the swing plane rotates, giving you a satisfying visual confirmation. Space the pins at 3 to 5 degree intervals around a circle whose radius matches your swing amplitude. Alternatively, print or paint degree markings on the floor and observe where the bob’s swing aligns over time.
Make sure the scale is centered precisely beneath the pivot point. Even a centimeter of offset will make your readings inconsistent as the swing plane rotates around a point that doesn’t match your scale’s center.