The pendulum pedestal was designed by architect G. Rietveld.

A Foucault Pendulum is a simple device to demonstrate the rotation of the Earth. The principle of the Foucault Pendulum was first demonstrated in Paris in 1851 by the noted French physicist Jean Bernard Léon Foucault.

While it had long been known that the Earth rotates, the introduction of the Foucault Pendulum in 1851 was the first simple proof of the Earth's rotation in an easy-to-see experiment . When hung from a fixed point, the plane in which the pendulum swings appears to rotate due to the rotation of the earth beneath it. At the latitude of New York, the pendulum takes approximately 36 hours and 45 minutes to complete its rotation cycle.

The Foucault Pendulum at the UN was designed and built in the research laboratory of Koninklijke Philips N.V.

The UN pendulum is a 200 pound (91 kg) gold-plated sphere 12 inches (30 cm) in diameter partially filled with copper and suspended from a fixed point in the ceiling, 75 feet (23 meters) above the ceremonial staircase, by a stainless steel wire. A universal joint allows it to swing freely in any direction. The sphere passes directly over a raised metal ring at the centre that contains an electromagnet which induces a current in the copper inside the ball. This supplies the necessary energy to overcome friction and air resistance and keeps it swinging uniformly.

The shaft supporting the electromagnetic ring bears the following message from Queen Juliana: “It is a privilege to live this day and tomorrow. Juliana”

Left to itself, air resistance will slow down any pendulum until it stops swinging. A grandfather clock uses a wound-up spring to add a little energy at every swing to sustain the pendulum motion.

Very large Foucault pendulums in museums of science around the world use electrical energy. Most use a mechanism that pushes the suspension wire near the top suspension point. See Academy Pendulums, who built their first Foucault Pendulum for the California Academy of Sciences in 1951 and their third for the Boston Museum of Science in 1957.

The UN pendulum is unusual in that it uses an electromagnet beneath the pendulum bob to push on a copper plate embedded in the bob. The design is based on R. Stuart Mackay's article "Sustained Foucault Pendulums" in the America Journal of Physics, 1953, vol.21, p,180.

J. A. Haringx and H. van Suchtelen of Koninklijke Philips N.V. described the engineering design in their article "The Foucault Pendulum in the United Nations Building in New York" in the Philips Technical Review, 1957/58, No.7-8, vol.19, p.236

The following is adapted from their article.

The moment at which the coil is energized is determined by the moment at which the suspension wire touches a circular ring near the top suspension point. A delay circuit, which also controls the duration of energization, ensures that the current in the coil is switched on at a specific time after the centre of the bob passes the axis of the coil.

The anode load of tube II in the circuit includes the DC winding of a transductor T). The AC winding II> of this transductor is connected in series with the drive coil L2 and a capacitor C". In addition, another capacitor C' is connected in parallel with Ll, When the DC. coil of the transductor is not energized, the circuit Ll-C' is in resonance and represents such a high impedance that the current through L2 is only 85mA. (Note that the voltage applied to the coil is 127V AC 60cps.)

When the transductor is energized by the anode current of tube II, the self-inductance of Ll is reduced by about a half. The capacitance of C" is such that the whole circuit now comes into series resonance, whereupon the current through L2 rises to 240 mA. The ratio between operating and quiescent currents does not seem particularly large, but it must be remembered that the energy transferred is proportional to the square of the current. In this way about 0.035 joule is supplied to the bob in each period, which is sufficient to provide an amplitude of swing of the required value. The power applied to the coil is 127V x 240mA = 30.5 watts (or joules/second).

If the energizing duration is 1 second, the energy input to the coil is 30.5 joules, but only 1/100th of this (0.035 joule) is applied to the bob by magnetic induction. Or perhaps the energizing duration is only 1/100th of a second?

Stuart Cody at the Foucault Pendulum

But the electronics then had another failure in 2025. Cody is exploring replacing the 1955 vacuum tube solution with modern solid state electronics.