Foucault (28) and Bravais (8) pendulums

A unique pendulum combining two different experiments carried out in 1851, each demonstrating in its own way the rotation of the earth.

The best known is Foucault’s pendulum, whose plane of oscillation changes because the earth rotates beneath it.

The second isAuguste Bravais‘s conical pendulum, which is so discreet that it is difficult to find echoes of it, and which no one seems to have reproduced since. It showed that the speed of revolution of the pendulum could not be the same if it turned clockwise or anti-clockwise, because of the earth rotating underneath it. My first attempts to resurrect this forgotten experiment date back to 2004. From trial and error, I finally came up with the idea of combining these two separate experiments into a single object.

The pendulum:

It’s a 4-kilo block of bronze suspended from a gimbal by an Invar wire. It has a small magnet underneath which is driven away by repulsive electromagnetic impulses. A simple switch allows you to choose between the Foucault and Bravais experiments. Switch on, and the pendulum oscillates. With the switch down, it rotates.

Foucault pendulum	Bravais pendulum
Propelled by an electromagnet located on the axis.	Propelled by 9 electromagnets arranged in a circle.
Oscillates	Rotates
Its plane of oscillation rotates because the earth rotates beneath it.	The time it takes to make a revolution depends on the direction of rotation, as the earth rotates beneath it.
The period of revolution depends on the latitude: 24 hours at the North Pole, 33 hours 13 minutes in my workshop, 48 hours in Cairo, infinite at the equator.	The difference between hourly and anti-clockwise rotation times depends on latitude: 0.000287 seconds at the North Pole, 0.000220 seconds in my workshop, 0.000143 seconds in Cairo, 0 seconds at the equator.
An observer on Earth sees the pendulum turning. An observer outside the solar system sees the pendulum oscillating without rotating, and the earth rotating beneath him.	An observer on Earth sees the pendulum rotating more slowly in one direction than the other. An observer outside the solar system sees that the speed of rotation is the same in both directions.

De Foucault:

It is propelled by an axial electromagnet. If we let go of the pendulum towards the centre, the electromagnet will maintain its oscillation indefinitely.

As the earth rotates beneath it, an observer will gradually see its plane of oscillation turn clockwise if he is located in the northern hemisphere. Its speed of revolution depends on the latitude where it is located: it rotates in 24 hours at the poles, but not at all below the equator.

De Bravais:

It is propelled by nine electromagnets arranged in a circle. If the pendulum is thrown in the shape of a circle, these electromagnets maintain its revolution indefinitely: it is a conical pendulum.

With the earth rotating beneath it, an observer in the northern hemisphere will see a faster rotation clockwise but a slower one anticlockwise. Its natural speed of revolution is only found below the equator, and it differs more and more the closer you get to a pole.

This one and only pendulum thus shows us, through two completely different approaches, that the earth rotates, that it is round, and also tells us the latitude where it is placed.

Time lapse of two weeks of rotations of the Foucault pendulum, where we can see that its accuracy was 1% over the two weeks of operation visible in the video.

Those who prefer technical details should go here.