Polarity

You are invited to come and see the Earth turn’. If I’d been in Paris in 1851 and got that through the post I’d have been right there in the front row. Louis Napoleon Bonaparte, fond of science and history, had authorized the physicist Léon Foucault and the engineer Gustave Froment, to use the dome of the Panthéon to conduct an experiment.

They suspended from the dome a steel wire 67 meters long and a sphere of brass and lead weighing 28 kilograms. A device of wood and sand was installed under the pendulum, allowing to visualize the explanations of Foucault live at each swing: the stylus fixed at the bottom of the pendulum causes a mark in the sand which increased hour after hour. The experiment was a great success. Yes, were talking about Foucault’s Pendulum.

Foucault's Pendulum demonstrated, 1851

So what is it and what does it measure? Imagine you are in a museum located at the North Pole and that the museum has a Foucault Pendulum suspended from the ceiling at a point exactly over the pole. When you set the pendulum swinging it will continue to swing in the same direction. The earth, on the other hand, will rotate once every 24 hours, more or less, underneath the pendulum.

The pendulum always rotates clockwise in the Northern Hemisphere with a rate that becomes slower as the pendulum’s location approaches the Equator. Foucault’s original pendulums at Paris rotated clockwise at a rate of more than 11° per hour, or with a period of about 32 hours per complete rotation. The rate of rotation depends on the latitude. At the Equator, 0° latitude, a Foucault pendulum does not rotate. In the Southern Hemisphere, rotation is the other polarity so is counterclockwise.

Another way of thinking about this phenomenon would be to imagine you are standing on the floor of a building housing a pendulum you will naturally think that the floor is stable and the pendulum is moving. This is because we naturally assume that the base on which we stand is stable unless our eyes or sense of balance tells us otherwise. If our base moves slowly or accelerates smoothly, we are easily fooled into thinking that another object we see is moving. You have probably experienced this in a car or train that begins to move very slowly and smoothly, and for a split second you think that a nearby car or train seems to move.

how Foucault's pendulum works

The rate of rotation of a Foucault pendulum can be stated mathematically as equal to the rate of rotation of the Earth times the sine of the number of degrees of latitude. Because the Earth rotates 360° approximately every 24 hours, its rate of rotation may be expressed as 15° per hour.

I wanted to write this blog about the Foucault pendulum as a couple days ago I was able to go into the newly opened and refurbished Harris Art Gallery and Museum in Preston and one of its central features has been the replacement of the original pendulum which had been in place since it was installed at the Harris in 1909 by George J. Gibbs who was the Honorary Curator of Preston Observatory.

Dr Brett Patterson, Pendulum Project Scientist and Senior Lecturer in Physics at the University of Lancashire who built the pendulum's sophisticated electronic systems, said: "This pendulum represents more than just a scientific instrument - it's a living demonstration of our planet's rotation and a testament to the enduring power of scientific curiosity. "At 35 metres, this is not just the longest pendulum in the UK, but a precision instrument that has taken months of careful calibration to achieve perfect operation."

the Harris Museum pendulum

Not only does it look wonderful, it also enables me to reprint this poem from years ago. The paintings mentioned in the poem are back as well. Hurrah.

Harris Museum and Art Gallery

You always pause at the Curiosity Shop,
Elwell RA ( 1929 )
face close to the frame,
part of the picture,
bringing a tea for Gran
telling her again
that I’d get rid of the nosy gits.
But I’m not there,
I’m standing in Grimshaw’s shoes,
ankle deep in leaves,
nodding to the woman and child
while my eyes half closed in the gold light
always return to the door,
the door in the high wall,
hiding that house and garden,
his secret while my secret
is breathing out slowly,
too softly to disturb the leaves,
till I feel part of it all,
not the Harris,1892,
but the other Harris,
the one with connections,
that’s part of the Louvre
and it’s part of us,
we’re part of the Tate
and it’s part of us,
part of the Met
and it’s part of us,
the pendulum has its small effect,
Foucault would be pleased,
though where it fits
is open to debate,
as long as it’s a quiet one.

First published in Equinox, March 2006.

On the Golden Olden Times, J A Grimshaw

Thanks for reading, Terry Q.