History
Jean Bernard L?on Foucault was born in Paris in 1819.
His parents had planned to make a doctor out of him but
he soon discovered that he couldn't even stand the sight
of blood and thus he quit medical school. His main interests
where optics and astronomy. Among other achievements he
took the first picture of the sun and along with Fizeau
they managed to measure the speed of light quiet accurately.
However, his most recognized achievement is proving the
Earth's rotation around its axis by using a pendulum.
It all began in 1848 when he made a remarkable observation.
Even if someone turns the point from which a pendulum is
hung the oscillation plane is not affected. Foucault realized
that this bizarre behaviour could be very useful for proving
the Earth's rotation. If the oscillation plane is not affected
by Earth's movement, it wouldn't rotate along with it; it
would remain fixed. Therefore, after some time the floor
underneath the pendulum would have shifted, compared to
the oscillation plane and thus proving the Earth's rotation.
In order to prove his theory he first had to overcome certain
obstacles. The pendulum's oscillation would stop after some
time due to air resistance. In order to perform the experiment
he would have to make sure that the pendulum will swing
long enough. His first attempt was made in his basement
where he hung a 5 kgr ball from a 2 meters long cord. His
pendulum didn't swing long, however it was enough for Foucault
to notice a very slight clockwise drift. He repeated his
experiment with a greater pendulum at Paris Observatory.
This time the wire was 11 meters long and the oscillation
lasted longer and thus confirming his first attempt's results.
In 1851 he constructed an even greater pendulum, this time
at the Panth?on in Paris. There, he had his first public
demonstration. The 67 meters long pendulum made the expected
clockwise drift causing the crowd's excitement.
How does it work?
A
pendulum consists of a thin cord attached to a fixed point.
A mass known as bob is hung at the other end of the cord.
Once set to motion the pendulum's bob swings back and forth
trying to get back to its initial equilibrium position.
Due to air resistance the oscillation's amplitude diminishes
as time passes and after a while it stops completely returning
to its equilibrium position. The time it takes the bob to
complete a full oscillation is called period (T). The number
of oscillations per time interval is called frequency (f).
The forces working on the pendulum are air resistance, tension
and gravity. Air resistance along with friction is the reason
the pendulum will eventually stop oscillating. Tension is
an external force that acts upon the pendulum and it is
always perpendicular to the trajectory of the pendulum's
bob. Thus, tension does not act upon the oscillation plane.
The last but not least of the forces acting upon the pendulum
is gravity. Gravity always acts in a downward direction
and does work on the bob but it cannot change the angle
of the oscillation plane. Gravity acts like a restoring
force that accelerates the pendulum back to its equilibrium
position. Gravity pulls the bob downwards to its equilibrium
position, but inertia prevents the bob from stopping, making
it swing upwards. When the pendulum reaches its highest
position, gravity once again pulls it downward.
The period of the swing depends on the length (L) of the
pendulum and the acceleration of gravity (g). Contrary to
common belief it does not depend on the mass of the bob.
However, for a non  ideal pendulum the mass of the pendulum
does affect the motion due to inertia. For small amplitudes
and given that air resistance is neglected (which is an
acceptable approximation especially for aero dynamical bobs)
the period of the pendulum can be considered constant and
equal to:
This is why in order to make the pendulum oscillate long
enough to prove Earth's rotation; Foucault needed the pendulum
to be as long as possible and have a quite heavy bob in
order to increase the inertia.
As mentioned above there are no forces acting upon the pendulum
that can change the oscillation plane. Thus, the drift the
pendulum's oscillation plane seems to have can only be due
to Earth rotating underneath it.
Does it always work?
Earth's angular velocity can always be analyzed into
two components, one vertical and one parallel to the plane.
These to components cause two different kinds of motion.
The first makes the plane rotate around a perpendicular
axis and the second component makes the plane move in a
circle around an axis parallel to the plane.
The first out of the two motions is the one witnessed while
observing Foucault's pendulum. Had this motion been observed
at the North or at the South Pole, the plane would make
a complete round within 24 hours. On the contrary, if the
experiment was held at the equator it wouldn't work at all.
This is because the plane at the equator is parallel to
the angular velocity vector and thus there is no vertical
component to cause the motion under discussion.
In any other case, the time it takes for a full twist depends
on the latitude (). It can be easily proved that the plane's
twist per day () can be calculated by using the following
formula:
