PROFESSOR: This insulating cylinder is free to rotate.
It's centered between these electrodes, which
will be used to impose an electric field transverse
to the axis of rotation.
The electrodes and cylinder are immersed in slightly
conducting corn oil.
As we apply the potential to the plates,
the threshold of about 20 kilovolts is reached,
and the cylinder begins to rotate.
Which direction is a matter of chance.
You need two more pieces of information
to explain this experiment.
The Teflon rotor is a very good insulator
compared to the corn oil.
And the oil has a relaxation time
on the same order as the period of rotation.
The rotor acts as an insulator in series
with the regions of conducting liquid
adjacent to the electrodes.
As surface charges build up on the electrodes,
they induce neighboring surface charges
of the same sign on the rotor liquid interface.
A perturbation of the static cylinder
carries these charges along.
The motion affects the field.
In return, there is a repulsion of the light charges that
tends to sustain the rotation.
The field affects the motion.
Steady rotation is maintained, because the convection
of the surface charge is just compensated by its relaxation
through conduction in the corn oil.
The cylinder begins to rotate only because there
is both an effect of the field on the motion
and of the motion on the field.
The rotation alters the charge distribution,
because the ratio of relaxation time
in the oil to the time required for a point on the cylinder
surface to make one revolution is about unity.
The field-induced rotations of the cylinder
also are responsible for this motion.
These cells of fluid are spinning on the free surface
of corn oil.
The fluid is contained in this box,
which has two electrodes lying flat on its bottom.
They're separated by about 1/4 inch.
Bubbles injected on the surface show how the rotations develop.
Passage of current through the oil
results in a potential drop along the interface.
From above, we see the charges on the electrodes
and on the interface.
Any slight fluid rotation results
in charge redistribution, which encourages further rotation.
The interface is unstable to cellular motions
in its own plane.
And the resulting cell pattern is
one which reduces viscous drag to a minimum.