PROFESSOR: This sphere is moving between electrodes
having a potential difference of about 25 kilovolts.
It is immersed in a slightly conducting oil.
The mechanism responsible for the bouncing motion
is basic to the interaction of electric fields and moving
media.
The applied voltage induces charges
on the electrodes and the contacting metallic sphere.
By virtue of the charges on its surface
the sphere is attracted toward the upper plate.
But as the sphere rises the charge induced on its surface
leaks off through the liquid, and eventually gravity
returns the sphere to the plate to renew the process.
Here we use corn oil with a relaxation time
of about one second.
Now we have transformer oil with a relaxation time
of over 10 seconds.
The sphere reaches the upper electrode
long before the charges leak off.
Upon contact it acquires charges of the opposite polarity
and is pulled downward by the electric field and gravity.
The sideways motion shows that now even charges on the wall
are important.
The oscillations of the spheres provide an explanation
of what we see if metal filings are
placed between the electrodes.
In corn oil, most of them lose their charge
and fall back before reaching the top electrode.
But in transformer oil they stream between electrodes.
Now if we increase the voltage the streaming
particles cause electrical breakdown.
Small particles in oil insulated high voltage equipment
can similarly lead to disaster.
Three streams, three trash barrels,
and three electrodes, each connected
to the adjacent barrel.
The drop deflections indicate that a large alternating
voltage is building up spontaneously.
They have an unusual form of a three phase high voltage
generator.
This explanation may occur to you with a little thought
as we study now the interactions of electric fields and moving
media.
Moving medium is ordinary water.
Replace the ring at one kilovolt in potential.
At a short distance from the nozzle
the jets break into drops which we see here in slow motion.
The water jets are grounded, and so charges on the ring
induce image charges on the jets.
As the drops pull away from the stream,
they do so with a net charge.
This charge is then transported to the pail
where it adds to the charge from the preceding drops.
This sphere attached to a conductor
can give an idea of the electric field intensity.
About 30 kilovolts per centimeter
is required to produce breakdown in air.
So we generate about 10 kilovolts
beginning with charges induced by two 510 volt batteries.
We have a voltage generator.
The relaxation time in tap water is far less than a microsecond.
This gives plenty of time for charge
to accumulate on the drop as it forms.
For the free drop the relaxation time of the air
is much greater than a second.
Enough time for the drop to reach the pail with its charge.
The electric Reynolds number based
on the properties of the air, the distance of travel,
and the drop velocity is large.
In a high voltage Van de Graaff generator
the falling drops are replaced by a moving insulating
belt. Charges are placed on the belt
by a corona source at the bottom.
They are removed at the top where
they accumulate on the dome.
This small Van de Graaff generator
can make 500,000 volts.
Large modern electrostatic machines produce as high
as 10 million volts.
So far, we've considered the effects of material motion
on the electric field.
This experiment emphasizes what we've learned.
You'll want to explain it yourself.
We placed a second stream of drops
next to the one we were using a moment ago.
This dream also has its inducer electrode
in a pail for catching the drops.
Now, however, there is no external source of voltage
on either inducer ring.
Rather the rings are connected to the pails
of the opposite stream, which causes
them to charge spontaneously.
The electrical breakdown makes it clear
that, with no excitation, this system
generates about 20 kilowatts.
Kelvin's water drop experiment demonstrates for us
that not only does the motion generate the electric field,
but that the field also has a strong influence
on the motions.
If we bear in mind that the major constituents
of this experiment, air, water drops, and electric charges,
are also in the atmosphere, it's not
difficult to imagine that the drop dynamics have a lot
to do with lightning.
Like our experiment, thunderstorms
are an electro hydrodynamic dynamo.
Of course, a much more complicated one than we
have here.
Lord Kelvin's dynamo took yet another form
in our introduction.
Three streams, with each inducer electrode attached
to the barrel of a neighbor.
The alternating streams show that the fields
are spontaneously building up.
This time, the voltage of each pail
increases in an oscillatory fashion.
The arrangement of electrodes, water, and air
is a three phase AC dynamo.