Van de Graaff Generators
Q: HOW DO THEY WORK?
Short Answer: A VDG is a charge pump. One or both rollers become charged
through contact with the belt. One roller sucks electric charges from its
adjacent metal comb and onto the belt. The other roller pushes electric
charges from the belt and onto the adjacent comb. When the belt is
cranked along, the device sucks charges in at one end and spits them
out at the other.
A VDG machine contains a flat circular belt running on a pair of rollers,
conveyor-belt style. The belt material must be an insulator; rubber or
plastic for example. A metal "comb" is placed adjacent to each roller,
with "teeth" pointing toward the belt surface. At each end of the belt,
each roller and comb is enclosed inside a hollow metal box or hollow
sphere. Each hollow box or sphere must be electrically connected to the
metal comb inside. One of the rollers is spun by an electric motor so
that the belt moves, and the other roller spins too.
To create a buildup of separated charge, the machine pulls one type of
charge ( either pos. or neg. ) out of one comb and places it onto the
belt's surface. The belt transports it to the far end of the machine.
The electric charge is then pushed off the belt surface and onto the other
metal comb, where it is sucked to the outside of the metal sphere. As the
charge-transport process continues, the voltage (electric potential)
between the two ends of the generator grows and grows.
Q: HOW IS A VDG DIFFERENT THAN OTHER ELECTRIC GENERATORS?
Like all other electric generators, a Van de Graaff machine is basically a
charge pump. It drives negative charge from one end to the other, and/or
drives positive charge the other way. The hollow metal ball acts as one
output terminal, while its metal base acts as the other. On some VDG
machines the upper sphere becomes negatively imbalanced, while the base
becomes positive. On other machines the polarity is reversed.
To drive home the idea that a VDG is like a battery or a standard power supply, it helps to imagine the generator like this:
Sphere Sphere _____ Belt & Column _____ / \ / \ | |------------------| | (+) | | | | (-) | |------------------| | \ _____ / \ _____ /A VDG machine is a bit like battery. All VDGs actually have a positive terminal and a negative terminal as shown above. However, most tabletop models lack the second ball. Instead of a sphere, they have a wire which connects the base of the generator to ground. Even the grounded-base type of generator actually has two spheres. One is small and metal, while the other one is 8000 miles across. If one end of the generator is connected to ground, then the whole earth becomes the generator's second terminal.
Batteries and VDG machines both act as charge pumps. However, a VDG is
different from a battery in one important way. Batteries produce constant
voltage with variable current, while VDGs produce constant current with
variable voltage. A VDG is similar to a battery, but the behavior of its
voltage and current are swapped, and everything works backwards. If we
short out a battery, we get an electrical overload. When short circuited,
a large current appears in the battery's connecting wires, while the
battery voltage remains the same. A VDG is the opposite: to overload a
VDG you don't short it out, instead you run it open-circuited with no
electrical load attached. When you overload a VDG you get a very large
voltage, but the VDG current stays the same. A VDG likes to be shorted,
but labors mightly when open-circuited. A battery is opposite: it likes
to be open-circuited, but labors mightly when shorted out.
Batteries can produce large currents, while VDG machines can produce large
voltages. A car battery is rated at 12 volts, and when a load is
connected to it, the battery can create any value of current between zero
and 500 amperes or so. A small VDG machine might be rated at 50
microAmperes current, and, depending on electrical load, can produce any
voltage between zero and 100,000 volts.
VDG machines are also different from common coil/magnet electric
generators. A coil/magnet generator pumps charge by sweeping a magnetic
field across a charge-filled conductor wire. This might seem magical,
with invisible magnetic fields causing an electrical pumping action which
creates invisible electric currents. A VDG machine is much more
down-to-earth. It uses a mechanical belt to grab the charge and
physically drag it along. A coil/magnet generator uses complicated
Maxwell/Einstein physics to pump charge, while a VDG machine is more like
a 16th-century waterwheel.
Q: WHERE DID VDG MACHINES COME FROM?
The VDG machine was invented in the 1920s by Robert Van de Graaff, an MIT
physics student who was inspired by large, unexplained sparks produced by
an industrial printing press. As paper in the press passed over high
speed rollers, both the paper and the metal printing press itself became
electrified. Robert's first machine was a few feet tall and made from
metal cans. In later years his large "professional" version was ??????
feet tall, with spheres which were ???? feet across.
The Museum of Science has a short history page.
Q: WHAT'S A VDG MACHINE GOOD FOR?
A: Yes, you can use a VDG to raise your hair, or to jump a large spark to
the knuckle of an overly-trusting science student. Or you can perform
many tricks and science demos. However, VDG devices do have many
professional applications. They were originally used as power supplies
for the early particle accelerators used in research into radioactivity.
This was in the days before the invention of the Cyclotron and Linear
Accelerator Ring. The early "atom smashers" consisted of a VDG
machine connected to a long vacuum tube. VDG machines still find use in
particle physics research, and many universities own large VDG machines
encased in huge pressure chambers filled with insulating gas. More
recently these have been replace with "Pelletron" VDG machines which use
a metal/plastic chain travelling in a vacuum chamber. Big VDGs are also
used to power high energy X-ray machines. If you want to treat cancer
with radiation, make X-ray photos of locomotive engines, or sterilze food
with gamma rays, you'll want to buy a Van de Graaff-powered X-ray
Closer to home are its educational uses. The Van de Graaff machine is an
excellent device for studying Electrostatics, the science of voltage and
electric charge. Yes, flashlight batteries are fine for studying electric
current and circuitry. But if you want to investigate voltage alone, then
get yourself a VDG electrostatic generator.
Q: DON'T THEY GENERATE "STATIC" ELECTRICITY?
A: Yes and no. A VDG machine is a Constant Current Source. It
generates a small, nearly-unstoppable electric current, and if this
current is blocked, extremely high levels of voltage potential or
"electrical pressure" will build up.
"Static" electricity is not electricity which is static and unmoving.
Instead, "static" appears when opposite electric charges are widely
separated from each other. But even this is not quite right, since
batteries and coil-type generators create separated charges as well.
Here's a better definition: "static" electricity is high voltage. For
example, when you rub your head on a balloon, you create up to 50,000
volts between the balloon and your hair. More specifically, "static" is
high voltage at low (or zero) current. So, since a VDG machine generates
high voltage at low current, we COULD say that it generates "static."
Myself, I prefer to avoid the term "static electricity" as much as
possible because it is misleading. If we really mean high voltage,
then we should just say "high voltage," and eliminate the misleading talk
of "unmoving charges."
Q: IS AN ELECTRIC MOTOR ABSOLUTELY REQUIRED?
No, it's just as easy to build a hand-cranked Van de Graaff generator.
I've always suspected that the electric motor caused misconceptions.
Since a motorized VDG machine is a closed electrically powered box, it
SEEMS to be doing something mysterious. In order to combat this
misconception, I bought several commercial VDG kits in 1988 for the
exhibit at the Museum of Science and had them modified for hand cranking.
With no motor and with nothing hidden, the workings become far more
obvious. Since then the idea has become popular, and several science
catalogs now sell hand-cranked VDG machines to the science education
Q: DO THE COMBS HAVE TO DRAG AGAINST THE BELT?
No. It's true that the combs act like motor brushes. However, the combs
operate by using high voltage to turn the air into a conductive corona.
It's this invisible, conductive air which actually touches the moving
belt. For best results, adjust the combs so their sharp points are close
to, but not touching, the belt. Or better yet don't guess about it.
Instead, measure the generator's output current with a microamp meter
connected between the upper comb and the lower one. Then just manually
adjust the comb spacing so the current is as large as possible.
Q: WHY USE A BELT AT ALL?
A: I always wondered what the belt was for. After all, if we want to put
50,000 volts on a metal sphere, why not just buy a 50KV power supply and
connect one wire to a sphere? In fact this would work fine. It would
lift your hair, make sparks, etc. (You might need to prevent electrocution
by wiring a couple of billion-ohm resistors in series with the power
supply connections!) VDG machines are charge pumps, but so are
high-voltage DC power supplies. Voltage is voltage.
The VDG belt performs an interesting task. It amplifies voltage by
physically stretching the e-field which exists between opposite charges.
The belt/rollers mechanism takes in opposite charges which are close
together, and spits out charges which are far apart. A VDG machine is a
field-line stretching device.
To produce a high voltage, we must take the opposite electric charges out
of matter and separate them. It takes work to do this. When a VDG is
operating, a bit of charge is placed on the belt. At the same time, a bit
of opposite charge is placed into the adjacent comb. As the belt is
cranked along, these opposite charges fight the belt's motion. They
attract each other, they "want" to leap together and rejoin. But the belt
draws them apart, it uses force to separate them farther and farther, then
it deposits the charge on the distant sphere and leaves the opposite
charge in the earth. If you've ever tried turning a hand-crank VDG
machine, you can feel the crank becoming slightly more difficult to turn
as the machine charges up. Mechanical work is being converted into stored
electrostatic energy as the positive and negative charges are being pulled
far apart. You're mechanically charging a capacitor.
It's true that VDG machines are equivalent to HV DC power supplies that
plugs into a wall outlet. However, small VDG machines can easily attain a
half-million volts, while a 500KV power supply would be big, heavy, and
VERY expensive. And without some large-value series resistors for
protection, a half-megavolt DC power supply would create a lethal safety
hazard. The low current and low energy-storage of tabletop Van de Graaff
machines make them safe for student use, yet at the same time they act as
inexpensive sources of extreme high voltage.
Q: CAN WE STEP DOWN THE OUTPUT TO RUN LIGHT BULBS?
A: This is probably possible, but I haven't tried it myself. A tabletop
Van de Graaff machine supplies a watt or two of electrical energy. For
example, a small VDG machine which produces 250,000 volts at 10 microamps
would act as a 2.5 watt DC power supply. If we could keep the wattage
the same, but step the voltage down and step the current up, in theory it
could run a flashlight bulb or some DC motors.
One possibility: we could repeatedly jump sparks to a large grounded
sphere or a mixing bowl (so the voltage is very high before the spark,)
then we'd break the ground wire and place a high-frequency high-voltage
stepdown transformer in the discharge path. A flyback transformer from a
TV monitor might work: route the discharge current through the high
voltage side and back to ground, then wind a few turns of wire around the
ferrite core, and rectify this low voltage output with some high speed
You might first have to use an oscilloscope to measure the AC voltage
coming out of the low voltage winding; to verify that its putting out a
few volts and can turn on the 1.4V of a diode bridge. (If it's too low,
wind more turns of wire on the flyback's ferrite core.) If you adjust the
two spheres to give a few sparks per second, the AC coming out of the
transformer could charge up a capacitor and light some LEDs. Since a
superbright red LED runs at 0.02A x 1.5V = .03 watts, you might be able to
flash a big wad of LEDs quite brightly, or even light a small incandescent
As a science project, this shows that "static electricity" generators are
no different than any other power supplies; they're just putting out their
electrical energy with low current at high voltage. "Watts is watts,"
and it really doesn't matter whether the voltage/current is low or high.
Also, there really is no such thing as "Static electricity." After all, a
conventional DC circuit is operated by surface charges on the conductors
which produce an e-field which cause currents in the conductors. You hear
me right: all circuits everywhere are run by surface charges. The "Static
electricity" we know and love is a misnomer, we should use it's more
accurate name: "high voltage." Rub a balloon on hair, and you produce
HIGH VOLTAGE charge-separation.
Q: WHY DID THE ON/OFF SWITCH ZAP ME?
A: Ah, you get that too? I wondered about this for quite awhile, but
then I eventually discovered a little-known feature of Van de Graaff
machines: they spew electric current into the air. The charge that
travels along the rubber belt doesn't just stop at the sphere. Instead,
the blocked charge causes the voltage on the sphere to rise until the
charge-flow is able to blow right across the barrier and into the air.
(Or in other words, the potential rises until corona discharge ignites,
providing a leakage path from metal to plasma to air.)
When you operate a VDG machine inside a draft-free room, the VDG sphere
spews a few microamps of current into the surrounding air. This
charge-flow follows the direction of e-field lines and tries to find a
pathway back to the earth. If it is intercepted by insulating or
ungrounded objects in the room, those objects become electrified. If
you stand next to an operating VDG machine, and if the humidity is low
enough that your shoe soles don't conduct, then your body will become
electrified. When you touch the grounded metal switch, zap!
The solution: hold a small metal object in your hand, then touch it
against ground, then turn off the switch with your other hand.
Either that, or grab a grounded wire when you turn your VDG machine on,
and never let go of that wire until after you've turned the machine off.