Van de Graaff Generators
Frequently Asked Questions

1998 William J. Beaty

Belorussian translation provided by webhostinggeeks.com

Q: MY BELT BROKE! EMERGENCY REPAIRS (also see VDG debugging article)

A: Belts can be temporarily repaired with cyanoacrylate (Crazy Glue.) But if this makes your belt too short, here's another trick: PLASTIC SHOPPING BAGS! First eliminate the destructive fingerprint contamination: scrub your hands thoroughly, or perhaps wear gloves. Cut up a poly shopping-bag to become a slightly-longer version of your broken belt. Stick it in your machine and connect the ends of the strip using any piece of tape. Does it work? (Tape a strip of paper towel to your sphere, to make a temporary electroscope.)

If not, then the polyethelene is a bad match for your present roller materials. I've succeeded at emergency fixes: BLACK VINYL ELECTRICAL TAPE, it works fine when used on the roller, paired with poly grocery bags. If one of your rollers is metal, don't put the black tape there, cover the *other* roller with vinyl tape. Doing this, I obtained two or three microamps, even in fairly humid conditions. But even better than electrical tape is plumbers white teflon, sold in any hardware or plumbing store. Stick one end to a roller with a bit of tape, wind the entire roller to cover the surface, then tack down the loose teflon end with tiny dots of crazy glue and a tiny bit of tape. FIFTEEN MICROAMPS BABY! Lots of crackles and pops, and any paper strips taped to the dome stand out perfectly straight. No fifty-dollar rubber belt needed. And it lasts practically forever, since ozone destroys rubber, but not PE plastic film.

But then ...for years my small portable generator said "Safeway(tm)" in big obvious letters on the belt. A lucky accident: it may convince students that no exotic materials are needed to build one of these machines. Some aluminum foil adhesive tape burnished down all over a plastic ball, a sewer pipe instead of expensive transparent acrylic, and a tiny motor from a broken record-player. And of course a poly Fred Meyer bag, etc.

Q: MY VAN DE GRAAFF MACHINE DOESN'T WORK!

A: First, is it humid out? Above 40%? Test humidity with an inflated balloon rubbed on your arm. If the balloon won't charge up and cause your arm-hair to rise when held near the balloon, then the humidity is way too high. Sometimes you can fix this by opening the dome and blowing a hair-dryer through the column with the belt running. The rollers in particular must be warm and dry. For some info about solving humidity problems, see THIS FILE.

If humidity is not the problem, see my article about VDG DEBUGGING and also about building electrostatic devices.

A:

KITS AND PRE-BUILT: Several sources will sell you VDG kits or complete devices, see the commercial suppliers page.

PLANS: There now are several DIY build-it websites for VDG machines, see the links to plans on my main VDG generator page. Or, you can check out my plans page for a list of books and magazine backissues. Also, Science First sells a set of plans for a few bucks.

A:

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.

Longer Answer:
Everyday objects are made up of equal amounts of positive electric charge and negative electric charge in almost perfect balance. We might say that ordinary matter is made up of "cancelled-out electricity." A VDG machine takes the mixed-up positive and negative charges of matter, sorts them out, then pulls them far apart from each other. A VDG is a "charge uncanceller" or "charge separator." The machine moves charge continuously, so it also acts as a mechanical charge pump.

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.

More stuff:

• THE SPHERE
• BELT AND ROLLERS

A:

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.

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.

A:

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.

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 generator.

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.

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."

A:

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 community.

A:

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.

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.

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 diodes.

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 pilot light.

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.

A: Yes, it's not impossible to reverse the voltage in a classroom VDG machine. Important concept: only the surfaces are important, since the "contact electrification" and charge-separation is produced by two differing surfaces. We can change the belt material, but it's even easier to change the roller surfaces. For example, if one roller surface is metal, and the other roller is plastic (polyethelene, or even nylon or teflon,) we could swap roller surfaces but without swapping rollers. Temporarily remove the belt (use gloves, or alcohol-scrubbed fingertips.) Then cover the plastic roller with adhesive aluminum foil tape bought from the hardware store. Cover the other roller with plastic tape, try black vinyl electricians' tape, or perhaps wind it with plumbers' white teflon tape (fasten down the ends with a tiny dot of crazy glue.)

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.