Wayne Schmidt's Van de Graaff Generator Page How to shop for them, build them, and interesting experiments.
INTRODUCTION
COMMERCIALLY
AVAILABLE VAN DE GRAAFF GENERATORS
THE
GENERATOR I PURCHASED AND WHY I CHOOSE THAT MODEL
EXPERIMENTS
AND DEMONSTRATIONS
HOW
VAN DE GRAAFF GENERATORS WORK
MY
HOME-MADE VAN DE GRAAFF GENERATOR
IMPROVING
THE PERFORMANCE OF VAN DE GRAAFF GENERATORS
ODDS
AND ENDS
This webpage chronicles what I've learned about buying, using and making Van de Graaff generators. Why would someone want to either purchase or build one? Because every self-respecting science nerd dreams of owning a Van de Graaff static generator. As the picture below shows of the first one I built, I'm no exception:
It worked, but just barely. This generator struggled to produce sparks 1/8-inch long, about 2000 volts. Anyone familiar with the design of Van de Graaff generators will instantly see several things I did wrong. We'll fix these problems farther down this page but for the moment I decided to buy a commercial unit so I'd have something reliable with which to play and help figure out what I'd done wrong with the home built generator.
(Please note: This page is still under construction and will have new material added to it as soon as possible. I'm posting it incrementally rather than waiting until the entire page is complete because the commercial generator list is unique and anyone interested in purchasing a Van de Graaff machine will find it very helpful. Please check back often to see the latest additions. Thank you.)
COMMERCIALLY
AVAILABLE VAN DE GRAAFF GENERATORS
Several days searching the Internet turned up 14 different models of Van de Graaff generators. The following list shows what they look like, how big they are, how strong a field they generate, how much they cost and who offered them at the least cost. The images of each have been sized to the same scale so you can get an idea how they compare. (Note: prices are in 2007 dollars.)
This is the most common Van de Graaff generator available on the
Internet. It's 18-inches tall, has a 7-inch diameter globe and
generates 200,000 volts. Prices average $200. The cheapest I found
was $140 at hobbytron.com.
The
second most common generator is this N100-V by Winsco. It is
extremely popular as a school demonstration unit. It's 30-inches
tall, has a 10-inch diameter globe and generates 350,000 volts. It
also has a variable speed motor control. Prices range from as little
as $400 (scienceenthusiast.com) to over $750.
You can pay anywhere from $179 (hometrainingtools.com) to $199
(anatomyphysics.com - includes discharge ball) for this 24-inch tall
Van de Graaff generator with an 8-inch diameter globe. It's rated
anywhere from 200,000 to 300,000 volts depending on who's selling it.
This largest production generator is 36-inches tall with a 14-inch
diameter oblate spheroid globe. It's rated at 400,000 volts and
typically sells for $499 (sargentwelch.com.)
This Van de Graaff generator is a close second in size. It's
35-inches tall and features a one-piece, mirror-finish stainless
steel globe rather than the more common spun aluminum globe. The
manufacturer claims this polished globe increases the voltage
4-percent. The odd thing is that in this picture of the generator,
borrowed from the seller's site, the "polished steel globe"
doesn't look very polished. In fact it looks like a plain two-piece
aluminum sphere.
These units produce 400,000 volts. Google.carolina.com sells a kit version for $495. Sciencefirst.com offers them for $499, but I don't know if this is a kit. Sciencefirst.com also offers an upgrade to a 14-inch diameter globe for $582, which they claim reaches 500,000 volts. These are large, good looking units but I found some inconsistency in the pictures used in the advertisements. Some showed an oblate spheroid in one image and the steel sphere in others.
This beauty is offered by unitednuclear.com. I believe it's identical
to the previous unit. This generator comes in two variants. The first
has a 14-inch globe and is rated at 400,000 volts ($665.) The second
costs $725, has a 17-inch sphere and a rating of 600,000 volts.
While it's possible that form follows function in Van de Graaff generators as much as with any device, the similarity of the base in each of the last three units suggest they may all be manufactured by the same company and simply repackaged for different distributors.
This 24-inch tall generator with an 8-inch diameter globe charges to
200,000 volts and sells for $165 at physicstoolbox.com
Sargentwelch.com sells this 19-inch tall Van de Graaff with a 9-inch
globe for $149. It's rated at 200,000 volts. (Note: it may be a kit.)
One very interesting feature about this units is that I believe it
may be battery powered.
This tiny Van de Graaff generator is only 13-inches tall with a
4-inch diameter globe. It generates 75,000 volts and sells for $207
at google.carolina.com.
This is a variable speed unit rated at 250,000 volts. It's 24-inches
tall and has a 9-inch diameter globe. If you're a teacher you can
purchase it for $549 from store.pasco.com. It features all-open
construction so that all of the workings can be observed.
Generating 200,000 volts, this 17-inch tall unit with a 7-inch globe
sells for $169 at Edmund Scientific.
This unique hand-cranked Van de Graaff generator can build up a 200,000-volt
charge. It's 28-inches tall, has a 10-inch diameter globe and sells
for $179 at arborscientific.com. I believe it is a kit.
This deluxe 400,000-volt generator is 30-inches tall and features an
11-inch mirror finish globe. It sells for $699 at sargentwelch.com.
This small Van de Graaff generator is 14-inches tall, has a 4 and
3/4-inch half-globe and produces 25,000 volts. It costs $59.95 at chaneyelectronic.com.
When shopping
for a Van de Graaff generator it's important to carefully read the
fine print. Some sellers charge more because they load you up with
bonus items like extra belts, discharge electrodes and even
demonstration kits full of pith balls and other items. Warranties
vary and are sometimes completely absent. Also be sure to check about
return policies. Finally, carefully consider what you're going to use
it for and where you're going to store it. A 36-inch Van de Graaff
generator with a 14-inch globe is a large, bulky item that is too
fragile to be thrown in a dusty corner of the garage and so large it
takes up too much room in a closet. Also be aware that while the
spark from even the largest unit may not injure you, it'll sting
quite sharply and leave you with a very uncomfortable electric
jitter. You may be a manly-man and able to laugh it off. For myself,
I'd prefer hitting my thumb with a hammer.
Regarding voltages: The actual voltage a Van de Graaff generator attains is effected by how clean the unit is, how dusty the air is, how humid it is and other factors. A unit rated at 350,000 volts in a very clean, dry location may only produce 200,000 in a dusted, humid one.
THE
GENERATOR I PURCHASED AND WHY I CHOOSE THAT MODEL
After considering all the options I decided to purchase the Winsco N100-V from scienceenthusiast.com.
The Winsco people have been making this unit for a long time and have worked out all the design problems. It's a sturdy unit with a history of reliability. It arrived completely assembled (even the dome halves were put together), worked perfectly the first time I turned it on and easily produced the rated 350,000 volts. I particularly liked the variable speed option that comes on this generator. It enables me to control the frequency of the discharges, a useful feature for photographing discharges.
EXPERIMENTS
AND DEMONSTRATIONS
WARNING!!!
Taking
pictures of the discharges from a Van de Graaff generator
is
extremely dangerous to camera equipment. The large electric
fields
can easily destroy the camera's internal electronics.
Arcs
The easiest of all demonstrations is to place a discharge electrode near a Van de Graaff generator to create an arc. After watching a few of these miniature lightning bolts it's easy to get bored with them because they all appear pretty much the same. However, taking a closer look at these arcs with the discharge electrode in different positions enables the user to create a wide range of arc types that defy explanation.
Placing a 5-inch diameter grounded electrode anywhere from almost touching the Van de Graaff generator's globe to 3 inches away creates a diffuse discharge tapered at both ends. Here's one at three inches: (Note, it's been enlarged to show detail.)
The negatively charged globe of the Van de Graaff generator is on the left and the grounded (positive) discharge electrode is on the right. See the softly glowing collar on the right? I have no idea what that is. It shows up in one quarter of these types of discharges.
Move the electrodes an inch more apart and interesting things start to happen.
A gap appears on the left side. The glow in this zone in much dimmer than the main body of the discharge. The oddest feature is that the transition from the dim to the bright zone is very sharp. I would have expected a gradual blending. Although this image again shows the mysterious donut on the right, it appears much less often in these longer discharges.
At a 5 inch separation things start getting strange.
One third of the arcs at this distance are solid cascade discharges like the one above. Seen in person the discharge is brilliant and followed by a sharp snap: miniature thunder. In photographs like this the main discharge is surrounded by soft, dim filaments, which aren't noticeable in person.
Some these discharges follow irregular paths.
Many of these discharges have breaks in them, like the following image:
Most
of the discharges at this distance consist of a complex matrix of
diffuse filaments.
Arc shape can
be sculpted by using oddly shaped discharge electrodes covered with
various materials. For example, the following diffuse arc was
produced by bending a length of insulated wire in a tight curve and
placing it 2 inches from a Van de Graaff generator:
The bent wire is the magenta curve on the right. The more diffuse a discharge is the dimmer it is. Although the arc above was clearly visible I had to digitally brighten the picture I took of it to show detail.
I have read
that charging two large spheres and attaching a ball bearing to one
of them makes it possible to create very long arcs. I tried this with
a commercial Van de Graaff generator with a 10-inch diameter sphere
and a 12-inch diameter grounded discharge sphere. The ball bearing
was 1-inch in diameter. This arrangement created discharges up to 9
inches long, far short of the 18 to 24 inches I read about.
Using a 12-inch diameter discharge electrode with no ball bearing produced many interesting arcs on the order of 9 inches long. Here are two of them:
Jets
While they aren't as large and dynamic as arcs, ion jets are still interesting and can create some fascinating effects.
Using a needle to create a tiny pin prick in a curved section of grounded wire causes a jet of ions to form. Here's what it looks like when it's 2 inches away from a Van de Graaff generator:
Moving it six inches away causes the jet to flare out.
These last two images had exposures times of 3 seconds. Increasing this to 20 captures additional detail in the jet.
I assume these jets consist of positive ions accelerating toward the negatively charged Van de Graaff generator to the left. As the ions stream toward the generator they collide with neutral atoms in the air, which get pushed along with them. This flow of air is strong enough to blow a candle flame over.
You can feel the draft on the back of you hand as you hold the candle. Moving the candle higher causes it to blow out.
Jet production
is tricky for me. I've attempted to make them using wires and needles
inside insulating tubes of various diameters and length but none of
these work. All I get are confined arcs. The same happens if the pin
hole in the insulator to the grounded wire is too large. It seems
that what's needed is an almost microscopic hole. I've also noted
that they only seem to form when grounded or possibly from a positive
Van de Graaff generator. Attached to a negative potential nothing
much appears. They are also sensitive to current flow. If it's too
low they degenerate into a normal arcing mode. Turning up the speed
on my generator to create more current tricks the arc over into a
jet. These jets produce a very pleasing sizzle when operating.
Once formed, they are quite stable and you can move in close enough
to use a jeweler's loop to investigate their structure. The hassle is
worth it because once you manage to create a good jet it is a
fascinating phenomenon to observe.
One great
disappointment was in attaching a sharp point to the Van de Graaff
generator's dome to create St. Elmo's Fire: a corona discharge.
Whether the point was on the generator or the discharge electrode and regardless of how close these were to each other this glow was exceedingly tiny and dim, barely a pinpoint of light. The image above is many times larger than the actual glow.
When attached to the negative electrode I did notice a substantial flow of air coming off the sharp point. You can easily feel it on the palm of your hand. Arcing to my hand wasn't an issue because the sharp point bled off charge so quickly to the air that arcing to another object (me) didn't happen. The same could not be said when the sharp point was mounted on a grounded discharge electrode. Although it too produced a breeze, because the electrode had to be placed within a foot of the Van de Graaff generator to induce a charge in it, arcing to my hand was an unpleasant issue. I assume if someone had a positive Van de Graaff generator this wouldn't be a problem.
(The photos
above were taken with a Canon 20D, 8.2 megapixel camera with a Canon
100mm macro lens working at f2.8. The camera's ISO was set to 3200.
I'd set the speed of the Van de Graaff generator's belt to produce
one discharge every second, open the shutter after a discharge and
use the next discharge itself to create the exposure, then close the
shutter. Needless to say this was done in a dark room. For the
continuous jets I used timed exposures ranging from 2 to 20 seconds.
Setting the color temperature to 2800 K seemed to help capture finer detail.)
Replusion
Demonstrations
Objects with the same electric charge repel each other. This phenomenon can be used to create a wide range of demonstrations using a Van de Graaff generator.
Flying
Saucers
Well... almost. Stack aluminum pie plates on top of a Van de Graaff generator and they will lift and float off one at a time.
Ten to twenty seconds after turning on the generator the top pan lifts off, fly up, flips over, and falls toward to floor as the next pan starts its liftoff. What's happening is that the top pan picks up a charge and is repulsed by the generator and the pans under it. As soon as it is gone, the next pan is also repulsed, and so on until the entire stack has taken off. A stack of eight takes about 30 seconds to fly off, some getting as high as one foot above the generator. The slower the generator's belt is turning the slower the pans launch themselves into the air. I tried several different sizes of foil pie plates and found the small, 5-inch diameter pans to work the best. This demonstration also works if the pans are right side up. But, they just lift off and quickly slide off the one side and fall the the floor. Placing them upside down gave me higher launches and their flipping over made the demonstration more interesting.
Rice
Krispies Volcano
Tape a small plastic bowl to the top of a Van de Graaff generator, put a small handful of rice krispies in it and turn on the generator. The krispies will shower up out of the bowl.
The rice krispies pick up a charge from the generator the same polarity as the generator and are repulsed. They fly upward as far as two feet. I tried small styrofoam balls, small fuzzy decorator balls, confetti, foam packing popcorm and even feathers. Rice krispies always worked the best. Typically the first burst is the highest. After that the krispies shoot upward sporatically. I found it best to start with a small handfull a repeat that several times rather than trying to load the bowl up with a large amount. (This is a messy demonstration so be sure to have a vacuum cleaner handy.) In theory a metal bowl would work. But, when I tried it with a foil pie plate, so much charge leaked off the edges of the pan that the krispies never took off.
HOW
VAN DE GRAAFF GENERATORS WORK
I haven't really started working on this section yet, but while tuning up the Van de Graaff generator I purchased I took a picture of the roller and comb assembly inside the aluminum sphere while the generator was running. I decided to post it right away because it provides an interesting image of the mechanism of charge transfer.
The following image captures the arcs between the belt and comb:
The belt in this picture is coming up from behind and is covered with electrons deposited on it by the comb at the bottom of the generator. As the belt comes up and over the top of the roller the belt's strong electric field pushes the electrons away from the tips of the comb, leaving the tips positively charged. The field between the belt and comb is strong enough to ionize the atmosphere between them. The freed electrons rush to the comb and charge the Van de Graaff's sphere while the positive ions travel to the belt. These ions, excited oxygen and nitrogen, attach themselves to the belt and are carried down. The persistent glow is because it takes a little time for the ions to attract an electron and give off photons as this electron falls down through the ion's potential energy shells.
This image suggests several ideas for improving performance. Note that the blue ion streaks are dimmer and shorter on the outside. This is because the top roller is tapered toward the ends to keep the belt running in the center of it. But, the comb, a piece of wire cloth, is cut straight. This means the outside corners of the comb are further from the belt than in the center. This greater distance reduces the current being transferred by the arcs. Curving the comb to match the curve of the roller should increase the current on the outer zones thereby increasing the overall current of the generator. There are also dark streaks where no ions are being transferred. Optimizing the spacing of the comb tips should again increase the overall current. Ideally the entire belt should have an even, bright, solid glow showing that every zone is being used.
Much more coming soon!
MY
HOME-MADE VAN DE GRAAFF GENERATOR
Coming soon.
IMPROVING
THE PERFORMANCE OF VAN DE GRAAFF GENERATORS
Coming soon.
Coming soon.
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