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Van de Graaff Generator
Hints, Demos, & Activities

©1994 William J. Beaty




MOTOR MAGIC
Build the pop-bottle electrostatic motor from plans elsewhere on my web pages. Use your VDG as a power supply for the motor. If you ground one bottle of the motor, stand on a plastic insulator, touch the VDG sphere, and point at the other bottle of the motor, the motor will start to turn. It is powered by the charged wind coming from your fingertip. Initially hold your finger a couple of inches from the bottle until you get the demo working. I managed to slowly increase the distance to the motor, retuning the motor brushes each time for best operation, until I could point at a motor which was four feet away! By instead using a paperclip taped to the VDG sphere and bent so it pointed at the motor, I managed to run the bottle motor from 10 feet away! This was in dry weather, when the VDG was working very well.
 

VDG VERSUS PC

A VDG is a constant current source. During normal operation there is a large e-field around the device, but there is also a flow of charge between the sphere and ground. This flow is composed of charged air, and while some of it manages to get to ground, much of it is attracted to surfaces, and lots of it travels far beyond the region immediately around the machine. If the humidity is low, it will build up on insulating surfaces where it creates large e-fields and high voltages. In other words, an operating Van de Graaff SPEWS CHARGED WIND which wanders around charging up EVERYTHING in the room, including the walls, ceilings, people, etc. The fans on your PC will suck the charged air into the case, where it will electrify all non-metal surfaces, cause huge electrostatic fields and sudden sparks, and generally trash circuitry left and right. It also tends to collect on ungrounded plastic parts such as keyboards. It is wise to avoid doing VDG demos in the same room with an operating computer. If this cannot be avoided, then keep your demo short, keep the VDG as far as possible from your computer, VCR, etc., keep the VDG turned off or shorted with a ground clip except during actual use, and avoid days with extremely low humidity. Better yet, do a Tesla coil demo instead!

"Giant" VDG Lightning

If you have access to a second large VDG sphere, you can create immensely long, dim sparks. (Remember, spark length is only limited by the voltage if the radius of electrode curvature is much larger than the gap between electrodes, and small electrodes can create VERY long sparks.) Connect the second sphere to ground, and position it about 6in from the VDG sphere terminal. Affix to the grounded sphere a 1/4 in. ball bearing, or an "acorn" type 8-32 nut with a spherical head. Position the spheres so that the nut is in the gap between terminals. When the VDG is run, the nut will initiate the spark, and the field between the terminals will provide energy to allow quick growth. Spark brightness decreases as length increases, so turn off the lights. In a darkened room, increase the separation between the spheres until you have long sparks. In this way I've occasionally managed to produce 24 in. sparks from a Science First 14" sphere. If you instead glue a ball bearing to a thread and lower it between the spheres, it will trigger the lightning in the same way that aircraft can trigger lightning when flying near storm clouds.

Turn off VDG without pain

Turning your Van de Graaff machine *on* is no problem, but how can you touch the metal switch to turn it off without being zapped? There are several methods. If you carry a metal object in your hand, such as car keys, you can touch the switch with the metal object, hold it there, then turn off the switch with fingers. The painful spark hits the metal, not your skin. Another way: hold onto a grounded wire when you turn the machine on, and never let go of ground. All objects near a VDG will become electrified because of the charged wind emitted by the metal sphere, but if you keep touching a ground wire, you will stay neutral, and will not receive a shock when touching the switch. (Note that it does not work to simply ground the VDG power switch, it is YOU who must be grounded.) And, in a pinch, it helps to just whack the switch with the palm of your hand. You'll still get zapped, but it hurts less when it's not your fingertips, and when you do it fast.

Sky Pie

Place a small aluminum foil pie pan upside down on the dome of your VDG. When you turn on the power, it will levitate and fly off to the side. Alike charged objects repel each other. Big thrills? But wait, what if you place TWO pie pans nested on your VDG? When you turn on power, the top one holds the bottom one down, until the top one flies off, which then allows the bottom one to take off. SOOOO, place an entire stack of thin foil pie pans upside-down on top of your generator, and get ready for a pan storm. When run, your generator will loft each pan in sequence and fling them in various directions. This works best with those little "pie tart" pans about 10cm dia.

DANCING BUBBLES

Blow soap bubbles at your VDG terminal. They will initially be attracted, but then will become charged by ion wind and will then be violently repelled from the generator sphere. They will also be attracted to any other object. With practice you can hold your hand above a charged bubble and keep it aloft by attraction.

ELECTROSTATIC LEVITATION

Place a large metal sheet or foil-covered cardboard on the ground. It should be at least 2 to 3 times the diameter of your VDG sphere. Connect this sheet to earth-ground. Place some small crumpled pieces of foil on the center of the sheet. Pick up your entire VDG machine, turn it on, and while holding it by its base, move the sphere down towards the crumpled foil. With practice you can get the foil to levitate and hang in the air between the sphere and the ground plate. The VDG attracts the grounded foil, but then the corona discharge from the edges of the foil chunk will form a conductive path in the air which allows the metal to acquire a like-charge from the sphere, which increases the repulsion force. As the foil drops away, it loses its charge via corona, and is again attracted upwards. At a particular distance you can get a piece of foil to hang unmoving in space with balanced attraction/gravity forces and continuous corona leakage. (Note: there may be ion-wind filaments associated with this phenomenon. Someone should do the foil-lifting experiment in front of a Schlieren system and look for grey lines in the air.)

MAGIC BLINKING WAND

Connect a small capacitor (.01uF, 250V) in parallel with a small neon pilot light (NE-2 or NE-2H). Hold one wire between fingers and bring the other wire towards the sphere of an operating VDG. The wire will intercept part of the ion current flowing from the sphere, and the bulb will begin flashing. DON'T touch both leads at once, or you will get a shock from the capacitor. For safety, you can connect the two leads of your device in series with 1-meg resistors, then cover everything except the floating resistor leads with insulating caulk. For dramatic effect you can mount this assembly in the tip of a metal wand, with one lead connected to the metal. The closer you bring the wand to the VDG, the faster the neon bulb will flash. Use a larger capacitor for slower, brighter flashing, or a smaller one for a fast, dim flicker.
                     .01 uF 250v

                   ______| |______
                  |      | |      |
     O---\/\/\----|               |----\/\/\---O
         1 meg    |_____/| |\_____|    1 meg
                        \| |/

                      NE-2 BULB



FAUX - DOO

Sometimes the humidity is too high, and even though your machine does give sparks, the "hair raise" demo doesn't work. There is just too much leakage to ground. All is not lost, you can cut up some strips of tissue 2cm x 15cm each and tape them all over your VDG sphere. When operated, the tissue strips stand out just fine. Note that sometimes this will fail during *low* humidity because the paper strips become good insulators, so they attract to the metal sphere and do not become alike-charged. I discovered that drawing a line on each strip with india ink can help. India ink is somewhat conductive.

Placing tissue-strips on the sphere is also useful as a visual indicator of voltage. If the strips do not rise, you know that something is wrong. If you are experimenting with e-motors, or designing your own VDG, etc., tissue strips are just as good as a voltmeter for detecting heavy loads, total shorts, bad rollers, etc. If you are de-humidifying a dead machine with a hair dryer, attach tissue strips then operate the machine while warming the belt. When the tissue strips suddenly rise, you know you're successful.

Sometimes the humidity is nice and low, yet your audience will have no long-hair 'victims.' In this case simply whip out your halloween costume 1960s "Cher" wig and stick it on the sphere-terminal. And if the *demonstrator* lacks hair, this opens up an opportunity for a variety of humorous banter and setting your audience up by donning an unobtrusive long-hair hairpiece before the demonstration... ahem!


CRACKLE'S HAILSTORM

This is the hands-on version of "Volta's Hailstorm". Pour a small pile of Rice Crispies on top of your VDG and turn on the power. Be prepared for a mess! If your machine is fairly powerful, you can try standing on an insulator, touching the VDG terminal, then extending a handful of cereal (flat palm, fingers spread hard) and they will levitate and fly to the nearest uncharged surface (your audience!) Note: a totally fresh box of Rice Crispies will have such a low humidity that the cereal will be insulating and won't acquire a charge from the terminal. If you open a new box of cereal before a demo, spray a bit of water into the box and shake well to distribute the moisture.

PLUS OR MINUS?

To determine VDG polarity, turn off and discharge your machine, then connect a sensitive ANALOG current-meter between the sphere and the base. (Note that Digital microamp meters can be destroyed by accidental discharges, so use a moving-needle meter instead.) When running, the VDG will produce a few tens of microamperes in the same direction as the high voltage polarity. If you lack a microamp meter, a tiny NE-2 neon pilot light will serve. When connected between sphere and base, one electrode will glow orange when the machine is run. The orange electrode is the negative one, the dark electrode is positive.
Analog microamp meters are useful for other things: when repairing a VDG or building a new one, measure your machine's output current rather than the voltage. Connect the microamp meter leads to the upper comb and the lower comb (or simply tape the meter leads to the sphere and the grounded motor assembly.) Run your machine and tweak the comb spacing for maximum meter reading. The higher the current, the faster the recharge rate, and the better it will work during humid days. Ever wondered if other types of roller or belt materials would work better? Well, try the alternate materials and see if they give higher microamp readings! FYI, typical readings are 5 uA (microamps) and up. A healthy VDG might give 10 to 20 uA. A fast machine with a very wide belt might give as much as 300 uA.

PAPERCLIP RAY

Tape a short piece of wire or an unbent paperclip to the side of the sphere of your VDG machine. Bend the wire so it points outwards. When the VDG is running, a stream of charged wind spews forth. This stream is a genuine Ion Beam. It will electrify distant surfaces, charge whole people if they are standing upon an insulator, and will run e-motors and fluorescent tubes at a distance. When this air gets on your clothes and the humidity is low, it makes them cling to your body as if wet. Also, it feels surprisingly cold. The air is attracted to conductive surfaces, and this disrupts the usual "boundary layer" of air which insulates the surface (ionized air has larger "wind chill" than normal.) Warning: never direct the ion beam towards a computer, it can induce electrostatic discharges INSIDE the computer case and keyboard.

CONDUCTIVE "INSULATORS"

A surprising number of "insulators" behave as conductors when used with a VDG machine. Wood, cardboard, paper, twine, floors, and shoe soles can all behave as conductors as far as a VDG machine is concerned. Why? Well, consider a 12-volt, 1-amp, 12-watt flash lantern. In this device the light bulb has a resistance of 12 ohms and the wires contribute nearly zero ohms. As far as the flash lantern is concerned, material which is far more conductive than the 12-ohm lightbulb is behaving as a conductor, while anything far less conductive is behaving as an insulator. Now look at a Van de Graaff machine. A typical output is 30 microamps at 300,000 volts, giving a load resistance of V/I, or 10,000,000,000 ohms. As far as your VDG is concerned, "insulators" must have far more resistance than this. And "conductors" have far less. If a piece of wood has a billion ohms of resistance from end to end, your VDG machine will "think" that it is a conductor!

THREADLIKE FLOWS OF ELECTRIC WIND

When placed in a strong e-field, human hair, eyelashes, and other sharp objects create tiny coronas which emit "electric wind". These invisible flows of air are extremely narrow and rapid, and their effects can be made visible by using dry-ice fog. Materials:
  - VDG machine
  - wire and tape (or clip-leads)
  - Tray of warm water sitting on insulator
  - chips of dry ice
  - dark paper (submerged in the water for contrast)
Procedure:
Drop several CO2 chips in the water so that a thin layer of fog forms.

Use tape and a wire to connect the tray to the sphere of your VDG. Charge the tray with respect to ground.

Move your hand slowly over the fog, keeping your hand a few inches above it. You'll see small mysterious furrows being carved in the fog by the invisible, narrow threads of "electric wind."

If your hands are extremely clean (no sharp microscopic defects), try wetting your fingers and brush them across fuzzy clothing to pick up some microscopic lint. Or instead try waving a torn bit of paper over the mist. The sharp paper fibers seem to generate these "threads" of charged air fairly well. If humidity is very low, then perhaps the paper should be made moist.

diagram of 15KV supply, water tray w/dry-ice chips Wave your hand fast, and the spots in the mist will follow your hand's motions. Pull your hand back, and the spots still appear.

Form a "thread", then wave a charged object near it. The spot in the mist moves, indicating that the "thread" is being deflected.

Use a soda straw to blow hard across a "thread". The corresponding spot in the mist will move only a small amount!

Drop some short (1cm) pieces of hair onto the charged water surface. They will stand on end, emit "threads" upwards, and narrow flows of entrained mist will be seen to project upwards from the fog layer.

Also see Air Threads article.

ZAP GUN

If you can locate a zerostat, a "Zerostat (tm)" record-cleaning gun (Discwasher Inc.), you can perform the following. Tape strips of tissue all over your VDG sphere. Turn it on so the strips stand out, then turn it off. The strips remain standing. Now "shoot" the sphere with the Zerostat gun. The strips will collapse!. This "gun" contains a Barium Titanate piezo crystal connected to a sharp needle in the gun's tip. Squeezing the trigger send a few microamps of charged wind out through the gun's tip. Ionized air is a conductor, so the presence of ionized air near the generator allows the charge on the sphere to leak away.

THE TERM "STATIC ELECTRICITY"

When explaining Van de Graaff machines it's probably a good idea to avoid the words "Static Electricity." A VDG machine is simply an electric power supply which has a characteristic of high voltage and low current output. This is in contrast to a dry cell battery. Dry cells are electric power supplies which give high current and low voltage output.

While it's true that electric charge, charge imbalance, voltage, current, power, and energy exist, it is NOT true that there is a "stuff" called Static Electricity. Just because voltage and current may vary, that's no reason to invoke a new kind of "electricity" called "static."

Van de Graaff machines and batteries do not differ as much as we might think. After all, if enough VDG machines are connected in parallel, their currents add up and they can light a normal incandescent bulb. And if enough dry cells are connected in series, their voltages add up and they can attract lint, raise your hair, charge your body, cause corona discharge, and make giant sparks.

Electrostatics, or "Static Electricity," is a class of effects in the same way that "biology" or "weather" are classes of effects. Your hand is "biology", yet your hand is not made out of biology. Clouds are "weather", yet clouds are not composed of weather. And, while scuffing your shoes on the rug involves "static electricity," scuffing your shoes does not create any substance or energy called static electricity. If we always call it "electrostatics" instead of "static electricity", we won't be so confused about it's nature. If we say "surface charge", we won't be so surprised when it moves or flows ("Static" must be unmoving, right? Wrong, surface charges can and do flow.) Assume that the words "static electricity" breed confusion and ignorance, then avoid speaking them.


EDUCATIONAL USE

The Van de Graaf machine is a fun demo tool in museums, but it's also useful in science teaching. It can be used to demonstrate two important things: electric fields and electric forces. Everyone encounters magnets and magnetic fields, but few are aware that *electric* fields exist. These fields are usually hidden under the label "static electricity" and are ignored. This is unfortunate, since knowledge of electric fields leads to the understanding of sparks and lightning, voltage and circuits, and even the physical basis of chemistry and biology! In the functioning of the everyday world, e-fields are MUCH more important than magnetic fields, yet all the emphasis is placed on the latter. Students have difficulty understanding voltage because voltage *IS* electric fields, and if we don't understand electric fields, we will be befuddled by "voltage." The Van de Graaff machine is extremely useful because it produces electric fields which are strong enough to be measured, manipulated, felt directly, played with, and finally grasped at an intuitive level.

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