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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.
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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