From: SFPhysics@aol.com
Date: Tue Jan 08 2002 - 03:32:46 PST
From: SFPhysics@aol.com Message-ID: <d2.11ed4b09.296c32de@aol.com> Date: Tue, 8 Jan 2002 06:32:46 EST Subject: Van de Graaff
> Hi Pinholers, I just inherited a van de graff
> generator and am not sure how to use it..any dangers?
> any good demos? thanks Dan
Hi Dan:
You didn't say what size generator you have. The small diameter 3" units are
virtually harmless and you can draw sparks to your knuckles with no problem.
The 7" diameter units now being made and sold can give you a good jolt but
nothing that has enough current to injure you provided you don't have a
pacemaker. You can draw a spark and not even feel it if you hold a metal
object to take the concentrated discharge instead of using your bare flesh.
The 3" unit can hold a charge of about 75 kV, the 7" unit can reach 200 kV,
and the 10" unit can go up to 400 kV which gets into a dangerous range not
because of the voltage but because the current goes up with the extra storage
area of the 10" collector dome. (Good for teaching Coulomb's Law) For
discharging the largest unit a discharge electrode with a ground wire is
highly recommended.
To get the highest voltage out of the unit the insulating column must be kept
clean of fingerprints and oils. Likewise the rubber belt must be kept clean,
not handled with bare hands, and the rollers should be kept clean with
Windex. The dome may also be cleaned with Windex. Rubbing alcohol contains
oils and should not be used to clean the belt, rollers, column, or other
parts. When not in use it is best to take the tension off of the belt by
unhooking the belt from the rollers. Dust kills the operation of the unit
and so the generator is best stored in a plastic bag that covers it
completely. Scratches on the dome collector will also degrade performance
markedly. A drop of high quality machine oil (not 3 in 1) on the motor
bearings is advised periodically and the roller bearings should not be oiled
unless absolutely necessary. Top and bottom charge collector wires should be
close to the belt without touching, 1 mm is a good distance. The output will
vary with the humidity but a clean unit will still give decent service even
on rainy days.
Experiments are many and all are classic. On a good dry day a student
standing on a plastic step stool can get their hair up in the air while
holding onto the dome. Start with the unit off and discharged then have the
student put their hand on the dome after which the motor power is turned on.
The charge will build and if the child's hair is clean the static will make
it fly up. Similarly, the Electric Pompom can be made using party store
metalized mylar "icicles" connected to a suction cup. Volta's Hail Storm can
be done with a plastic jar filled with pieces of plastic foam, cork pieces,
or pith chunks, etc. A string of NE-2 neon lamps can show a discharge as
well as making your own lightening plate by pasting aluminum foil on a
plastic sheet. Smoke put into a plastic bottle will magically disappear when
a charge is placed on a pin driven through the cap. If you are handy with
tools you can even make an electrostatic bell ringer where a keychain with a
small metal ball is suspended between two brass bells and a charge from the
Van de Graaff will make the ball alternately strike each bell. A cut up
aluminum pie plate can be made into an electrostatic spinner by balancing it
on a pin driven through a suction cup placed on the top of the collector.
The aluminum is "S" shaped with the points of the "S" being sharp points
which will cause an electrostatic wind and drive the spinner around its
centered pivot bump sitting on the pin. The pin by itself is good to show
St. Elmo's Fire in the dark where the concentrated charges leave the end of
the pin like Benjamin Franklin's Lightening Rods. You can also feel the
electrostatic wind from some distance away. The things you can do with a
high tension static machine are extensive and fascinating. Check out a Cenco
or other science catalogue to see the simple electrostatic items they sell
for a goodly sum but that you can make from cheap items almost for free.
Best of luck,
Al Sefl
SFPhysics@aol.com
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