1. ANCIENT METHOD FROM 1950s:
Buy a steel world-globe. Cut a hole in the bottom. Sand off all the
paint. A variety of various sizes of metal world globes can be found,
including 1in diameter pencil sharpeners which are excellent for building
ultra-tiny rubberband belt VDG machines.
2. BETTER: USE MIXING BOWLS
Purchase two metal mixing bowls. Cut a hole in the base of one. Join
the bowls rim-to-rim by winding black electrical tape around the rim
several times. There may be some corona leakage from the bowl rims.
This can be reduced by applying a thick (1/4") bead of silicone caulk
to the outside edge of the bowl rims.
Mark W. recently found these excellent rimless mixing bowls at IKEA stores:
Blanda Blank.
They even have wooden ones for antique generator Prime Conductors.
3. IMPROVED METHOD: BUNDT PAN(S)
Connect two identical ring-cake pans (bundt pans) rim
to rim. The smooth
central hole of the bundt pan removes the need to cut a hole in a steel
bowl bottom. If you search around, you might even find a metal mixing
bowl which matches the size of a bundt pan. Good-will stores are an
excellent source for Bundt pans under $2.00. Note: if you're using
two Bundt pans, test-fit before buying. Sometimes the bundt pan's
central column sticks up past the rim. It will prevent the rims from
touching. You want the rims to touch together without
a large gap between.
If you're ambitious, you can increase the maximum voltage attainable by
the mixing-bowl sphere by removing the rims. Cut or grind them off,
grind and then sand the edge of each bowl so it will sit flat (test it
against a flat tabletop.) Be sure to file off all burrs from the edge.
Temporarily attach the bowls edge to edge with tape on the inside.
Obtain some adhesive aluminum foil tape from a hardware store. This is
foil-type duct repair tape with a peel-off waxpaper back. Lay down the tape
on the joint and burnish it down with a plastic spoon. Avoid making
any creases.
Another way to connect the bowls is with conductive epoxy, or epoxy
followed by a layer of conductive paint or aluminum tape. Place the
bowls edge to edge
and wind a couple of layers of masking tape along the gap to seal.
Working through the hole you've made in one bowl, fill the gap with
epoxy, then add a temporary
layer of tape over the epoxy to keep it from running too badly. You'll
probably have to do this in sections, letting one section harden before
starting the next. Remove the tape from the outside and sand down the
epoxy. If you're
using normal (nonconductive) epoxy, paint the epoxy with a coat of
conductive paint (Nickel Print is available from some TV repair stores
and mail order electronics catalogs, see below.)
4. CONDUCTIVE PAINT
Various plastic globes can be found, and these can be coated with
conductive paint. An older source is carbon-alcohol paint called "dag"
or "aquadag." A more modern paint is G.C. Electronics "Nickel
Print",
Number 22-207. It is fairly expensive, but take a look at their Silver
paint, whew!, like thirty bucks for a couple of ounces! Once source
is Allied Electronics (mail order), see their Catalog site
Note: the paint only needs to be very slightly conductive. India
ink or a mixture of carbon black and wood glue will work. However,
if you want to create high-energy sparks, the resistance must be
low, therefore use metal. OR PERHAPS... use carbon paint for
the major part of the sphere, then add a disk of aluminum foil with
rubber cement, and burnish down the foil edges
onto your onto your inexpensive globe. The foil acts as the
spark-launcher On the other hand, if you
want your VDG to be entirely
safe, use highly diluted carbon paint (or perhaps mix some carbon
powder into standard latex or epoxy paint.) That way the most painful
sparks will
simply be impossible, and kids can put their hands on the machine
in full operation; raising their hair without getting "zapped."
5. POP CANS
Cola cans have fairly smooth bottoms. IF you remove the pull-tab of
two cans and tape them mouth-to-mouth with electrical tape, you'll end
up with a very smooth metal object. You can use this to create a good
VDG upper terminal. Take 12 or 16 empty cola cans, bind them mouth/mouth to
make six pairs, then connect them side by side with tape and glue, so
that you make a large cylinder with hollow middle. This forms your VDG
terminal. As long as your upper VDG brush assembly remains entirely
within the hollow part, the cans will function the same as a hollow
metal globe.
6. PAPIER MACHE AND INK
Here's an untried method for sphere construction. I found that india
ink is a fair conductor. If you make a papier mache sphere from
glue-soaked newspaper strips on a balloon, a coating with India Ink
should make the surface become conductive. While the conductivity
might be too poor to generate impressive sparks, it should serve well
for e-field demonstrations (hair raising, etc.)
7. QUICK AND DIRTY
Contrary to what you might think, a sphere ISN'T required in order to
make a passable VDG. All that's really needed is a metal shield around
the upper pulley/comb assembly. This can be accomplished in a variety
of ways: one inverted mixing bowl, a large metal coffee can, four 90deg
elbow furnace ducts mated together (or eight 45deg ducts), a bent piece
of sheet metal, etc.
I highly recommend taking the quick and dirty path, just to speed the
initial construction of your device. Don't let the unavailablity of a
perfect polished sphere stand in your way, use a rusty tin can at the
start!
Some of the above methods give sharp edges which lead to severe corona
leakage and reduced generator voltage. One simple cure is: thick beads
of silicone caulk. For example, if you use an inverted coffee can as
a terminal, simply build up several layers of silicon caulk on the top
and bottom edges of the can. A 1/4 in. thickness is good. Use care
to avoid getting bubbles or leaving gaps in the caulk. Another simple
cure: adhesive foil tape from hardware stores, or even rubber cement
and aluminum foil. If you use rubber cement, coat both surfaces and
let the cement dry, then stick it on.
8. LANCE F. JERALE'S METHOD:
Buy a large balloon from a party/costumes store. Cover it with many
layers of papier mache' (made from newspaper strips dipped in diluted
white glue, mix glue with an equal amount of water.) Many layers are
required, otherwise the surface will be too flexible. When dry, remove
the balloon, repair any folds and loose paper strips with more paper and
glue, dry thoroughly, then cover the sphere with adhesive aluminum
foil tape (aluminum foil duct repair tape from a hardware store.)
Burnish it down with an old spoon, and you've got a beautiful silvery
VDG sphere.
If you're ambitious, you can shape an involute hole in your papier
mache'. Build many layers on a partially inflated balloon. Keep them
wet and soft, push it in to form a deep smooth valley, arrange some sort of
bracket or dangling weight to keep the valley in place until the mache'
dries, then repair the creases and folds with scissors and more mache.
Lance Jerale managed to build a FOUR FOOT DIAMETER sphere using this
method. He used a partially-inflated weather balloon as the form. His
finished machine produced bright & wicked lightning discharges
approximately 3ft. long!
9. Giant foam-stack sphere, by Mark Massey
I am having fun with a home built van de graaff. I am building a sphere
for the collector. It is of an unusual construction. first i layed out
the shape and size on CAD (computer adided design). It is 48" in
diameter and built up from rings of extruded polystyrene foam (pink
house insulating foam).
The rings are cut to follow the shape of the sphere, then epoxied
together (don't use anything but epoxy -other glues will dissolve the
foam). The top and bottom rings have a solid center (not rings but
disks) with a 1" hole. I then inserted a 1" bar through the whole
assembly (at this point it's a stepped sphere). I then mounted the
stepped sphere and axle bar on two sets of saw horses so the sphere
could be rotated against a sanding disk on an electric drill. I used
the drill to sand out the steps in the sphere. I tested the radius with
a form cut from cardboard.
After final shape is achieved, paint the thing with epoxy glue - this
seals the surface and adds dent resistance. Then use "bondo" (auto body
filler) to fill in any depressions and make the surface smooth. Next
use aluminuim foil duct tape to cover the surface.
My sphere is at the bondo stage now. it took 2 days to cut and glue the
rings in to a stack. It took two days to build the sanding jig (axle
bar, sawhorses, etc) and another two days for epoxy coating and bondo
work. I have $65 in 4 sheets of 2" x 48" x 96" foam (from builders
store). $65 in epoxy, $30 in bondo, $10 of sanding disks. the conclusion
is that a HUGE custom sphere can be built for cheep, using only common
hand tools. I am writing u in hopes that u can add this method to the
appropriate websites so others can take advantage of this construction.
It looks to be a lot better than the wire frame method. please mention
my name if u post this methiod (also please excuse the bad spelling -
its late and i have a cold). Also be sure to use the best dust mask u
can buy when sanding - polystyrine foam is not good for your lungs.
- mark massey
10. Oliver Schaefer's mache' and plaster spheres
Hello,
Here some comments to the papier mache -method for building VDG-spheres.
I tried to build some small spheres (d=19 cm) based on a balloon. At
first I marked a half sphere on the balloon, then I covered it with
vaseline at first and then glued some layers (1 ort 2 mm) of paper onto
it. Beeing still a little wet I removed the papier mache half sphere
from the balloon and dried it. Same procedure with the other half
sphere. Next I cut a disk out of 2 cm wood (inner diameter of the
spheres)
and drilled a hole into the center. Herewith you can fix the sphere on a
bar with two nuts. In the top of ONE half sphere I drilled a similar
hole to get the bar through it. So the hole thing looks like this:
----(-| )
I put the two half spheres over the disk and fixed them with some
cramps. Don't forget to fix the bar at the disk before assembling the
sphere! Next step to get a nice round sphere is to use plaster. There
are types on the market which you can use for 10 minutes and some
(doweling-mass) which can be used up to an hour. To get this creamy
stuff in shape i used a stencil made of plastic with the proposed
diameter of the sphere:
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After drying I got it a little more in shape with sand paper. HERE you
should insert a step I didn't think of: cover the hole sphere with
paint, because whenever the plaster gets a little moist it will get
corrosive to metals!!!
Now the conducting surface: I used aluminium foil for that, which I cut
into "orange-shells". I calculated a little and found that they have to
follow a function like:
y=Pi*r*sin(x/(n*r))
with r=d/2 and n the Number of the strips.
I used 30 strips for the 19 cm sphere, because these strips are still
flat and not 3D! I glued the strips of aluminium foil onto the plaster
sphere (also wallpaper paste) very cleanly. On the top I couldn't resist
to set a little circle formed piece of aluminium foil (I wasn't as clean
as I should have been... :)
The result is a 21 cm sphere with a holding bar which is quite
stable(and heavy, ~ 3 or 4 kg) This kind of spheres is quite usefull for
bigger measurement spark gaps and so on.
These days I trie to build a ~60 cm sphere with involute hole.
For this I built up a wood construction with 6 corners.
Onto the corners I fixed some thick (4mm) steel wire, in the shape of
the final sphere. so I get a wire sphere.
Next I used some wire rings (steel, 2mm)of different diameter (soldered
together at the ends). and fixed them in position with a little bit of
smaller electric wire, twist or whatever.
Then I covered this network of wires with some not too soft wire mesh
cut into pieces looking somehow like the "orange-shells". I fixed the
wire mesh by sewing. I just covered it with some layers of newspaper
ripped into strips of ~15x50 cm with much paste. During this night I'll
dry it and then use plaster to get it in real good shape. Maybe I'll
send a picture of production...when I'm not producting too fast ;-)
A last idea for involute holes: use a swimming ring as basic form - also
I didn't try it yet (because I started to use the more professional
wire-and-wood- construction, used in theatres and elsewhere) it should
work well.
I do not recommend to glue the aluminium foil directly onto the papier
mache
- it is very rough
- it is quite instable (in thin layers)
Maybe one can get better results by covering the papier mache with thick
layers of paint at first, but I don't know...
Always keep in mind that great constructions require stable
underconstructions for example the famous Roundhill-VDG (built among
others by van de Graaff himself) had two spheres (d=4.55m) weighting 5.5
tons! They where mounted on Textolith-columns of l=6.7 m d=1.83m and
wall thicknes 1.6 cm!
Using papier mache and plaster I found to be the best method (except of
using complete metal spheres) to build well formed spheres for
electrostatic purpose, when it is done carefully and with quite a
quantum of time.
With best regards,
Oliver Schaefer, Germany.
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