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VAN DE GRAAFF ELECTROSTATIC GENERATOR
Main Terminals

(c)1996 William J. Beaty

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MAKE YOUR OWN SPHERE-TERMINAL

There are a number of ways you can build a sphere terminal (also called a toroid or oblate)
          BOOK:
Electrostatic Lightning-bolt Generators
1992 Walt Noon, ppbk pp91.
from the infamous Lindsay Books

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.

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.

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.)

CONDUCTIVE PAINT

Various plastic globes can be found, and these can be coated with conductive paint. One type 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 use rubber cement to burnish down a large disk of aluminum foil 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 diluted carbon paint, that way the painful sparks will simply be impossible, and kids can put their hands on the machine in full operation; raising their hair without getting "zapped."

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.

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.)

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.

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!

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




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:
             |      _      |
             |    /   \    |
             |   |     |   |
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

    1. it is very rough
    2. 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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