Lightning in Your Hand (Oct, 1946)
Lightning in Your Hand
EXPERIMENTAL MODEL OF A VAN DE GRAAFF ELECTROSTATIC GENERATOR CAN DELIVER OVER 100,000 HARMLESS VOLTS
By Loren W. Gay
NUCLEAR physics, the unpredictable baby of the sciences, is about 50 years old. For the first two thirds of its short life it crawled patiently along on all fours. Then, without bothering to walk, it started to run. Just where it’s running to, no one knows, but it has already revolutionized man’s conception of his universe without even stopping for breath.
One of the mileposts in this swift race was the original of the electrostatic generator shown here. Neither surprising nor dramatic in its origin, the device was invented simply because it was needed: physicists wanted a controlled source of high-voltage D.C. in order to bombard the nucleus of the atom.
In 1931, Dr. Robert J. Van de Graaff went back to first principles to build a machine capable of delivering nearly 2,000,000 volts. Instead of rubbing amber and silk as the Greeks had done 2,000 years before, he revolved an endless belt in contact with metallic brushes.
The remarkably powerful model described in this article was designed chiefly as a laboratory-demonstration instrument. It is very simple in construction, and its exposed lower section permits a clear view of operation as well as effects. Before starting to build, procure the roller shafts, ball bearings, and the bakelite tube. If these vary from the dimensions given, you may have to alter some of the other specifications so that the parts will fit together properly.
Hard composition board was used for the two sides of the ground-pulley housing. Saw two pieces roughly to size, clamp them together, and drill and jigsaw the windows, slots and holes in both at once to insure perfect alignment. Slots for the pulley shaft should make a snug fit to prevent vibration, and the slots that will hold the ground-inductor glass must be cut so that the glass will be perfectly parallel to the silk belt and in light contact with it. It may be easiest to cut the glass first and fit the slots to it.
Cement and screw the base, sides, tube, and the two lower wood rings together. The belt tighteners and the uprights for the sphere pulley are also made of composition board clamped together and finished in pairs.
For the rollers, use a good dry wood that is not subject to warping or cracking. Rough them on the lathe, then screw to the faceplate a scrap piece of 1″ stock, and in it turn a hole that will make a tight press fit on one end of the roller. With the pulley chucked firmly in this hole so that you can work on the free end, turn a centered opening just large enough for the ball bearing to be pressed into place. Bore the shaft clearance hole part way through, using a skew chisel. Repeat these operations on the other end, and complete the shaft hole with a drill.
Prepare the other pulley in the same way. Test both rollers to make sure that they run true. Press the bearings and shafts in place, and clamp one shaft tight between the lathe centers. Belt the pulley to a motor so that it will spin on its bearings, and carefully turn the wood down to a uniform diameter of 2″. Turn a groove in the ground pulley for the driving belt; the sphere roller, of course, needs no groove. If the pulleys are out of true in any respect, it will be worth while to discard them and try again. Quality ball bearings good for a speed of 6,000 r.p.m., with an outside diameter of are recommended.
When assembling the pulleys on the shafts and on the machine, use washers and short pieces of metal tubing to space them out between the supports. Align the rollers with extreme care, and cement the upper ring on the tube.
Pure silk ribbon would be the ideal choice for the belt, but you’ll probably have to settle for rayon. Set the tighteners to allow take-up, place the ribbon around the pulleys, and cut it to size, allowing about lap. Remove the belt, cement the ends together, and clamp the lap between blocks overnight. When the mechanical parts are in satisfactory running order you can proceed with the electrical parts—the brushes and inductors. Inductors are made of sheet aluminum or similar metal, cemented to glass plates. For the sphere inductor, cut a 2-1/4″ by 2-1/2″ piece of aluminum, bend it to a right angle, and cement it to a 2-1/2″ by 3-1/4″ sheet of thin window glass. Both are then cemented to the edge of a piece of composition board which is slotted to fit between the pulley-supporting uprights. Position this piece so that the glass plate will be parallel to and make light contact with the belt. A brush, which must make electrical contact with the inductor, is bolted to the same composition-board strip, and touches the ribbon at the opposite end of the roller. The sphere brush completes the upper assembly; it is located directly behind the inductor and is separated from it by the ribbon and glass.
To assemble, install the sphere brush, and drop a loop of the belt through the bakelite tube; then slip the inductor assembly in place between the uprights, and place the sphere pulley and shafts in the retaining slots. Insert the ground pulley through the belt, and adjust and tighten to run true. The ground inductor and its brush are made in one piece and cemented to the lower glass plate, which is placed in slots in the housing and held firm by wood disks screwed to the case to overlap the edges of the glass.
Brushes are made of sheet aluminum or copper (medium-size variable-condenser plates will do nicely) and short pieces of extraflexible stranded-copper wire or tinsel. Drill an even number of small holes parallel to one edge and spaced about 1/4″ apart. Loop a length of the wire through each pair of holes, and bend the metal back on itself to clamp the wire in place. Trim the edges to a straight line. Brushes should be cut to come as close to the belt as possible without rubbing or catching appreciably.
The sphere is a 7″ metal map globe with an opening 4″ in diameter cut in the bottom. File the edge of the opening so that the globe fits the groove in the upper ring; if the fit is good, no other fastening is needed.
Shellac or varnish all wooden parts of the machine; avoid paints, since some pigments are metallic and will reduce efficiency. When the generator is ready for trial, screw it to a solid base and belt the lower pulley to a 1/4-hp. motor so as to drive it at about 3,000 r.p.m. Connect the copper ring and the ground brush or its terminal to a good external ground.
With the motor running, touch the belt lightly at the “charging point” with a rolled edge of celluloid or photo negative. Friction will knock free some of the electrons in the belt, imparting a positive charge. The missing electrons, however, are immediately replaced by the inductor brush, creating a potential between the latter and the ground brush. Between these oppositely charged plates, the glass acts somewhat in the manner of the dielectric of a condenser. Free electrons are attracted from the ground onto the ribbon, which promptly carries the negative particles upward through the tube and into the sphere. Three or four complete revolutions may be necessary to build the charge on the sphere inductor to its saturation point, after which most of the electrons are carried around to the sphere brush where, by repulsion of the sphere inductor, they are kicked off into the metal globe. The celluloid charging sheet may then be removed and the generator will continue to build its own cycle.
Once the sphere is fully charged, a number of interesting effects may be observed. Some are illustrated on these pages; others may come to you as a bit of a surprise. If yon venture close to the generator when it is in proper operation, the tingling sensation you may feel will probably be due to hair standing up on your head or forearm. You can draw a spark to your finger or to any metal object held near the sphere. In a darkened room you will be able to see the static electricity streaming from the ground brush onto the belt, and from the ground inductor up over the glass plate to the belt, or from one side of the belt to the other.
Measurement of actual voltage is extremely difficult, so no attempt was made to obtain a proper rating of this unit. Under favorable conditions the spark effects that are observed suggest an output in excess of 100,000 volts.
Favorable conditions, by the way, include a dry climate, since moisture encourages the charge to leak off into the atmosphere. If you live in high, dry territory, you can get results the year round. If not, you’ll have to do your experimenting when the weather man gives you the nod, or when winter’s steam heat cooks the moisture out of the air.