Midget Van de Graaff GeneratorDevelops Up to 50,000 Volts

Build the world's tiniest workingmodel for $10 and run it on drycells to perform electrostatic ex-periments for your science fair

project

By HAROLD P. STRAND

Craft Print ProjectNo. 315

Repulsion and attraction forces at work. Many strips of facialtissue taped to the sphere get a similar charge from the gen-erator and stand rigidly apart. But point your finger, whichhas an opposite charge, and they'll reach over and grab it as

though they were alive.

SCIENCEUNTIL we see a

smaller one that ac-tually works, we willcall this the smallestelectrostatic generator

in the world! Standing but 6-1/2-in. high, itwill develop 30,000 to 50,000 volts, dependingon humidity. Yet the current is so small,there is no shock hazard.

Operating on 3 volts from two dry cells, itwill perform the fascinating "satellite" and"electric wind" experiments as surely as itsbig brothers (400,000-volt model, Craft Print301, and the 150,000-250,000-volt model, CraftPrint 283). With the midget, you can demon-strate many variations of repulsion and at-traction as in Figs. 1 and 2. And by adding aspark gap electrode and wire capacitor youcan produce a continuous series of lightning-

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like discharges as in Fig. 3.If you're used to working with small parts,

construction will not be difficult, using main-ly hand tools. Many of the pieces may wellbe in your scrap and you can obtain the slot-ted plastic tube and other plastic parts for$3.95 (see Materials List). Cost of all mate-rials for the generator alone should averageabout $10, plus batteries.

Dr. Robert J. Van de Graaff invented thegenerator which bears his name in 1931. Eversince, laboratories and research organizationsthroughout the world have used it in variousforms and sizes, including the multi-million-volt machines employed in atomic research.

How the Midget Works. Since staticcharges are created whenever two dissimilarmaterials are repeatedly brought togetherand separated, the subminiature model devel-

SCIENCE AND MECHANICS

ops its charge by passing a rubber belt overan aluminum foil-covered plastic pulley (Fig.4). Charges are carried by the belt inside theplastic tube and up to the sphere where theyare picked up by a comb brush and directedto the outer surface. The charges build upto a high potential until they are either dis-charged by jumping the spark gap providedat one end of the base or taken off and di-rected elsewhere for experimentation.

Constructing the Generator. Cut out thebase from aluminum sheet as in Fig. 6, drillall holes indicated and bend to shape. Attachsmall end tabs to sides with small rivets tohold lower corners together. Shape a clampfrom soft brass or aluminum for a snug slidefit with a 1/8 in. rod as in Fig. 6A and rivet itto the end of the base nearest the columnlocation.

Now invert the base and position a #55Cer-Mag d-c motor as in Fig. 7A, so that a3/8-in. section at the end of the shaft is cen-tered over the 7/8-in. hole. Locate and drillmounting holes, then attach motor to the basewith 4-40 x 1/4-in. binder head machine screwsand nuts. Install insulated banana jacks inthe two 1/4-in. holes on one end and solder amotor lead to each as in Fig. 7A. Insert a6-32 x 1/4-in. binder head machine screw inthe tapped hole between jacks to hold thewire capacitor when an experiment calls forits use.

Make the small bracket for theground brush as in Fig. 7B and solder ashort piece of copper or bronze screenwire at the free end. Mount brush as-sembly to side of box as in Fig. 7A.Return base to upright position for in-stallation of the column.

Column and Pulleys. If plastic partsare easily available, you can cut andshape the generator column and pul-

Husky spark, 3/4-in. long and representing about50,000 volts, occurs when spark gap electrode andcapacitor accessories are attached to energized gen-erator. Electrode alone produces thinner spark, dem-

onstrating effect of capacitance.

MAY, 1961

leys as in Fig. 8A and C; however, you canget all the pieces slotted and drilled for in-stallation in a package deal for $3.95, (seeMaterials List). The same source offers analternate package at slightly higher cost tosimplify the installation by eliminating theneed for stop rings.

Place lower end of the column, with plasticstop rings cemented in place as in Fig. 8Aor ends turned as in Fig. 8B, in the base holeto make a tight press fit as in Fig. 5. Addsome Pliobond cement at the joint to assurea tight job. Turn the column in the hole be-fore the cement sets to align pulley slots atthe top with motor shaft below the hole.

Pulleys will be cut to correct length andcenter drilled as required, if you order oneof the plastic kits specified, but you can makeyour own as in Fig. 8C if you have 3/8 " plas-tic rod stock and can drill the shaft holesperfectly centered. Cut a 7/8 " length of 1/16-in, dia. smooth steel rod for a press fit in theupper pulley hole. Center pulley on shaftwith spacers of brass tubing about 5/32-in.long to leave just enough of the shaft exposedat each end to fit in the column slots,

Cover the upper pulley with a turn ofblack plastic adhesive tape and butt ends asin Fig. 8C. Add two additional turns of thetape cut 1/8" wide and applied the same way

at the center. Thiscrowns the pulley tohelp keep the belt inplace. Apply tapesmoothly by rubbingit down with a smallround object.

The lower pulleyshould be drilled for apress fit on the motorshaft, but before in-stalling, cover it inthe same manner asthe other pulley, sub-stituting aluminumfoil adhesive tape forthe black tape. If youdo not have the ad-hesive product, usestrips of aluminumfoil cemented on withPliobond. You cannow press the pulleyon the motor shaft, be-ing sure to center itunder the column. Im-portant: Wipe all fin-ger marks off the foilwhen pulley is in

148 SCIENCE AND MECHANICS

place.Make the belt out of

thin rubber sheet as inFig. 8E and install iton the pulley so thatoutside of cementedlapped joint will nottend to catch on thelower ground brush.When stretched inplace, the 7/16-in. widebelt will narrow downto pulley width. Ifnecessary, trim wire atend of lower groundbrush to maintain 1/32in. clearance from thebelt.

Now is a good timeto test the rotatingparts. Apply a trace oflight grease to upperpulley shaft for lubri-cation. Hook up twoNo. 6 dry cells in se-ries, connect leads tobanana plugs and in-sert them in the jackson the base. The beltshould travel clock-wise around the pul-leys—ground brushmay be on either side.If not, transpose theplugs to reverse themotor rotation. Marka plus sign on the basebelow the jack con-nected to the positivepole of the battery.

Make sure the beltrides centered on thepulleys by adjustingalignment or thecrowns on the pulleys.The motor should runat high speed and allparts work smoothly.

Next, make the up-per brush assembly asin Fig. 8D. Curve athin, 2-13/16 " long stripof hard brass or phos-phor bronze into asmall spring ring,overlapping the edges. Expand the ring justenough to slip over the column and fit snug-ly against it, as in Fig. 5. If ring still overlaps,cut off excess to butt ends.

Cut out a 3/8 x 3/8 " piece from thin brassshim stock for the brush and solder one endto the edge of the ring as in Fig. 8D. Turnring in position until brush is parallel to thepulley on the side where the belt travels up-ward. Now bend the brass shim at a right

Small world globe makes a good sphere (top to bottom). After paint is removed,hole in bottom half is enlarged with duckbill tin snips to about 13/l6-in. diameter.Pair of hammers and a homemade die put a smooth, well-rounded edge on the

hole for a press fit on plastic column.

angle toward the pulley. Trim the edge asneeded to clear belt by 1/16 " and cut teethin it with a small scissors.

High Voltage Terminal. A small metal-globe pencil sharpener (Fig. 9) commonlysold in 5 and 10 stores and at stationers for19 to 39 C is adequate for the all-importanthigh voltage sphere. Look for one about 1-5/8in. in diameter and with smooth sides, ratherthan a raised ridge, at the equator.

MAY, 1961 149

in Figs. 5 and 12B. This should be a press fit.If sphere is loose, tap the metal in or out asneeded to tighten the joint. Replace topbrush, slipping ring support down against thesphere. Complete the generator assembly byrejoining the two halves of the sphere.

No switch is provided to control the motorbecause of the limited space in the base. In-stead, you can start and stop the motor byinsertion and removal of one of the plugs inits jack, while the other plug remains con-nected. If desired, you can make the unitmore compact by using a Burgess #F2BP 3-volt battery. It can be hidden in a small

Pointed rotor of fhin aluminum spins rapidly whilebalanced on needle placed near the charged sphere.

Holder is small plastic stick.

After removing the globe from its plasticbase, you can separate the two halves with abit of prying. Soak them in paint remover orlacquer thinner for about a half-hour, thenwipe off the softened paint with a clothdampened with the solvent. Now, enlargethe small hole in the half with the inside edgeas in Fig. 10, using tin snips.

Since this opening will be the point wherethe sphere fits over the column, it is very im-portant to have a well-rounded edge to mini-mize leakage at this joint. We solved thisproblem by making a simple die in a wood-turning lathe from a piece of scrap hardwoodas in Fig. 12A, then driving in and shapingthe edge with two hammers as in Fig. 11.Fill the small hole in the other half of thesphere with solder, file flat and then polishboth halves to a high sheen, using rouge ortripoli (dust-like silica) on a buffing wheel.

Any dents or small ridges in the metalcaused by the die-stamping process should beremoved. You can do this by holding a pieceof round metal inside and using a small ham-mer in much the same way as an auto bodyman removes dents. Best results depend ona smooth and highly polished surface.

Now remove the top brush and fit the low-er half of the sphere on top of the column as

150

Small wads of tissue are drawn to the sphere whenthrown, then bounced violently away as soon as they

touch it.

Tissue ball suspended on thread and allowed to touchthe sphere will then swing like a pendulum when

held in the right position.

SCIENCE AND MECHANICS

High speed strobe light "stopped" this phenomenon—a satellite inorbit. Small, specially-shaped piece of aluminum foil revolves on its

own axis as it travels around lower edge of sphere.

wooden box placed under the generator base,with leads to connect with the jacks. How-ever, the dry cells will last much longer.

Vivid Demonstrations of electrostatic forceare easy to conduct with this pigmy gen-erator. Make an "electrified plume" by at-taching several tissue strips to the spherewith cellophane tape. When you turn on themotor, the charge will be transferred to thestrips. Since like charges repel, the stripswill stand rigid and try to get away fromeach other as in Fig. 1. They'll even remainstanding when power is shut off, until thecharge slowly leaks off the sphere. Turn onmotor again and point your finger. The stripswill be attracted by its opposite charge andflock around it.

Electric Wind. Cut a strip of thin alumi-num to the shape of a reaction rotor as inFig. 17A. Press one end of a needle in asmall hole near the end of a strip of plastic.Now balance strip on the upraised needle.As you bring it close to the charged sphere(Fig. 13), the rotor will react to the "electricwind" effect of corona discharges at pointsby rotating like a high speed pinwheel.

Bouncing, Swinging Tissue. Roll up piecesof facial tissue into small balls and tossthem at the sphere. They are first attracted,then violently repelled as in Fig. 14, becausethe charge in the paper changed on contact.Tie one of the balls on the end of some threadand suspend it to touch the sphere. It willswing away immediately with considerablerepelling force. If held in the right position,the ball will emulate a pendulum as in Fig.15. This experiment is often more effectiveif the ball is coated with powdered graphiteto make it more conductive.

Satellite in Orbit. Cut a small piece ofaluminum foil from a cigarette pack to shape

as in Fig. 17B. Hold it in your fingers withthe broad end facing the lower section of thesphere. After some experimenting, you'llget it to float a short distance from the spheresurface as in Fig. 16, apparently defyinggravity. It will then orbit around the sphereand rotate on its own axis.

Adjustable Spark Gap. For lightning-likedischarges, make the spark gap electrode asin Fig. 17C and the wire capacitor as in Fig.17D. Attach each to the base as in Fig. 3,shaping capacitor in the fingers to fit in place.Plug in battery lead jacks. The resultant fatspark across the gap between the electrodeand sphere is intensified by the capacitor.

Without the capacitor, sparks are muchless intense but occur with greater frequen-cy, because a short time is required to chargeup the capacitor. Spark gap rod can be bentto secure gap desired. Results will be bestwhen the room's relative humidity is about30 per cent or less. Moisture causes leaksdown the column, at pulleys and belt, as wellas from the sphere to surrounding air.

To dry out accumulated moisture, it issometimes helpful to direct warm air from aportable hair dryer on the parts under thebase and also on the column. Dust particleson the sphere also cause poor dischargessince they form corona points from whichthe charges can escape and thus prevent thefull voltage from building up.

• Craft Prints in enlarged size for building Van de Graaffgenerators are available at $1.00 each. Order by printnumber. To avoid possible loss of coin or currency in themails, we suggest you remit by check or money order. (NoC.O.D.'s or stamps.) Now available, our new illustratedcatalog of "196 Do It Yourself Plans." IOC SCIENCE ANDMECHANICS, 450 East Ohio Street, Chicago 11, Illinois.Please allow three to four weeks for delivery.

MAY, 1961