Taming your VandeGraaff Generator

(c)1996 William J. Beaty

Wouldn't it be great if you could easily control the output voltage of your classroom VandeGraaff machine? By reducing the voltage you could make it safe and non-scarey for the youngest preschooler. If you could lower the voltage, you wouldn't risk damaging small instruments or nearby computers with the quarter-million-volt overkill. And if you could easily crank the voltage up and down, you could perform demonstrations which illustrate the effects of various levels of e-field, or even demonstrate slowly varying e-fields.

The secret to controlling your VDG voltage involves the creation of a variable-gap gas discharge voltage regulator comprising a small-diameter electrode adjacent to a flat plate electrode. But since we live our lives immersed in an appropriate gas, it's simple. The only parts required for this are:

Piece of wire
Cable ties, or rubber band, or tape

Simply do as shown in the diagram. Connect a piece of wire to the electrical ground terminal on the base of your VDG machine. (If it has no ground terminal, tape the bare end of the wire to the metal base of the generator.) Run the wire up along the column to where it barely touches the base of the sphere. Temporarily fix it in place with plastic cable-ties (zip several together if yours are too short) or use rubber bands, or simply tape it in place.

The voltage of your VDG machine can now be controlled by varying the distance between the tip of the wire and the upper sphere terminal. When the wire touches the sphere, the voltage of course is zero. If you pull the wire slowly downwards, the gap ("D" in the diagram) increases slowly, and the voltage on the sphere increases. To set the voltage to maximum, move the tip of the wire down the column all the way to the base. To set the voltage to approximately half-maximum, move the tip of the wire down the column midway between the base and the sphere. It's not really necessary to place the wire against the column. So, if you hold the grounded wire in your hand and move the tip slowly towards and away from the sphere, the output voltage will slowly decrease and increase. Move it in and out, and you silently broadcast low frequency electromagnetic sine waves out into space! :)

SUGGESTED USES

You can elmiminate any accidental shocks to students during the "Hair Raising" demo. First adjust the wire tip upwards for zero voltage. Run the generator, let your victim place his/her hand on the sphere terminal, then pull the wire down to increase the voltage. When your victim has had enough fun, push the wire back up into contact with the sphere, which leaves their body without an excess charge. He/she won't get zaps from the floor, other students, etc., upon climbing down from the insulating stool. And the next victim won't get a zap from terminal, since it was left with zero excess charge at the end of the demo.
Eliminate accidental shocks to yourself by always holding a bare voltage-adjustment wire during demonstrations, and stand so the tip of the wire is between you and the VDG sphere. This lets you easily raise and lower the voltage, and when it comes time to turn off the VDG, you won't get a nasty zap from the power switch if first you use the grounded wire to adjust the sphere voltage to zero.
Tape a bunch of strips of paper tissue (2cm x 20cm) all over the sphere-terminal of your VDG machine. Set the voltage to zero, turn the machine on, then slowly raise the voltage. The tissue strips will slowly stand on end. Move the wire rapidly in and out, which lowers and raised the voltage, which makes the tissue strips respond. Try this with soap bubbles. Try this with homemade electrometers. How far can you "broadcast" these waves of changing e-field?
Build the Pop-bottle motor detailed elsewhere on my website. Connect it to your VDG, and use the voltage adjusting wire to set the motor speed to any value desired. Or, if you are using your VDG to drive various student-built electrostatic devices, you can use the voltage-adjusting wire to slowly raise the applied voltage (thus avoiding sudden discharges.)
Obtain a professional rotary-plate Electrometer or "field mill," then practice your graph-making ability by calibrating the distance of the wire-gap "D" against the actual measured output voltage of your VDG machine.
End confusion over the speed control knob found on some VandeGraaff machines. The speed control does not control the voltage, it controls the speed of the belt, which controls the electric current going up the belt, and so controls the rate of charging of the sphere terminal. A low setting on the speed control will make the sphere terminal charge to maximum voltage ...but more slowly. To really control the output voltage, turn the speed up medium-high, then move the grounded control-wire up and down.
Ambitious people with large VDG machines can even add a remote-control voltage regulator in the form of an automobile retractable FM radio antenna having a sharpened tip. Provide a 12V supply for the motor, and an "up" and "down" button, and you can control your big nasty machine from a safe distance.

HOW IT WORKS

If a sharply-pointed ground wire is held near the sphere of an operating VandeGraaff machine, a tiny corona discharge or "St. Elmo's Fire" will be created on the wire tip. This corona discharge emits positive and negative air ions, which makes the surrounding air conductive. The conductive air stream flows between the ground wire and the sphere, and it acts as a resistive "short circuit" path to ground, which shorts out the VDG machine and drops the output voltage to a very low level. But that's only part of it.

If the wire is held very very close to the sphere terminal, the voltage falls almost to zero. But why does the voltage rise higher for a large gap between the wire and the sphere surface? I don't have a ready answer for this. I suspect that it involves a complicated feedback process akin to the one which occurs within gas-discharge voltage regulator tubes, and in Zener voltage regulator diodes.

But here's my attempted explanation anyway!
As the ground wire is pulled away from the sphere, the strength of the e-field at the tip of the wire drops below a level which supports the "avalanche" process which keeps the tiny plasma-flame ignited. The corona discharge comes close to winking out, and its rate of ion emissions drop. This increases the overall resistance of the invisible conductive path between the wire and the sphere, which lets the sphere's voltage rise higher. The higher voltage again increases the corona discharge at the tip of the wire. The VDG machine is producing approximately a constant current, so as the air-path resistance increases, the VDG output voltage increases. As the voltage increases, the e-field at the wire tip increases just enough to keep the same ion-current going within the corona all the time. Or put more simply: as the distance between wire tip and the VDG sphere is varied, the rate of corona discharge at the wire tip remains constant, and the current in the air path remains constant, while the resistance of the air path and the voltage on the sphere will go up or down. Variable resistor and constant current? Ohm's law says that this gives variable voltage.

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