Electrostatic and VandeGraaff Generators: Solving Humidity Problems
(c)1994 William J. Beaty
UP: All Electrostatics Demos, Physics Demonstrations
Blow-dryerAlways carry an electric blow-dryer with you when doing electrostatics demos in high-humidity locations. When your VandeGraaff machine fails, the usual cause is adsorbed water on the rollers and belt, which prevents the initial charge-separation from occurring where belt touches/peels from the roller surface. Pop open the sphere or the base, run the machine, then direct warm air upon each of the rollers until the upper sphere starts making sparks when touched. Don't give up, sometimes it takes 5 minutes or more to dry the cloth rollers used in the base of the WINSCO Inc. VDG. Another common problem is fingerprints and grime which make the belt and roller become conductive on humid days. Replace the belt with a fresh one. Or, remove the dirty belt, clean both sides with plenty of rubbing alcohol, then pat it dry with a clean paper towel. You want to flush the oil away, not just move it around, so let lots of alcohol dribble off. If one of the rollers is plastic, wipe it off with alcohol too. [WARNING: don't wet acrylic plastic with alcohol, it will crack.] Fully dry it by operating the belt in the machine while applying the blow-dryer. Don't wet the felt type of roller, it takes too long to dry again.
"Hotbox" for storageIt is not impossible to perform electrostatic demonstrations in a very high humidity environment. One secret is to build a "hotbox" storage device for your equipment. A cabinet with a tight-fitting lid can be heated with a small light bulb inside. Anything stored in this box will have its conductive surface moisture evaporated by the low heat. In a pinch you can heat your equipment in a trash can containing a light bulb. One science teacher revived a long-dead VDG machine using this method. WARNING! FIRE DANGER! Try different small lightbulbs to find one which produces approx. 110F heat in the box. Too large a bulb in too small a box can start a fire. Also, it is wise to mount the bulb near the top of the box. This makes it less likely that a flammable object will fall against the bulb.
DessicantIt is also possible to use baked silica-gel dessicant instead of a lightbulb to dry the contents of a sealed case. If you can obtain a couple of pounds of color-change silica gel dessicant, placing the stuff in your box will lower the air's humidity and act to dry your equipment. The dessicant eventually turns from blue to pink as it gets full of moisture. Hours of oven-baking at low temperature according to its supplied instructions will restore its dessicating properties.
Dirt and greaseOccasionally wipe any grime from the vertical column of your VDG using a paper towel slightly dampened with rubbing alcohol, then dry the column thoroughly with the blow-dryer. DO NOT WET THE PLASTIC, it will cause instant cracking. Avoid getting any alcohol near the mounting screws on the column, since the internal stress in the plastic plus alcohol can crack the plastic. [Some people suggest using soapy water only, then blow-drying thoroughly.] It seems like grime builds up on electrostatic devices faster than on any other object. It's no illusion. Electrostatic generators create ion currents in the air around them, and this charged air in turn charges all the dust motes and air pollution particles, which then seek out any available surface. If you live in a big city, your VDG machine will quickly acquire a black coating as all the car exhaust is extracted from the air and coats itself on the machine!
Hi-volt Supplies Still WorkSometimes the humidity is so high that no amount of debugging can fix your demo. In this case, sometimes a DC power supply can be used in place of a VDG machines. I have several 20,000-volt power supplies bought from surplus mail-order suppliers. They convert 24Vdc to high voltage. They will give a mild shock, but are not as bad as a Leyden jar. Another high-voltage supply: negative ion generators. These devices are actually DC power supplies having 10KV or higher output, but with large current-limiting resistance in series. Touch their output and you feel nothing, but connect the ionizer brush to a pop-bottle electrostatic motor, and it spins like mad. Negative air ionizers are pricey, but I see them for $5.00 frequently at garage sales.
Here's an electrostatic demo which will even work when submerged
RED AND GREEN ELECTRICITY, This one works because no actual charges
are involved. It's really a visual analogy for electrostatics, which uses
red and green transparancies to simulate the + and - charge within matter.
This article might not look like much, but I highly recommend that you
mess around with the red and green plastic anyway. I would estimate that
the depth and solidity of my own understanding of Electrostatics was
DOUBLED by messing around with these simple plastic sheets.
Sharp dust-motesOne last possibility: if the voltage on your machine seems to drop suddenly during operation, try wiping down the sphere with a damp cloth. Sometimes your machine will attract a sharp, conductive dust mote which then spews charged wind. This presents major current leakage and partially shorts out the sphere voltage. With luck, wiping down the sphere will dislodge it.
Humidity's EffectsWhy does high humidity affect electrostatic devices? Simple: it makes surfaces conductive enough to "short out" the devices. Under high-humidity conditions, insulating surfaces can behave as a dead short, almost like metal, even though their resistance remains too high to easily measure. A paradox? No.
effects deal with high voltage at little or no current. Because the
voltage is high and the current low, even a fairly insulating object can
act like an conductor when used with electrostatic devices. Ohm's Law
tells us that high voltage at low current implies high resistance. For
normal, non-electrostatic circuitry, low voltage at high current implies
LOW resistance. Therefor
we can say that "conductivity" in the electrostatic realm involve enormous
values of electrical resistance, while "conductivity" in normal circuitry
For example: A generator that produces 1 microamp at
10,000 volts has an inherent series source impedance of 10^10, ten
(Compare a 1.5v D-cell, which has far less than one ohm internal series
If the source-impedance of an electrostatic generator is ten billion ohms,
then any material having much less than this resistance will act as a good
conductor, while materials having much more resistance will be insulators.
(Compare to a D-cell, where 'conductors' must be lots less than one ohm,
and 'insulators' must be much greater.)
A dirty surface in a humid environment might have hundreds of millions of
ohms across its surface, yet it will act as a good conductor and can short
out your generator. You must clean and dry the insulating surface so that
its resistance rises to a range of *thousands of billions* of ohms across
a few inches, so the generator will see it as an insulator. A wire will
have a resistance of less than one ohm, yet a dirty surface, a surface
which is more insulating than metal by hundreds of millions of times, will
still act as a good conductor. Very strange, no? Electrostatics shows us
that the meaning of "Conductor" and "Insulator" is not fixed: it varies
depending on the nature of the generator or power supply, and the nature
of the electrical load. Electrostatics is not the only place this occurs.
For example, in superconducting systems, copper can be used as an
INSULATOR. In AC power transmission, stepup transformers are used to make
the long power lines act "more conductive." And in electrostatic systems,
humid string (in Ben Franklin's kite) can act as a conductor.
Too dry?!I encountered one instance where the humidity was TOO LOW for a demonstration: I opened a fresh bag of rice-crispy cereal, intending to produce a big messy "Volta's Hailstorm" effect using a VandeGraaff generator and a pie pan, filling the pan with cereal. Fortunately I tried out the demo just before the kids arrived. It didn't work. The rice crispies were too dry, acted as insulators, and refused to pick up a charge and be repelled, and so were attracted instead. Sprinkling water into the cereal box and giving it a good long shaking cured the difficulty.
Another place where humidity can be too LOW: when performing the
VDG hair-raise demo, sometimes the VDG generator works fine, but long hair
simply won't rise. This is probably because the hair has become an
insulator, and charge is unable to leak along the hair shafts and give
them an imbalance of alike-charge. Slightly humid hair is required for
success; totally dry hair will cling to the head by induction and will not