Benjamin Franklin, 1747
On a warm summer day when the relative humidity (RH) starts to climb, the surface conductivities of insulating materials such as NylonTM, PVC, and PlexiglasTM, etc., begin to rise. Triboelectric charging still occurs, but the separated charge apparently dissipates more rapidly and the performance of many electrostatic devices, from the electrophorus to the van de Graaff generator, suffers. Above ~60% RH, performance starts to deteriorate noticeably, and above ~70% RH, it is probably best to defer your demonstrations for another day, and perhaps head for the beach.
When the relative humidity is between 60% and 70%, and the temperature is not too high, there are a few simple measures that can be taken to combat the effects of humidity.
As temperature and humidity rise, all of these measures inevitably begin to lose their effectiveness; therefore, the serious hobbyist or lecture demonstrator will need to experiment with various countermeasures to find the ones most effective for a given apparatus and given set of conditions. Anyone wishing to demonstrate electrostatic phenomena had best be prepared for occasional embarrassment, when the apparatus does not work well, or genuine humiliation, when nothing works at all.
What humidity does is to increase the
adsorption of water on to the surface of insulators (such as glass or plastics) and the
absorption into many other materials. This moisture has the effect of decreasing the
resistivity, which allows charge to be conducted away and reduces maximum attainable voltages
and electric fields. Paper is an example of a very common material that is highly susceptible
to the effects of humidity. The developers and designers of xerographic copy machines learned
very early about this trait of paper. The graph below shows the influence of relative humidity
on bond paper at room temperature.
For certain electrostatics demonstrations, some very specific measures can to be taken. For example, in the case of the vapor ignition demonstration, the hand-held piezoelectric sparker can be used as an ignition source on warm, humid days when the capacitive discharge obtainable from an electrophorus is too anemic. Sometimes the problem is moisture inside the ignition chamber that has condensed out of the humid air by the cooling action of the evaporating flammable solvent and shorts out the sparking electrodes. To avoid this trouble, use a hair drier to dry the inside of the chamber.
One approach to the problem of humidity, suitable for laboratory demonstrations though perhaps not so convenient for the lecture hall, is to store all the equipment in a heated enclosure. The air within the enclosure, warmed to about 10 deg C above ambient, has reduced relative humidity; and this measure is usually sufficient to control surface conduction on insulating materials. Lichten and McGrath made a large enclosure, about one cubic meter in size, out of PlexiglasTM and, never removing the equipment, performed the demonstrations inside it [Lichten and McGrath, 1984]. They reported success in demonstrating electrostatic phenomena in humid summer weather.
Though few electrostatics demonstrators will ever be heard to complain about low humidity conditions, there are certain demonstrations where problems can arise in very dry weather. For example, a slightly conductive property for the bench or tabletop upon which rests the apparatus is usually beneficial [Bakken Library, 1995]. The electrophorus is a good example of such a case. Here, when the tabletop or bench is plastic-coated or if the atmosphere is very dry, so that the table is made insulating, then the reliability can be enhanced by making the bench into a ground plane. A sheet of conductive plastic or aluminum foil, or wiping the table-top with a damp cloth, will serve this purpose.
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additional useful suggestions for dealing with humid conditions.
Bakken Library and Museum, Sparks and Shocks, Kendall/Hunt Publishing Co., Dubuque, IA, 1995, pp. 78-80.
W. Lichten and P. McGrath, "Electrostatics demonstrations at high relative humidity," Am. J. Phys., vol. 52, January, 1984, p. 86
E. M. Williams, The Physics and Technology of Xerographic Processes (Wiley, New York) 1984, pp. 213-216.