Coaxing better performance from electrostatics demonstrations in humid weather

If there is no other Use discover'd of Electricity, this, however, is something considerable, that it may help to make a vain man humble.

Benjamin Franklin, 1747

On a warm summer day when the relative humidity (RH) starts to climb, the surface conductivities of insulating materials such as Nylon^TM, PVC, and Plexiglas^TM, 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.

• First of all, a small electric hair dryer should be standard equipment for anyone serious about electrostatics demonstrations. The dryer can be used to drive off moisture from insulating surfaces. For real effect, surfaces must be perceptibly warmed to the touch by the hair dryer.
• It is always a good idea to keep critical insulating surfaces clean and free of dirt, hand oils, etc. A small bottle of methyl alcohol (or another suitable solvent) can be kept for this purpose. Note that the solvent must be completely removed by air drying.

• A heat lamp is another way to provide quick warming to reduce surface conduction, but this measure is effective only if the object is heated so that it is warm to the touch.
• For those who have a carrying case for their demonstration apparatus, another trick is to place some silica gel in a Nylon^TM stocking and keep it inside the case. Periodically, the silica gel will have to be heated in a dry oven to drive out the collected moisture.
• Removing any sharp edges and nicks from metal surfaces will help to retain electric charge. For apparatus that can achieve high voltages (e.g., the electrophorus), it is a good idea to polish the electrode.
• In the summer, try to set up your demonstrations in a room that has air conditioning. In really humid conditions, it sometimes works out better if the air conditioner is turned up so that the room is uncomfortably cold.
• A rather handy accessory is a small, battery-operated humidity meter. This instrument can be used by a lecture demonstrator to anticipate the need for special measures well in advance of a presentation.

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 Plexiglas^TM 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 a very light application of water on the table-top, will serve this purpose.

Click here for additional useful suggestions for dealing with humid conditions.

Library references

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.

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