IN DECEMBER I WROTE ABOUT the electrical sparking that can be seen (in a dark room, with dark-adapted eyes) as adhesive tape is peeled from a solid surface. The separation creates electrically charged patches that discharge to one another. What you see is a faint blue or blue-white glow that marks the moving line of separation between the tape and the surface.

After reading the article David K. Donald of the Hewlett-Packard Research and Development Laboratories in Palo Alto wrote to me about similar experiments he has carried out-and that indeed have been done by many people for more than 200 years-to identify the location and electrical sign of charged patches on a surface. In this century such experimentation figured prominently in the development of modern photocopying machines that depend on electrostatics to attract a "toner" powder that makes the copies. Indeed, much of Donald's work was done while he was with the Xerox Corporation.

You can repeat Donald's experiments in your kitchen (when the humidity is low enough). All you need is a smooth nonconducting surface, adhesive tape and an assortment of powders: flour, powdered spices or herbs, or the toner from a photocopying machine. A strip of the tape is affixed to the surface and then peeled off, leaving charged patches on the surface. If an appropriate combination of two of the powders is then quickly dusted over the surface, one type of powder collects in negatively charged patches and the other type collects in positively charged ones. If the powders are different colors, they provide a map of the charged surface. A few of Donald's results, made with a combination of ground cinnamon and toner from a Kodak photocopying machine, are shown below. In each example the tape has been peeled toward the right.

To prepare for an experiment, Donald pours a small amount of the powders into a plastic squirt bottle and adds "shaking chips"-small glass beads, metal nuts or other such objects-that help to break up clumps of powder. When the bottle is shaken vigorously, the toner particles become negatively charged and the cinnamon particles become positively charged. Then, in a typical experiment, he applies some kind of sticky tape to a plastic tool case. (The experiment must be done on a nonconducting surface such as plastic so that the charged patches are not immediately neutralized by conduction.) After he has shaken the bottle to make some of the powder airborne, Donald peels the tape and then squeezes the bottle to blow powder over the case.

As the dust drifts over the surface, the electric fields from positively charged patches pull the toner dust out of the air and those from the negatively charged patches gather the cinnamon. The application takes about a second, during which the plastic loses little of its charge to moisture in the air or through conduction. When the dust has settled, the region from which the tape was peeled is colored brown by the cinnamon, indicating that the tape left the plastic negatively charged. The rest of the plastic surface is black (the color of the toner), presumably because when Donald wiped the case prior to the experiment, he left positively charged patches on it.

If you repeat the experiment, clip the nozzle of the bottle so that its opening is wider than normal. You might also clip the tube that extends into the bottle so that airborne dust is more easily blown out through the nozzle when you squeeze the bottle. Hold the nozzle from 10 to 20 centimeters above the surface to be dusted. Avoid releasing too much dust. If you do, tilt the plastic and tap it to knock off some of the dust, or blow gently across the surface.

Donald cautions that the experiment is guaranteed to make a mess, and indeed my kitchen and clothes are covered with multicolored dust. Be careful: the dust will ruin computers and floppy disks, and the toner will ruin clothing. To avoid a mess, you can do the experiment inside a large plastic soft-drink bottle. Put the dust in the bottle, peel tape from the outside surface of the bottle and then shake the bottle. The electric fields established by the charged patches on the outside surface will attract the powder to the inside surface. The pattern may, however, lack the fine detail achieved by the messier standard method.

Moreover, a major disadvantage of this approach is that the dust patterns cannot be preserved by Donald's usual

technique, in which the powdery map is transferred to fresh layers of tape. Donald applies several strips of transparent sticky tape to the dusted surface, with the strips slightly overlapping one another. When he lifts the tape array gently, much of the dust remains stuck to it, and he then presses the array onto a sheet of paper. (Usually the paper is white, but sometimes, depending on the colors of the powders, he chooses an appropriately colored paper.) In transferring the powder to tape it is important to avoid sliding the strips of tape over the dusted surface. I stick each end of a strip to a finger of each hand, lower the tape to the dusted surface, press my thumbs down on the middle of the strip and then slide the thumbs outward toward the fingers.

The success of the experiments depends on the immobility of the charges on the surface from which the tape is peeled. Glass, a moderate conductor, functions poorly: by the time the dust settles, the charges have largely been dissipated by conduction. High humidity also ruins the experiments. Even if the room air is dry, the tape may be slightly "wet" if it has been in humid air in the days preceding an experiment.

Toner powders from other photocopying machines or from laser printers can be substituted for the Kodak powder, but in some cases the electrical polarity of the cinnamon and the toner powder may be reversed. (There is a simple test, which I shall describe below, that indicates which of two powders is negatively charged.) Instead of cinnamon and a toner you can use parsley, sage, thyme, cumin, various flours or a variety of other common kitchen staples as long as they are ground to a fine powder. The best results are obtained when the grains are tiny and quite uniform in size, so that the tape strips pick up all the grains and retain fine details in the map.

Donald enjoys working with unbleached wheat flour because it lacks the magnetic properties of some toners, which can smear a charge map to some extent. Several other kitchen powders do not work well. Mustard powder is too transparent; coffee and corn meal are too coarse even after grinding. You should also avoid any powder whose index of refraction matches that of the tape, because the dust is then invisible in the lifted map. Some powder combinations function better than others because of the way the powders interact when they are charged by shaking. Crushed paprika has a striking color but works only with toner; when it is shaken with flour, for example, the oppositely charged grains attract each other so strongly that they ignore the charges on the surface and settle uniformly over the test site.

When tape is peeled from plastic small avalanches of electrons may jump along the surfaces without creating a true spark (which is a complete column along which the air is ionized). To promote sparking, Donald put a narrow strip of aluminum foil, the right-hand end of which was pointed, under the tape. He reasoned that when he peeled the tape and foil, the electric fields at the edges of the foil, and in particular at the pointed end, might be strong enough to ionize the air.

When Donald dusted the plastic, little of the powder was attracted to where the foil had been, indicating that the plastic was electrically neutral there. Throughout most of the region where the tape had peeled from the plastic the color was brown from the cinnamon, as in his previous trials. Where the edges of the foil had been, however, he found forked paths resembling lightning strokes, colored black by the toner. Sparks must have developed in the air just above the plastic, draining electrons from the plastic and leaving the positively charged paths. In some trials' Donald found fan-shaped black patterns where the edges of the foil had been. Such a pattern is due to a corona: an ionization of the air that does not develop into a spark.

By repeating Donald's corona and spark experiments you can always tell which powder in a combination of powders is negative. It is the powder that collects to form either forked paths or fan-shaped patterns where ionization of the air has left positively charged regions. I tested this technique on a combination of two DayGlo powders, Neon Red and Saturn Yellow. Several experimenters had found that Day-Glo powders, which are brilliant in color, define charged patches effectively. When I dusted my powders over plastic from which tape and foil had been peeled, the red powder collected along a forked path, indicating that the powder must have been negatively charged. (Five-pound packs of the powders can be bought from the Day-Glo Color Corporation, 4515 St. Clair Avenue, Cleveland, Ohio 44103.)

Donald produces a quite different distribution of charge when he peels sticky tape from a film of cellulose triacetate. Part of the charge map that he lifts from the acetate is black from. toner, but there are also pale brown— almost clear—petal-like regions. The left side of a petal is narrow and curved; the right side is wider and straight. (Remember, the peeling is from left to right.)

Donald explains that during the peeling the acetate becomes positively charged, the tape negatively charged. When the charge difference is large enough, electrons begin to flow from the tape to the acetate, almost neutralizing it. The discharge begins in a narrow region-which will be the left side of a petal when the acetate is later dusted. As the discharging extends across the width of the tape, the area that is almost neutralized widens. When the charge difference between the tape and the acetate gets to be small enough, the discharge stops abruptly, forming the right side of the petal. The continued peeling then rebuilds the charge difference until discharging is reestablished and another petal is formed.

On some surfaces sticky tape peels in a stick-and-slip manner, giving rise on the charge map to a series of striations that run across the width of the strip from which the tape was peeled. Interesting charged patches can also be created when tape is peeled from tape, as when a length of tape is pulled from a dispenser roll. After pulling off some tape, dust it in the usual way and then lay another strip over it for preservation. (You must do the dusting quickly, because the tape is partially conducting.)

As I mentioned above, the technique for dusting samples that have undergone discharge and sparking goes way back: it was discovered in 1777 by Georg Christoph Lichtenberg of the University of Gottingen. He noticed that when dust drifted over a cake of resin that had been sparked, dust grains aggregated to form beautiful patterns, which now bear his name. Soon afterward someone recognized that a mixture of two powders can reveal the polarity of the charges on a surface that has been sparked. When sulfur and minium (red lead) are sifted through the mesh of a muslin bag, the sulfur colors the positive regions yellow and the minium colors the negative regions red.

In this century Lichtenberg figures have helped to demonstrate how discharging and sparking proceed. In many experiments a narrow electrode is placed above a wide, flat electrode covered with a thin insulating material, and the two electrodes are connected to a device that can produce a sudden discharge between them. Then the insulating layer is dusted to reveal Lichtenberg figures.

The shape of a figure depends on the nature of the discharge. If the narrow electrode is an anode (that is, if it is higher in voltage than the other electrode), the figure, called an anode pattern, consists of many forked, branched lines radiating from a point directly under the anode. If the narrow electrode is a cathode (lower in voltage than the other electrode), the resulting cathode pattern is a circular patch consisting of many lines, which are often too narrow to resolve individually.

With Donald's guidance I set out to make some Lichtenberg figures in my university library. During the low-humidity days of winter the library is notorious for the sparks that result when one walks over the carpeted floor and then reaches for a metal door or bookshelf; the spark is h often strong enough to be felt and even heard. I decided this nuisance could finally be put to some use. Is my finger an anode or a cathode? I could tell by arranging to have the G spark create a Lichtenberg figure.

With strips of sticky tape I attached a small square of Mylar to the side of a large metal bookshelf. Then I walked about the floor, shuffling my feet and also swinging my arms so that my body became charged by the brushing of clothing against my skin, as well as by my movement over the carpet. Then I returned to the patch of plastic film and brought my finger up to it, causing a mild discharge. When I quickly squirted Xerox toner and cumin onto the Mylar, I found a black pattern about a centimeter wide. It was complex but appeared to have forked lines radiating from where I had touched the film, indicating that I had acted as an anode. I lifted the pattern with sticky tape and filed it in my notebook.

In repetitions of the experiment I held either a key, a straight pin or the point of a scissors blade up to the Mylar, thinking that the sharp end would provide a stronger electric field and a more vigorous discharge than my blunt finger. Indeed, audible and visible sparks sometimes formed in spite of the layer of plastic film. In each case the dusted Mylar revealed an anode pattern: fine, forked lines radiating from where the point had touched. Apparently, then, as I walked about the library my body had lost electrons and become positively charged. Back in my laboratory I was able to produce cathode patterns on squares of Mylar with electrostatic charging devices that are common to a physics classroom.

The anode and cathode patterns differ because of the way charge flows over the Mylar during the discharge. When a narrow anode is positioned above the film, electrons from the film and the air just above it flow to the point directly under the anode and then up to the anode in a series of avalanches or sparks. Positively charged channels of sluggish or immobile positive ions develop on the film's surface and in the air just above it, snaking outward from the point under the electrode. As the channels lengthen, the strong electric fields at their tips collect even more electrons, which then pass on to the anode. After the discharge the pattern of crooked, positively charged channels remains on the Mylar. The initial voltage difference between the anode and the cathode under the film determines the width of the pattern: larger voltage differences produce wider patterns.

If instead a narrow cathode is positioned above the Mylar, the discharge consists of a series of avalanches or sparks that send electrons from the cathode down to the film where they are driven outward radially. As electrons from the film and the air just above it join in the outward flow, the Mylar under the cathode becomes positively charged and begins to mask the cathode's electric field from the departing electrons.

Depending on the actual distribution of charges, the peripheral electrons then undergo forces that are not radial, and they spread outward in many directions. No channels form. When the discharge stops, the positively charged region of the Mylar under the cathode is approximately circular, with a diameter that depends on the initial voltage difference between the cathode and the anode.

The library experiments gave rise to two perplexing patterns. One involved the straight pin. When the head of the pin was held toward the Mylar, the normal anode figure developed. When the pointed end was held toward the film, however, the anode pattern had a clear center. I suspect that the point may have pierced the film, enabling electrons from the bookcase to neutralize the center of the pattern. The other puzzle was that a sharp point often left two patterns, one within the other, with a clear zone between them. Both patterns were anode figures. I think an initial discharge produced the larger pattern; a bit later, as my unsteady hand moved the point closer to the Mylar, pushing the plastic film against the bookcase, a smaller second discharge produced the narrower pattern. The electron collection in the second discharge may have obliterated part of the first pattern.

You may want to challenge my interpretations of these strange patterns. You might also enjoy searching for evidence of extra discharging in the Lichtenberg figures. Sometimes clouds of electrons on the perimeter of a pattern are discharged just as the voltage difference between the electrodes disappears. For example, consider the usual arrangement for making a cathode pattern. When the voltage difference drops toward the end of the discharge, the electrons outside the pattern might sometimes travel into positive regions within the pattern, leaving positive trails on the Mylar surface. Such discharging could be only slight or it could be an actual spark.

There are a host of experiments to be done with electrostatic charging. Charged patches are often left when plastic food wrap is peeled from the roll or some other surface. Laundry dried in a machine without an antistatic material often exhibits sparking. Can you produce a Lichtenberg figure with the help of a cat that you have charged electrically by rubbing its fur? I shall be glad to hear about your experiments on this timeworn and dusty bit of physics.

Bibliography

CONTACT ELECTRIFICATION OF INSULATORS AND ITS RELEVANCE TO ELECTRETS. D. K. Donald in Journal of the Electrochemical Society, Vol. 115, No. 3, pages 270-272; March, 1968.

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