What are Lichtenberg figures?
A bit of history...
Credit to http://www.capturedlightning.com/frames/lichtenbergs.html#what
for compiling all this information!
"Lichtenberg figures" are branching, tree-like patterns that are created by the passage of high voltage electrical discharges along the surface, or through, electrically insulating materials (dielectrics). The first Lichtenberg figures were actually 2-dimensional "dust figures" that formed when airborne dust settled on the surface of electrically-charged plates of resin in the laboratory of their discoverer, German physicist Georg Christoph Lichtenberg (1742-1799). Professor Lichtenberg first observed this in 1777, demonstrated the phenomenon to his physics students and peers, and reported his findings in his memoir (in Latin): De Nova Methodo Naturam Ac Motum Fluidi Electrici Investigandi (Göttinger Novi Commentarii, Göttingen, 1777). The English translation of the title is, "Concerning the New Method Of Investigating the Nature and Movement of Electric Fluid". Lichtenberg's translated paper is contained in Appendix A of a Masters thesis by Mark A Payrebrune ("Experimental Morphology of Lichtenberg Figures", McGill University, Montreal, Canada, 1979). The translated document (by Dr. J. Blain, Classics Department at McGill University) contains the following passage that describes Lichtenberg's initial discovery:
"At the beginning of spring 1777, after the completion of the new Electrophore, everything in my little room was still covered with extremely fine resinous dust that had settled, between the scraping and the shaving of the instrument's base or stand, on the walls and books. As soon as a draft in the air arose, the dust fell, much to my annoyance, on the conducting disc of the Electrophore. Often afterwards, when I held the disc suspended from the ceiling of my room, it turned out that the dust, as it settled on the base, did not cover it completely, as it previously had covered the disc, but only in certain areas. Much to my great joy, it gathered to form little stars, dim and pale at first, but as the dust was more abundantly and energetically scattered, there were very beautiful and definite figures, not unlike an engraved design. Sometimes there appeared almost innumerable stars, milky ways, and great suns. There were arcs, unclear on their concave side, but radiant on their convex side. Very glittering little twigs were formed, similar to those which frozen moisture produces on glass window panes. There were clouds of different shape and shadows that were visible in varying degrees ... But the most pleasing sight presented itself to me, when I saw that these figures could not be easily erased, as I tried to wipe away the dust with a feather or a rabbit foot. I could not prevent these same figures, which I had just erased, from shining forth once more, and somehow, more brightly. Therefore l placed a piece of black paper smeared with a viscous material on the figures and pressed down lightly. I was able to produce imprints of the figures, six of which the Royal Society has seen. [Note: see figures below]. This new kind of Typography has been extremely satisfying to me, hastening as I was to more remote preoccupations and having neither the time nor the inclination of sketching the figures or destroying them all."
During his subsequent studies, Professor Lichtenberg used various high-voltage electrostatic devices to electrically charge the surfaces of various insulating materials including resin, glass, and ebonite (hard rubber). He then sprinkled mixtures of finely-powdered sulfur (yellow) and minium ("red lead", now called lead tetroxide) onto the charged surfaces. He found that powdered sulfur (which becomes negatively-charged by rubbing against its container) was more strongly attracted to the positively-charged regions on the surface. Similarly, frictionally-charged particles of red lead acquired a positive charge and were attracted to negatively-charged regions. The colored powders made previously-hidden regions of stranded surface charges, as well as their polarity, clearly visible. We now know that these charged surface regions were previously deposited by small sparks of static electricity. The sparks deposited isolated patches of electrical charge onto the surface as they flashed along the surface of the insulator. Once deposited onto the insulator surface, the charges remain stranded for a very long time since the insulator prevents them from moving and dissipating. Lichtenberg also discovered that the appearance of positive and negative dust figures was markedly different. Discharges created by a positively-charged high-voltage terminal were star-like, with long, branching paths while discharges from negatively-charged terminals were shorter, rounded, and fan-shaped or shell-shaped. By carefully pressing a piece of paper onto the dusted surface, Lichtenberg found that he could transfer the images onto a piece of paper, demonstrating what eventually became the modern processes of xerography and laser printing. The underlying physics that created Lichtenberg's dust figures evolved to become the modern-day science of plasma physics.