Carl Noggle wrote:
The Lichtenberg figure forms often when sparks slide over the surface of a dielectric. It is typical of dendritic figures that form when water or some substance has to spread out (or be collected from) a surface (or in the case of Bert Hickman's figures, or tree roots). In this case, charge has to be distributed. If the dI/dt is fast enough, the high dielectric constant of the water in the body will dominate over the resistivity and the current will flow over the surface. dI/dt can be as high as 10E11 amps per second in the vicinity of a lightning strike. This is probably why only about of 10% of lightning strike victims die. Power line electrocutions, however, are very often fatal, since the dI/dt is much slower and the current can flow into the body. This is basically a skin depth argument in the time domain and applied to a very lossy dielectric. We would not expect figures on the skin of a power line electrocution victim.
Good information, Carl. Lightning hitting a human is similar to lightning hitting a bag of salty water - fortunately most of the discharge current flashes over, instead of going through, the victim. If the victim is sweaty or rain-soaked, the surface flash can create a steam explosion that blows off the victim's clothing or shoes. Because of the higher peak current and persistent tail current (10's or even 100's of milliseconds), an unfortunate victim of a positive lightning strike usually doesn't survive.
A similar effect has been observed by experimenters playing with HV discharges to water surfaces. As Carl's note suggested, the shape and horizontal excursion distance of the transient Lichtenberg figure can be controlled by adjusting the water's electrical conductivity. The purer the water, the longer the branches. Water with higher electrical conductivity develops shorter, brighter branches. Some examples of transient figures ("sliding sparks") on water:
http://volcaniclightning.tripod.com/lichtenb.htm http://www.youtube.com/watch?v=8h0PEbqlYnc http://www.capturedlightning.com/frames/Z02A.jpgAnd, in response to Jim's question, a few years ago a rather well-known HV experimenter (who shall remain nameless) succeeded in creating a small star-like Lichtenberg figure on his hand by accidentally becoming part of his 60 kV Marx generator's discharge circuit... :^)
Bert
On 1/7/12 4:08 PM, Bob Wroblewski wrote:On the CBS network television show A Gifted Man (Friday night 1/6/2012) the storyline featured a young boy with a perforated eardrum. Upon removing his shirt to check for other injuries there was a prominent Lichtenberg pattern on his back. I immediately said to my wife he was hit by lightning. The doctors on the show were puzzled and searched online to reach the same conclusion soon after. My astonished wife said how did you know that before the doctors? So I related a story of several Rochester Teslathons where Bert Hickman made and sold Lichtenberg figures in various acrylic forms and I have several on display in my office cubicle. They are also viewable at his Teslamania.com website. Anyway the episode should be available at the cable tv on-demand or network websites or hulu or elsewhere on the interweb by now.I had a similar experience at a family gathering where one of the guys was an ER physician, and he had some journal with a "what causes this?" mystery puzzle in the back. It was the photo of a man's back with a great Lichtenberg figure on it, prompting speculations about some sort of weird dermatitis, or a surface skin infection that had spread. I saw and instantly said, Lichtenberg figure, he was hit by lightning. That's what it was, of course. Here's a TC list question about which I've been wondering... Would such a figure occur with an ordinary HV incident (e.g. contact with a power line). Or would it have to be pulsed power with very high peak current/voltage? Lichtenberg figures are similar to the dendritic treeing that occurs in dielectric, but not from the same cause, as far as I know (and treeing is a long duration aging kind of effect), although the fractal nature IS similar (and for the same reason.. field distributions) _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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