Lau, Gary wrote:
I hope this isn't viewed as too off-topic - I'll argue that the same physics apply to TC sparks ;-) I was giving a demonstration of various HV toys to a 4th grade class yesterday. Among the devices was a Jacobs ladder, powered by a 15/30 NST. The two 1/8" x 3 ft steel electrodes appeared to have been excited into a mechanical oscillation, bouncing towards and away from each other, at very roughly ~ 1Hz. One of the students asked my why they were moving, and I had to admit that I didn't know the source of the force that was moving them. The period of the oscillation was much faster than the arc travel time up the electrodes. It's clear that the period was that of the free-standing rods, and that the exciting force between them varies as a function of their separation, but I don't see the source of the attraction or repulsion between them. Any theories?Thanks, Gary Lau MA, USA _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
Hi Gary,I've seen this on ladders where the lower part of the ladder is mechanically fixed but the top portion of the electrodes are free to move.
I suspect that it's due to electrostatic attraction between the electrodes. The attractive force becomes particularly strong during the brief interval between one arc extinguishing at the top and the next one restriking at the base of the ladder. The short time lag causes a brief impulse of electrostatic force that can shock excite any mechanical resonances of the electrodes. There may also be secondary effects due to increasing electrostatic attraction as the arc climbs the ladder, since the increasing positive column length causes a larger voltage drop across the arc and increasing E-field between the electrodes. However, since the open circuit voltage is considerably higher than the arc voltage drop, the largest attractive force undoubtedly occurs during the brief interval between quenching and reignition.
BTW, I have encountered some folks who think that one reason why the arc rises in a Jacob's Ladder is due to Lorentz forces. Because of the relatively short circuit current, I suspect that NST and even ballasted pig arcs rise primarily via thermal effects. This is NOT the case for high current arcs, such as phase-phase arcs on power lines. One very interesting video clip shows a circular Jacob's Ladder created by a higher current (3 kV, 500A) arc. The circular loop has a gap to prevent the arc from "hanging" half way around. Any German speaking folks who can translate the speech on the video?
http://www.youtube.com/watch?v=6hGiAIRcn98
Bert
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