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Re: [TCML] Spark dynamics on Jacobs Ladder
Gary, I long ago decided that it was electrostatic attraction and
repulsion through performing the same experiments you just did!
I also noticed that the first movement accompanied the spark switching
from top to bottom. I doubt it has anything at all to do with the AC
On Jan 18, 2008, at 8:21 PM, bartb wrote:
Good test. I've always seen the oscillation on JL's simply due to a
single end connection. I personally have no clue why this occurs
(funny, we've all built JL's and don't have a definitive answer on
this simple thing).
From memory (two Halloweens ago), it seems repulsion of the wires
was the driving force and when the wires came close together it
appeared due to the mechanical "let go" of the repulsion. It's only
an old memory now, but I do remember thinking about this very issue.
As the arc travels up the ladder, the motion is somewhat captured
but not completely (it appeared to be repulsion with mechanical
energy wanting it to go back in). When the arc is quenched at the
top, the mechanical energy swung the wires inward where the arc
started again. The wire swing did get stronger as the heat enabled
the arc to roll up faster along the length. I think heat is directly
involved in this mechanical action by it's travel time from start to
quench, but there is a limit to how high the temp of the wires can
get and this is what "appeared" to stabilized the amount of swing.
Just my regurgitation of that particular Halloween when I had the JL
in full swing. I was controlling the coil running behind the ladder
which just made for a cool effect of the whole night.
Lau, Gary wrote:
I would have bet money that you were right about the EM Lorentz
force force being the answer, but a couple of quick experiments I
just did make me think otherwise.
I took a length of thin #30 wire to connect the free tops of the JL
electrodes, so that they can still move unencumbered, just
shorted. I pulsed the AC power to the 15/30 NST to match the
mechanical period of the electrodes, but I couldn't get any kind of
sympathetic motion. This makes me think that EM is not the force.
Then I removed the #30 wire, and moved the electrodes just slightly
further apart so that no spark occurred between them. Now by
timing the AC pulses to the NST, I WAS able to get the electrodes
to oscillate, although for some reason, my timing mostly seemed to
be OUT of phase with what would increase the amplitude.
Electrostatic attraction is definitely at work.
Looking more closely at the motion in the operating JL, the motion
seemed to get its kick principally as the arc broke at the top,
although I still saw oscillation if I positioned the electrodes
such that the arc stayed at the top - not sure why that might be.
Regards, Gary Lau
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of Bert Hickman
Sent: Friday, January 18, 2008 1:20 PM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Spark dynamics on Jacobs Ladder
Possibly, except that Peter is correct - the EM force on the
is actually repulsive and not attractive since the current is
opposite directions. The magnitude of the repulsion force scales
the square of the current and is inversely proportional to the wire
BTW, for a graphic example of what these forces (at tens -
kA) can do to substation bus bars and standoff insulators during
accidental short circuits, see:
It would be interesting to compare the magnitude of the magnetic
electrostatic forces for a typical 15/30 NST to see if they are of
same magnitude... I'll look into this a bit more.
A simple explanation, understandable by elementary school kids,
is this: A
wire carrying a current has an EM field around it. The two wires
currents in opposite directions will thus attract each other.
When the spark
first forms at the bottom, the two electrodes form effectively
very short wires.
As the spark rises, their lengths becomes effectively longer and
attraction stronger. The maximum attraction occurs when the
spark is at the top.
When the spark breaks at this point, the restorative force of
the spring tension
of the electrodes pulls them apart again and they try to
oscillate at their
natural frequency. If you could get the voltage, current, spark
and distance just right, you would see significant resonant gain
movement (The pushing-a-swing analogy). The college-level
explanation is here:
Hope this helps,
In a message dated 1/18/08 11:06:32 A.M. Eastern Standard Time,
I hope this isn't viewed as too off-topic - I'll argue that the
apply to TC sparks ;-)
I was giving a demonstration of various HV toys to a 4th grade
yesterday. Among the devices was a Jacobs ladder, powered by a
1/8" x 3 ft steel electrodes appeared to have been excited into
oscillation, bouncing towards and away from each other, at very
1Hz. One of the students asked my why they were moving, and I
had to admit
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-
and that the exciting force between them varies as a function of
separation, but I don't see the source of the attraction or
them. Any theories?
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