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Here is another thought about spark initiation. Arc initiation which ought to be the same thing is related to the creation of fine fingers of metal that lift off the surface in the high field, creating even higher fields leading to field emission and field ionization which assist in starting the spark. It is probably the case that cosmic rays play a part in the initiation as well.
If the presence of fine fingers of metal is important, you would expect a larger number of them on a blunt tip then you would a sharp tip.
just a thought,
joe
Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
On Jun 4, 2006, at 6:01 PM, Michael Edmiston wrote:
A few people have responded to me directly rather than the list. One
person commented that it might make sense the blunt lightning rods get
hit more often than the sharp ones because they don't prevent the
lighting as well as the sharp rods. The person then asked if you would
rather protect your house with blunt rods that will get hit more often
or sharp rods that will prevent strikes better.
That reasoning was my initial reaction. However, Charles Moore and the
Langmuir Laboratory for Atmospheric Research folks are saying the blunt
rods actually protect your house better. They view lightning protection
from the "attract it" approach rather than the "prevent it" approach.
* * * Quotation from Charles Moore * * *
The undeniable facts are that "dissipation" devices do not prevent the
occurrence of cloud-to- ground lightning strikes and that they are not
designed nor intended to be the preferential receptors of the lightning
strikes in their vicinity. Accordingly, such devices serve no useful
protective purpose in the prevention nor in the reception and conveyance
of lightning to Earth.
In my opinion, the "Charge Transfer System" method does not have
technical merit for lightning protection and there is little probability
that it will acquire any technical merit in the future.
It would be a disservice to the public for the IEEE to issue a standard
supporting the use of these devices as being suitable for protection
against lightning.
Sincerely,
/s/ CBM
Charles B. Moore
Professor Emeritus
Atmospheric Physics
New Mexico Institute of Mining and Technology
* * * End quote * * *
Ludwik mentioned radioactive rods. Moore's group has also tested those
and says they don't work.
Their theory is that the lightning strike actually starts high, usually
at the base of the cloud, and this is so remote from any rods or other
surface objects that surface details have no bearing on whether the
strike will occur or not. Therefore, since strikes cannot be prevented,
the best approach is to look for the best receptor rather than trying
for prevention.
Based on Moore's recommendations, some organizations such as National
Lightning Safety Institute are beginning to recommend blunt rods of 1 to
2 cm diameter rather than sharp rods.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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_______________________________________________
Forum for Physics Educators
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https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l