Study shows Ben Franklin Design
reported by Melissa Lovin
Among Benjamin Franklin's famous accomplishments
was inventing the lightning rod. But, according to a recent Associated
Press article, a new study says his design was flawed, and the rods
work better if they are blunt-tipped instead of being sharp. Researchers
in New Mexico tested both types of lightning rods, along with some
new devices called "early streamer emitters,'' which manufacturers
claim are even better than the rods at attracting lightning. Rods
with blunt, rounded ends worked best, the scientists report in the
May 15 issue of Geophysical Research Letters.
In fact, when they left rods with
various tips on the 12,000-foot summit of South Baldy Peak in the
Magdalena Mountains of central New Mexico, the blunt tipped rods
were the only ones that managed to attract lightning. The research
team from the Langmuir Laboratory for Atmospheric Research at the
New Mexico Institute of Mining and Technology was led by now-retired
professor Charles B. Moore.
Franklin based his lightning rod on
the discovery that electrified objects could be discharged by approaching
them with a metal needle. At first he thought that lightning might
be prevented by sharp rods, which could cause it to discharge silently,
without a spark. In fact, however, his rods were struck by lightning,
and Franklin realized that grounding the rod with a wire provided
lightning a preferential path, away from the structure.
Moore launched his lightning studies
in the 1950s working with Bernard Vonnegut, a pioneer in developing
cloud seeding. Over the years, Moore said, he became curious why
sharp-tipped rods he was using to attract lightning didn't seem
to do that as well as he expected. "When you see a a paradox of
nature its always intriguing to try to find out why,'' he said in
a telephone interview. "I'm a strong believer in Franklin's lightning
rods, but they could be made better.'' The team exposed sharp-tipped
rods, blunt ones of various sizes and the new early streamer emitters
on a mountaintop and waited to see what happened.
"After seven years of tests, none
of the sharp Franklin rods or of the so-called 'early streamer emitters'
has been struck, but 12 blunt rods with tip diameters ranging from
12.7 mm to 25.4 mm have taken strikes,'' they reported. "Our field
experiments and our analyses indicate that the strike-reception
probabilities of Franklin's rods are greatly increased when their
tips are made moderately blunt.'' "We have found no evidence suggesting
that sharp-tipped lightning rods are effective strike receptors
when similarly-exposed, moderately blunt rods are in their vicinity,''
they said. They stressed, however, that lightning does strike sharp
rods when no competing blunt ones are nearby.
They concluded that "Franklin's method
for providing (lightning) protection has been made less effective
than it could be by his urging that the tip of lightning rods be
sharpened.'' Working with Moore on the project were researchers
Graydon D. Aulich and William Rison. Geophysical Research Letters
is published by the American Geophysical Union.