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Ben Franklin Design Flawed
By RANDOLPH E. SCHMID
Associated Press Writer

WASHINGTON (AP) MAY 15, 2000 — Among Benjamin Franklin's famous accomplishments was inventing the lightning rod. But 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.

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On the Net: New Mexico Institute of Mining and Technology: http://www.nmt.edu

American Geophysical Union: http://www.agu.org

 
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