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Blunt Lightning Rods Are Better, May 30, 2000


SOCORRO, N.M., May 30, 2000 -- Among Benjamin Franklin's many useful contributions to science was his invention of the sharp-tipped lightning rod--a simple device which was originally intended to discharge thunderclouds and, thus, actually prevent lightning.

However, when Franklin tried out his novel idea by erecting iron rods beneath passing thunderclouds, he found that his rods did not prevent lighting, but instead were struck.

Since that time, the role of "Franklin rods" has been to help protect buildings, structures, and equipment by receiving lightning strikes and providing a preferential path to Earth for the natural electrical discharges.

Following Franklin's original suggestion, it has become traditional to use sharp-tipped lightning rods; although, the advantage of the sharpness has never been established.

In field tests conducted during the past six years on various configurations of differently shaped lightning rods, New Mexico Tech scientists determined that rounded, blunt-tipped rods were preferentially struck by lightning during thunderstorms, while sharpened Franklin rods exposed nearby remained untouched by lightning.

C. B. Moore, William Rison, James Mathis, and Graydon Aulich, all researchers at the university's Langmuir Laboratory for Atmospheric Research, recently published the results of their definitive study and measurements on the tips of lightning rods in current issues of both Geophysical Research Letters and The Journal of Applied Meteorology.

"Although lightning rods have long been used to limit damage from lightning, there are currently no American standards for the shape and form of these devices," the Tech researchers noted. "Following tradition, however, sharp-tipped Franklin rods are widely installed despite evidence that, on occasion, lightning strikes objects in their vicinity."

The field portions of the study were conducted on the 10,787-foot summit of South Baldy Peak, near Langmuir Laboratory, in the Magdalena Mountains of central New Mexico.

In the course of their investigation, the New Mexico Tech atmospheric scientists arranged a "lightning-reception competition" among sharp and blunt lightning rods on a mountain ridge over which thunderclouds frequently form during New Mexico's "monsoon" summer months.

Twelve of the blunt rods were struck by lightning, while none of the nearby sharp rods were ever hit.

The results of the study and of a theoretical analysis urther suggest that not only do moderately blunt metal rods serve as better lightning strike receptors than sharper rods, but they also are better receptors than very blunt rods, as well.

The findings from the study have been included in the National Fire Protection Association's 2000 standard for the installation of lightning protection systems (NFPA 780).

 

 


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