91av

KING George III was right and Benjamin Franklin was wrong, at least when it
came to the tips of lightning rods. The American scientist and diplomat believed
lightning rods, which he invented in 1749, should have pointed tips, and his
design has been used for over two centuries. But tests on a New Mexico mountain
top show that blunt lightning rods—which George III decreed be used on
royal buildings—are actually more effective at attracting lightning.

Electric charge builds up during thunderstorms, and Franklin’s original aim
was to prevent lightning by dissipating this charge, says Charles Moore, a
retired atmospheric physicist formerly at the New Mexico Institute of Mining and
Technology in Socorro. Franklin knew from his indoor experiments that earthed,
pointed rods could discharge a nearby charged object without a spark. But when
he placed a rod outdoors, he found that instead of preventing lightning strikes,
it attracted them. Undaunted, he realised that attaching a conductor to the rod
could divert the strike harmlessly to earth. This remains the basis of lightning
rods today, although a perfect design has never been agreed.

In the 1930s, researchers showed that when a sharply pointed conductor is in
an electric field, it is “protected” by a surrounding cloud of ions that reduces
the local field strength. Moore wondered if rounded tips would better attract
lightning because they lack these ions.

“We ran a competition between sharp and blunt rods,” with tips from 0.1 to 50
millimetres in diameter, he told 91av. During seven summer
thunderstorm seasons, he found that the ones struck most frequently had tips
around 19 millimetres in diameter. The rounded rods invariably attracted
lightning away from the pointed rods, none of which were hit. Moore calculates
that the electric field strength above a 19-millimetre blunt rod is higher than
that over an otherwise similar sharper rod, making it better at attracting
lightning.

George III had no great insight when he insisted on blunt rods, Moore notes.
“He did it out of political pique,” because he was angry at Franklin for his
support of American independence. “Franklin was right in putting up a
conductor,” he adds. “We just fine-tuned him.”