Benjamin Franklin invented lightning rods in the 18th century, and the devices have been protecting buildings and people from the destructive forces of lightning ever since. But the details of how lightning rods function are still the subject of scientific research.
Although modern lightning protection systems involve extra equipment that makes them more efficient, the lightning rod itself is quite simple: a copper or aluminum rod set above the highest point of a building, with wires connected to the ground. When lightning strikes a building it will preferably pass through the rod — the path of least resistance — and then through the wires into the ground, protecting the building and its contents from the extremely high currents and voltages produced by lightning.
But a rod doesn’t wait for the lightning to strike. Less than one millisecond before the lightning touches it, the rod, provoked by the presence of the negative discharge of the lightning, sends a positive discharge up to connect to it.
Brazilian researchers recently got lucky, photographing this event with high-speed video cameras at very high resolution. They captured the electric action in Sao Jose dos Campos, a city northeast of São Paulo.
The scientists were in the right place, at the right time, and with the right equipment to capture 31 of these upward discharges as they happened. With their location, about 150 yards away from the lightning strikes, and their camera, a device that records 40,000 images a second, they were able to take clear photographs and a slow-motion video of what happens in that instant before the charge from the rod meets the charge from the lightning bolt. The scientists’ study and photos were published in Geophysical Research Letters in December.
It was not only lightning rods producing these discharges, but also various corners of the buildings and other high spots. In fact, “Any person standing in an open area can similarly launch an upward connecting discharge from their head or shoulders and be injured by lightning even when not directly struck by it,” said Marcelo M.F. Saba, a senior researcher at the National Institute for Space Research in Brazil and an author of the study.
Does one kind of lightning rod provide better protection than another?
“Some salesmen say that their lightning rods are better than the rest,” Dr. Saba said, “but this is just sales talk. There is no solid research on that. There are standards that have to be followed by those installing lightning rods. That’s the best we can do for now.”
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