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Small sparks pack a big punch

October 28, 2005 By Philip Ball This article courtesy of Nature News.

Sparks made in the lab are found to emit X rays like lightning.

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Studying lightning in the lab could be much easier than previously thought. You don't need to make huge sparks to get the same kinds of effects seen with lightning bolts, researchers report: a simple electrical discharge in the lab can do the trick.

That's good news for researchers still puzzling over how lightning works.

Lightning bolts are thought to trigger a process called 'runaway breakdown'. As the surge of electric charge passes through the air, it ionizes atoms and accelerates electrons to speeds that approach the speed of light, leading to the emission of X-rays and gamma rays.

Lightning remains a mystery.
Joseph Dwyer
Florida Institute of Technology.
"It is like what happens in a particle accelerator, or in the X-ray machine at the doctor's surgery," says Joseph Dwyer, a physicist at the Florida Institute of Technology in Melbourne, Florida. Sparks generated in laboratories were thought to involve 'conventional' breakdown, in which the air is still ionized but the electrons don't get accelerated to such high speeds.

Charged up

"People assumed that conventional electrical discharges weren't like lightning," says Dwyer. "It turns out we were wrong." There is a striking similarity between the two, he says.

He and his colleagues generated sparks in the laboratories of Lightning Technologies in Pittsfield, Massachusetts. This involved charging up a bank of capacitors to about 100,000 volts each, and then allowing them to discharge between two metal spheres held 10 centimetres apart.

In every case, Dwyer and his colleagues detected X-rays emitted by the spark that resulted. They report their results in the journal Geophysical Research Letters1.

Dwyer says that the X-ray intensities he has seen aren't sufficient to pose a health hazard. But the findings could help unravel what lightning actually is.

Enlightening rays

"Lightning remains a mystery," says Dwyer. It doesn't travel from cloud to ground in one step, but advances in a series of hops. Each hop produces X-rays. "So understanding the production of X-rays can tell us about how lightning propagates," Dwyer says.

Now it seems that researchers may be able to do that with simple lab equipment. But, he adds, not every electrical spark is a source of X-rays. He and his colleagues took their X-ray detectors to the Boston Science Museum in Massachusetts, where an immense van der Graaf generator produces a simulated lightning display for visitors every day. They couldn't detect any X-rays at all from these artificial blasts, implying that the museum's 'lightning show' isn't so realistic after all.


  1. Dwyer J. R. et al. Geophys. Res. Lett., 32. L20809 (2005).


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