Friday 19 September 2014

How lightning victims were saved by the bright sparks

Lightning used to be a real menace – until someone had the brilliant idea of locking a Frenchman into a wooden box


Here in southern France, the thunderstorm season is upon us. A hot, steamy day, and over the mountains, towering clouds, flashes of light, and a threatening rumble. Fortunately I am safe, for some time ago I bought a battered old lightning conductor covered with decorative spikes from the local scrapyard and fixed it to my roof.

So universal are such devices today that the horrors of thunderbolts have largely been forgotten. But once, they were a real menace. The highest – and hence most vulnerable – point in a town or village was usually the church spire. To scare off the demons in the sky, its bells were rung at the height of a storm. That could lead to disaster. Again and again, spires were struck, leading to the death of thousands of bell-ringers and believers. All this discommoded the clergy, for God seemed to direct his wrath not at the sinners, but at the faithful.

The very first lightning conductor was erected in May 1752 here in France. A man was placed in a sentry box, from the roof of which protruded a long metal rod that rose into the air and descended into the box itself. As storm clouds passed overhead, the unfortunate subject would be ordered to hold a piece of iron and move it closer to the rod. Sparks might then pass between the two as the charge grounded itself through his body. The experiment was tried on a squally day at Marly, a village 20 miles north of Paris, and was a great success. The idea had in fact come from a book by Benjamin Franklin, who a few weeks later flew his own famous kite with a key attached to the wet and conductive string. (Still, had the rod been hit by lightning, the subject in Marly would – like a German physicist studying such a device just a couple of years later – have fried.)

A lightning flash is an electrical spark a kilometre or so long. It is generated by the churning of the atmosphere, which causes hail and raindrops to generate an electrical charge as they rub against each other, just as someone who walks on a nylon carpet may pick up enough static to get a nasty shock when they touch a doorknob.

In a thunderstorm, hot air shoots upwards. As the air rises, it cools, taking with it billions of tiny ice crystals. Its water may also freeze into larger hail particles, which may either prove heavy enough to fall or stay at a constant altitude. The two elements rub against each other and – in most clouds – a positive charge borne on the ice particles rises to the top, while lower down there is a matching negative charge.

Lightning flashes within the cloud when the imbalance between the two becomes too great, and – less often – may strike the ground when the charge at the base of the cloud gets large enough, and close enough to the surface, to send down a “leader”. Sometimes, the leader has a positive charge instead, so a rare flash can travel from ground to air, usually from tall buildings or mountain tops. They often emit a menacing hum before the spark itself.

We live on a lightning-plagued planet, with 10 million strikes a day and, at any time, around 2,000 thunderstorms. No more than one strike in 100,000 kills someone – a proportion far smaller than in the old days, but still notable (men are several times more likely to be killed than women, not because they attract thunderbolts, but because they spend more time outdoors).

The science of lightning has been filled with controversy. What is better, a pointed rod or one with a rounded end? The British (and the French) felt that a ball welded to the tip improved its efficiency, while the Americans stuck to Franklin’s sharp points. A decade or so ago, it was shown that the Europeans were right. As a leader sparks down from the skies towards its target, it generates a “streamer”, a matching torrent of charge that flows up from the rod, meeting its partner to complete the circuit. A rounded tip with a diameter of about a centimetre generates a longer streamer and hence makes the contact fractionally earlier. Some models incorporated radioactive materials into the tip to make even bigger streamers, but that idea has been abandoned.

My own rod has a design fault – for, elegant as it is, my own idleness (and the cost of copper) means that it is not connected to the ground, and is hence useless. Time, perhaps, to ring whatever bells are in the house at the sound of distant thunder.

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