LIGHTNING, SAYS SCIENCE, STRIKES TWICE

Dominion, Volume 30, Issue 143, 13 March 1937, Page 26

LIGHTNING, SAYS SCIENCE, STRIKES TWICE

One Hundred Thunderbolts A Second

ttecciil observation Ims proved the falsity of the old adage Unit lightning never strikes twice in I lie same place. Indeed, in the case of many skyscrapers, eye-witnesses have seen a

single building struck again and again. During a violent thunderstorm in New York one morning before daybre.ak, the Empire State Building was hit 15 times iti as many minutes. Five of these magnificent strokes were photographed by an alert photographer from his apartment window. No damage was done to the building, for its steel framework is so large and so thoroughly grounded that it can take care of the heaviest discharge with perfect case. Not only does the deep-rooted steel skeleton of this tallest tower provide complete protection for itself and its occupants, but it serves as an effective lightning rod for the surrounding neighbourhood. Laboratory tests show that Hie zone of protection afforded by a steel building is cone-shaped. If you take (lie centre of the ground floor as the centre of a circle whose radius is two and a half times the height of the building, and describe a cone connecting this circle on the ground with the top of the tower, all that lies within the cone may be regarded as safe from ordinary lightning—i.e., from clouds one mite high or higher. Since the Empire State Building is 1250 feet tall, this means that everything on the ground within a radius of half a mile is protected—provided it does not protrude outside the cone.

Several years ago, during the brilliant play of lightning on the spire of the Woolworth Building in New York, a sudden bolt flashed between a cloud and the metal dome of the New York World Building, a considerably lower structure on the opposite side of City Halt I’ark. When the distance was computed it was found that although tlie base of the World Building was within the protected cone of the Woolworth. its dome protruded and thereby offered itself as art inviting path to any nearby overloaded cloud. The city dweller is safer from lightning than tlie country dweller, both because of the steel buildings ami because of the network of electric power, light and communication wires with their lightning arresters and oilier grounded conductors. Of 749 fires by lightning reported in lowa for a fiveyear period, only 153 occurred in towns; the remaining 596 were farm

dwellings and barns. Nine out of len of these buildings burned were not protected by lightning rods. Most of the deaths from lightning—and about 600 per year is tlie toll for tlie United States and Canada —occur in the open country.

The safest procedure when caught in the open in a thunderstorm is to lie Hat on level ground. Better to get wet. than to burn.

Both automobiles and aeroplanes have been struck, but very rarely, and neither of these vehicles is to bo considered a danger spot. If an aeroplane is in the path of a discharge between two clouds or between a cloud and the ground, it may get hit—lint a cautious pilot avoids mixing with thunderclouds. Automobiles owe their relatively low percentage of hits to tlie fact that they are close to the ground and thus are protected by trees, telephone posts and other taller objects around them —their safety probably lies in this more than in any insulation provided by their rubber tires. But don't park under a tree in a thunderstorm !

Not only can we say that lightning often strikes again in the same place, but we may add with the assurance of recent scientific research that it always strikes at least twice in the same place. That is to say, photographic studies show that every lightning bolt is in reality two—one down striking from the cloud, the other upstriking from tlie ground—and each of these iu turn appears to be a stepped series of very rapid strokes. The Hashes come in quick succession, the torrent of electricity which makes up each stroke travels thousands of miles a second, and the whole combination of performances is so almost instantaneous that the eye sees the complicated sequence as ti single vivid flash. These photographic studies were made possible by the use of a revolving, double-lens camera, the invention of the English physicist, C. J. Boys. The idea itself is not new. Indeed, Dr. Boys invented his camera many years ago, ail'd carried it with him 26 years in vain attempts to photograph a double image of .a thunderbolt. The lightning eluded him again and again, but finally. while he was on a visit, to the United States, his camera made its lucky shot. During a thunderstorm in New York State lie got the picture lie had long been seeking.

Ilie next act ill Ibis drama of research shifts to South Africa. There the thunderstorms are-extraordinarily severe, and several institutions have joined forces to seek the true secret of the mysterious lightning which is the cause of so much destruction of property and life. Professor B. F. J. Sehonland, of the University of Capetown, and engineer 11. Colletts, of tlie Victoria Falls and Transvaal Power Company, saw in tlie Boys: camera a strategic tool for their use. They improved the design of the camera, increased its speed, and witii the support of the South African Institute of Electrical Engineers placed several of these swift instruments out in the veldt. In two thunderstorms they secured revealing photographs of 18 lightning flashes, comprising 50 separate strokes. From these data they were able to work out a remarkable analysis of what is happening when wild electricity crashes its thunder and darts its blinding light. First of all, says the revolving camera, there is a thin spurt of electricity from tlie cloud downward to the ground. The first down-coming dart may run for 200 feet, then it disappears for about a fifty-millionth of a second, to be succeeded by tlie next streak which darts downward another 200 feet or so. Io another break, to be followed by a third streak. In this way. by a succession of IS to 20 steins, the downward stroke, or “leader,” as Dr. Sclionland calls it, reaches the ground. Tliis leader tunnels a path of electrification through the air, and immediately the next phase of lightning follows—along (be same trail but iu the opposite direction. For now a more intense flame-like .stroke surges from tlie earth upward to the cloud. A tremendous blasting of air particles by electricity moving under tlie pressure of millions of volts takes place. It is this main stroke-—which may be a succession of many upward-moving surges—that provides the spectacle and doubt lestlie main destructiveness of lightning. What the photograph shows is two distorted images of the lightning—but these displacements are tell-tale thumbprints to the scientific detectives. Knowing tlie velocities of tlie lenses, they are able to work out from Hie distorted lines of the photograph not onlx tlie duration of each lightning flash, but also its direction of motion and its speed. Speeds as high as 31,000 miles an hour have been recorded.