100 THUNDERBOLTS A SECOND.

Auckland Star, Volume LXVIII, Issue 49, 27 February 1937, Page 10 (Supplement)

100 THUNDERBOLTS A SECOND.

RECENT oliservation has proved the 1 faNity of the old adage that light- i ning never strikes twice in the } 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 Xew 1 ; York one morning before daybreak, the ; : Empire State Building was hit 15 times 1 . in as many minutes. Five of these' magnificent strokes were photographed ] by an alert photographer from his apart- ! , ment window. Xo damage was done to j ; the building, for its steel framework is so large and so thoroughly grounded that | it can take care of the heaviest discharge j with perfect ease. Xot 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 the zone of protection afforded by a steel building is cone shaped. If you take the 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 mile 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 a brilliant play of lightning on the spire of the Woolworth Building in Xew York, a sudden bolt flashed between a cloud and the metal dome of the Xew \ork World Building, a considerably lower structure on the opposite side of City Hall Park. When the distance wa« computed it was found that although the base of the World Building was within the protected cone of the Woolworth, its dome protruded and thereby offered itself as an inviting path to any nearby overloaded cloud. City Safer than Country. The city dweller is safer from lightning than the country dweller, both because of the steel" buildings and because of the network of electric power, light and communication wires with their lightning arresters and other 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 ten of these buildings burned were not protected by

lightning rods. Most of the deaths from lightning—and about 600 per year f r is the toll for the United States and £ Canada —occur in the open country. | The safest procedure when caught in | the open Li a thunderstorm is to lie | flat on level ground. Better to get wet I than to burn. 'i Both automobiles and aeroplanes have I been struck, but very rarely, and neither | of these vehicles is to be considered a danger spot. If an aeroplane is in, the path of \ discharge between two clouds or between a cloud and the ground, it I may get hit—but a cautious pilot avoids | mixing with thunderclouds. Automobiles owe their relatively low percentage i of hits to the fact that they are close t to the ground and thus are protected by l~ trees, telephone posts and other taller |. objects around them —their safety probJ ably lies in this more than in any insu- | lation provided by their rubber tyres, [g l»nt don't park under a tree in a thunderft a* or m! H What the Camera Shows. ■ Not only can we say that lightning K often strikes again in the same place, » but we may add with the assurance of Rr recent scientific research that it always w strikes at leart twice in the same place. W That is to say, photographic studies ■ show that every lightning bolt is in H| reality two—one down striking from the K cloud, the other upstriking from the B ground—and each of these in turn K/ appears to be a stepped series of very k rapid strokes. The flashes come in K-: quick succession, the torrent of electricity Hfe which makes up each stroke travels ■ thousands of miles a second, and the whole combination of performances is so H:; almost instantaneous that the eye sees the complicated sequence as a single vivid flash. These photographic studies were made 80. possible by the use of a revolving. double-lens camera, the invention of the Hp--' English physicist, C. J. Boys. The idea itself is not new. Indeed, Dr. Boys inTented his camera many years ago, and K; carried it with him 2ti years in vain attempts to photograph a double image of a thunderbolt. The lightning eluded him again and again, but finally, while he waa on a visit to the United States,

All Threats To Life And Property.

LIGHTNING, SAYS SCIENCE, STRIKES TWICE.

his camera made its lucky shot. During I a thunderstorm in Xew York State he got the picture he hail long been seeking. Series of Downward Darts. The next act in this drama of research .-hitts 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. Schonland, of the University of Caj>etown, and Kngineer H. Collens, of the Victoria Falls and Transvaal Power Company, saw in the Boys camera a strategic tool for their use. They improved the design j of the camera, increased its speed, and with 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 the cloud downward to the ground. The! first down-coming dart may run for 200 feet, then it disappears for about a tiftj -millionth of & second, to be succeeded by the next streak which darts downward another 200 feet or so, to another break, to be followed by a third streak. In this way, by a succession of 18 to 20 steps, the downward stroke, or "leader," as Dr. Schonland calls it, reaches the ground. This leader tunnels a path of electrification through the air, and immediately the next phase of lightning follows—along the same trail but in the opposite direction. For now a more intense flame-like stroke surges from the earth upward to the cloud. A tremendous blasting of air particles by electricity moving under the 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 doubtless the main destructiveness of lightning. Speeds Exceed Radium Shots. What the photograph allows is two distorted image* of the liirlitiiiup^—but these displacements arc tell-tale thumbprints to the scientific detectives. Knowing the velocities of the lenses, they are able to work otit from the distorted lines of the photograph not only the duration of each lightning flash, but also its direction of motion and its speed. Speeds as high as .'51.000 miles an hour have been recorded. Years ago C. T. R. Wilson suggested that the enormous electrical pressures or voltages associated with a lightning

discharge might impart high energy to I particles. Dr. Schonland finds that such is the case. In the lightning discharge air molecules are smashed, the subatomic particles known as electrons are knocked out of their atoms, and then sent hurtling off at terrific speeds. Because these electrons are electrically charged, they respond to the influence of the earth's magnetism, and so are swerved into curving paths whose final effect is to make them sweep down in a westerly direction, as though they were coming from the east. Their influence may be appreciable at a distance of 2000 miles from their place of generation, says Dr. Schonland. And since thunderstorms are raging somewhere continually, with an average of 6000 lightning bolts flashing through the earth's atmosphere every minute, we

may picture the thunderbolt rays as continually speeding through the air at velocities exceeding those of the particles shot out of radium. These "run away" electrons, as they have been called, acquire such high speeds that they will jienetrate thick plates of metal. They are detected by the sensitive instruments used for measuring the cosmic rays, which they closely resemble. In fact, C. T. R. Wilson has suggested that the cosmic j rays are made up. in part at least, of these run-away electrons; so that the cosmic rays may not be so "cosmic" after all. Growth of Protective Knowledge. Whether the electrical wild horsepower of the atmosphere will ever be tamed and harnessed is a question; but there is no doubt that science is advancing rapidly in its understanding of the thunderbolt, and that engineering is rapidly putting this knowledge to use in protective devices. Forty years ago it was customary for electric power plants to shut down during a thunderstorm. To keep the transmission lines open was to court disaster. But to-day systems operate at full capacity during the most severe lightning conditions, and breakdowns from this cause have become exceedingly rare. Only a few years ago lightning was one of the most serious menaces to oil fields and tanks where petroleum and gasoline are stored, but to-day protective- overhead systems conduct the incendiary bolts harmlessly into the ground. Perhaps it is not too much to expect that some day the forests—the most numerous victims of lightning— will be effectively protected against this greatest of all sources of forest fires.