Lightning No Menace in City

Nikola Tesla Says Individuals Are Safe

Electrical Inventor Declares Tall Steel Structures Act as Conductors

The summer thunderstorms that have swept over the city within the last few days have raised the question of the danger to human life by lightning in New York City. Incidents such as the shattering of the cornice of the World Tower Building by a lightning stroke give added interest to this question: Is the inhabitant of a congested city in greater danger from thunderbolts than a person in the country? Nikola Tesla, in an interview to-day, said there is no danger to life from lightning in the city. The metallic construction of modern city structures, which are veritable steel shells, acts as a vast protective system against lightning, he said, and for this reason the city dweller is entirely free of this danger, whereas a person in the open country during a thunderstorm faces a very real risk.

Dr. Tesla also cleared up many points in regard to lightning that have hitherto been unknown or in controversy. The power expanded in the average lightning flash would be sufficient to run a 5,000 horsepower motor for one year, he said, contrary to common belief all lightning does not discharge from clouds to the earth, but the most powerful flashes occur from the earth towards the clouds, a thunderbolt that appears to the eye as a single flash may in reality consist of hundreds or even thousands of separate flashes.

But those who dread thunderstorms will look most askance at Dr. Tesla's statement that without lightning there could be no survival of life on earth. Without lightning there could be no circulation of water from the ocean, through the atmosphere, and to the earth as rain, he asserted, and in this incessant life-giving circulation of water the electrical discharges actually control the precipitation.

Dr. Tesla, whose office is at 8 West Fortieth Street, is the inventor and discoverer of electrical and mechanical devices and principles of the first importance, among which are the Tesla transformer, Tesla turbine, lighting systems, and power-transmission systems.

"The average person does not suppose that we owe our very existence to atmospheric electricity," he said. "The sun evaporates the waters of the ocean, and the minute droplets are carried by air currents to distant re- gions where they are held in a state of delicate suspension until electrical forces cause them to aggregate in dense cloud masses. When the electrical tension becomes excessive, flashes occur, and a copious precipitation is the result. So it comes that lightning maintains a circulation of the water and thereby all life. "It would not be exact to say that there would be absolutely no rain without lightning, but it is a fact that lightning is the chief controlling agent.

Man Can Control Power

"Years ago it seemed beyond the power of man to generate electrical forces and disturbances comparable to those we witness in natural displays. That through gradual perfection of electrical methods and apparatus we have reached the stage where it appears that man will have it in his power to convert deserts into fertile land, and to produce lakes and rivers wherever desired, thus providing motive power and also greatly increasing the productivity of the soil.

"Many popular misconceptions exist pertaining to the power of lightning. When I began my investigations I was convinced that a lightning bolt involved no more than a few horse-power, but as I advanced in knowledge I satisfied myself that the power was immense. Ordinarily it is believed that the tension of the lightning discharge, as well as the current flowing through the arc, is moderate, but the fact is that the tension often reaches hundreds of millions of volts, and the currents are not infrequently of a strength of several million amperes.

"The energy of lightning is represented by half the product of the electrical capacity of the cloud and the square of the electrical potential. When the flashes take place the greatest part of this potential energy is dissipated in the form of electro-magnetic waves, another part is used in generating heat, and still another portion appears as sound and light. An idea of the energy involved in lightning may be had when I state that sometimes merely the sound wave, representing a very small fraction of the whole energy of a flash, would be sufficient to run a 200 horsepower engine one year. The entire power of a lightning discharge, however, is such that an engine of 5,000 horsepower could be kept running at full capacity for a year, and in some instances the power is still much greater.

"The reason for the enormous electrical tension lies in the fact that the curvature of the cloud on the underside is very small, so that a tremendous electrical pressure is needed to break down the air stratum between a cloud and earth. For example, spheres with a radius of about 40 centimetres must be charged with a pressure of more than 3,000,000 volts to produce an electric streamer from its surface. The required tension increases in direct proportion to the radius of the sphere, so that the discharge from such a body as a cloud, which is virtually flat on the underside, calls for a tension of billions of volts.

May Go From Earth to Cloud

"It is a popular belief that lightning always strikes from cloud to earth, but as a matter of fact the most powerful discharges occur from the earth towards the cloud. I have seen some which at a distance of fifteen miles appeared like gigantic trees of fire with innumerable branches springing from the heavy trunk connecting to the ground. According to my computation, based on experimental data obtained with a wireless transmitter, the current near the earth in these flashes must have been several million amperes."

 "About New York lightning displays are comparatively rare, but in certain regions they are common. To mention one instance, instruments at my wireless plant at Colorado Springs recorded nearly 13,000 discharges inside of two hours, and all within a radius of fifteen miles, on July 3, 1899. The energy involved in the displays for the time they lasted amounted to several billion horsepower.

"But an equally interesting fact that I have found is that lightning discharges sometimes occur that do not involve more than a few horsepower. On two or three occasions I have observed some so feeble that the tiny spark passing from the cloud to the earth was hardly visible, and the sound produced was not more than the feeble crack of a whip.

"We cannot think of lightning without remembering the man whose epitaph is Fulgurum eripuit caeli sceptrumque tyrannis, but Franklin made one mistake — perhaps the only one he ever made — and that was in believing that metallic points would discharge the earth, and so save the building equipped with his pointed rod. In those times he had nothing on which to base his conclusions save his observations with a static machine, which he knew would be discharged by a needle point. In the case of lightning it is just the opposite. The point causes a brush discharge which ionizes the surrounding air and attracts the lightning. But fortunately Franklin was right in the rest of his theory that the bolt would pass through the rod into the earth (or from the earth through the rod to the cloud) without damage. This is the case as a rule, but occasionally when the discharge is too powerful it overflows the rod and causes severe damage.

"Observations with a powerful electrical discharge which I produced with a wireless transmitter built on a novel principle have enabled me to devise a form of lightning protector which is virtually infallible. The underlying principle is to prevent the accumulation of electricity in the vicinity, so that the bolt will strike anywhere but the place thus protected. This device has proved its efficacy beyond any doubt, and no structure protected in this way has ever been struck.

"It is generally believed that a stroke of lightning consists of a single bolt, but there may be hundreds or even thousands of them in one apparent flash. Under favorable conditions it is possible to count them by observing the falling rain drops from a darkened window. Each separate flash illuminates a falling drop in a different position, so that the effect to the eye is that of a series of glittering drops in a vertical line. The series of drops is, of course, the same drop illuminated in different positions by the separate lightning flashes. Now if the drops in this series appear half an inch apart, and if the drop is falling at a rate of 100 feet a second, then during the lightning flash, lasting say one second, there have been 2,400 separate bolts.

"The majority of persons are afraid of lightning and generally do not know what to do in case of danger. People should know in the first place that in cities, with buildings practically all of steel girders, there is absolutely no possibility of being injured, no matter how intense may be the storms.

"But in the open country, if one is walking or driving in an automobile, precautions should be taken as soon as a thunderstorm appears. One is almost surely safe if one selects a depression in the ground and keeps away from trees or tall structures. He should not build a fire, since the heated air offers an easier path for a bolt from the earth to a cloud, nor should be expose himself in the open. And if one is in an open building one should keep in the centre of the room and away from all metallic objects."