Lightning Page 139

Nikola Tesla watched, elated, as the needle on the ammeter pinned. The room was filled with a roaring, crackling noise and the smell of ozone permeated the air. The scientist walked outside. High atop his laboratory, a metallic globe was alive with a hissing blue corona. While he watched, fascinated, a bolt of man-made lightning leaped from the globe to the ground. Seconds later, another bolt leaped out a little farther than the first; then another struck farther still. Tesla's machine even seemed to be drawing lightning from the air around it.

Suddenly the noise stopped. Silence closed in like a cloak as the machine went dead. Tesla rushed back into his lab. His machine was intact, but there was no power. The experiment had been terminated by a local power failure. Furiously he telephoned the power station demanding an explanation.

"You're not the only one who wants an explanation," the voice on the other end of the line retorted, "You've just melted the windings in three of our generators. It'll be a cold day in hell before we send any more power out to you!"

It is doubtful that even this brilliant scientist fully realized the awesome magnitude of power involved when he tried to produce lightning, for although it is usually dissipated with comparatively little damage, a single stroke of lightning can contain as much electrical energy as today's average family consumes in a week. A storm of moderate intensity can produce many times the energy of a nuclear bomb.

The actual amount of power in a lightning stroke is the subject of considerable disagreement in the engineering world. Some authorities say that the voltage represented by a lightning stroke is in the neighborhood of thirty million volts. Others claim the voltage can be as high as 900 thousand volts per foot. At that rate, the voltage of a stroke, which can be as much as ten miles long, would reach astronomical magnitudes. The current, on the other hand, is generally agreed by the various authorities to range from four thousand to a quarter million amperes. The duration has been measured at 5 to 50 microseconds.

The electrical charges that produce lightning are thought to be produced by friction between rapidly moving raindrops and the surrounding air. Thunderheads are known to contain vertically moving air columns which reach velocities in excess of 100 miles per hour. Raindrops and ice particles, moved about by these currents, develop electrical charges. The vertical movement accumulates these charges much in the same manner as the vertically moving belt in a Van de Graaff generator.

The turbulence within a thunderhead develops a strong negative charge in that region of the cloud where the temperature is between 0° and 32°F. A positively charged zone develops in the upper portion of the cloud, and the lower portion of the cloud contains both positively and negatively charged zones. From this it can be easily understood why 65% of all the discharges are within a cloud or between clouds.

Lightning strokes, either within or between clouds, or from a cloud to the ground, occur when the voltage between charges of opposite polarity exceeds the breakdown potential of the intervening air gap. This breakdown is of a different nature than the breakdown of a small air gap. There appears