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Lightning has occurred on earth much longer than man has been around. Because of this, an aura of mystery and misinformation has evolved that continues to mislead us today. The idea that man can do nothing to prevent or divert a lightning strike is a popular misconception. In most cases, lightning can be safely diverted or better yet prevented from striking. With all the mysticism stripped away, science clearly explains the causes and functioning of lightning. Let's review the current best practices to mitigate lightning damage.
The Franklin lightning rod was introduced more than two and a half centuries ago. Franklin was not at first looking to invent a device to capture lightning. He simply wanted to understand electricity in all forms. From his experiments and observations, he wrote a paper titled, On the Sameness of Lightning and Electricity. A friend presented this paper to the Royal Society in London. Their response was laughter and derision. However, within a decade they had to acknowledge that Franklin was right.
Scientists of this era were limited to storing electrical energy in a Leyden jar in the form of high voltage. The source of the voltage was primarily friction between various materials, such as shoe soles and nylon carpet. They soon learned that if the connection to the Leyden jar was in the form of a wire, it soon discharged itself into the surrounding air. To retain the charge, they learned to place a sphere often called a corona ball onto the end of the wire. A sharply pointed wire discharged the energy rapidly while the corona ball retained it. As the sphere became larger and larger in diameter, higher and higher voltages could be retained for longer periods of time.
Franklin observed that electricity actually flowed out into the air in this discharge, which he defined as a silent current. His idea for the lightning rod derived from this observation and his conclusion that that the "electric" in the area would be discharged via one or a few sharply pointed rods. Because the nature of electric current was not yet fully understood, he could not realize that each sharp point is limited to discharging just a small value, typically a few milliamperes. As a result, when the lightning rod industry developed during the 19th century, lightning rods were intended to intercept a strike and then conduct it to ground through a thick wire.
A relatively recent outgrowth of the Franklin rod is the ESE (early streamer emission) device. If a stream of ions is emitted from a sharp, elevated point, an upward streamer will occur in the powerful electric field accompanying a thunderstorm. An upward streamer is the normal means that initiates a lightning strike. In most cases, the streamer is positive in polarity while the cloud bottom is usually negative. When the field voltage is great enough, the upward streamer attaches to a downward negative streamer from the cloud.
Early ESE devices used a small amount of radioactive material to initiate the flow of ions into the air. However, because of concern over the radiation, ESE devices now contain a small, high-voltage source that initiates a stream of ions from a sharp, elevated point. These devices are quite popular in Europe and parts of Asia.
Some scientists have written that their effectiveness is questionable, claiming that the wind associated with most thunderstorms blows the ion stream away faster than it can be generated and flown upward.
Measuring the energy
The third method is predicated on the fact that a lightning strike is not inevitable. To understand it requires a study of the science behind a strike. For many years it was assumed that an average strike releases a massive quantity of energy. However, it has been learned over the last several decades that the quantity of charge (in coulombs) is relatively small. The reason a lightning strike is so damaging is because the energy in the lightning strike is delivered in an extremely short time period.
A coulomb is a unit of charge and may be simply defined as one ampere for one second. A coulomb is also defined as a specific number of electrons, regardless of voltage, current or anything else (approximately 6.24 x 1018> power electrons). Early researchers assumed that a lightning strike must contain thousands of coulombs of charge. Today, instrumentation is available that can measure the voltage and the current of a strike as well as the length of time the strike current flows. These values can vary widely in individual strikes, but the one that is most important is the length of time the strike current flows. A typical strike lasts from about 10 to 50 microseconds; although multiple strikes can occur down the same ionized channel until the charge is depleted.
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