IV.

Protective Measures

Buildings are protected from lightning by providing them with metallic lightning rods extending to the ground from a point above the highest part of the roof. These rods form a low-resistance path for the lightning discharge and prevent it from traveling through the structure itself. Power lines and radio sets with external aerials are protected against lightning by lightning arresters that consist of a small gas-filled gap between the line and ground wire. This gap offers a high resistance to ordinary voltages, but a lightning discharge, which has a potential of tens of millions of volts, causes the gas in the gap to ionize, providing a low-resistance path to earth for this discharge.

Three common and erroneous ideas about lightning ought to be mentioned. One is that lightning never strikes twice in the same place. Photographic evidence shows that skyscrapers and other tall structures may be struck many times in the course of a single storm. A second idea is that the safest place to stay in a thunderstorm is under a tall tree. Trees, because of their height, are apt to be struck by lightning and are, therefore, actually dangerous during violent electric storms. The safest places for a person outdoors in a thunderstorm are inside a metal-bodied car or lying flat on the ground in the open. A third misconception is that lightning is always clearly associated with thunder. Observers who listen for thunder as a cue may miss up to 40 percent of lightning strokes.

In the U.S. about 100 persons are killed and many injured by lightning each year, more than by tornadoes or hurricanes. Forty percent of all farm fires and 75,000 forest fires a year are started by lightning. Lightning is not all bad, however. The soil is enriched with nitrogen that is released from the atmosphere by lightning and carried to the ground by raindrops. Some scientists believe that lightning may have been a key element in the origin of life on earth, creating from simple elements complex chemical compounds that gave rise to living matter.