LIGHTNING ROD

metallic rod (usually copper) that protects a structure from lightning damage by intercepting flashes and transmitting their current to the ground. Because lightning tends to strike the highest object in the vicinity, the rod is placed at the apex of the structure; it is connected to the ground by low-resistance cables. In the case of a building, the soil is used as the ground; on a ship, the water is used. A lightning rod affords protection because it diverts the current from the nonconducting parts of the structure, allowing it to follow the path of least resistance and pass harmlessly through the rod and its cables. It is the high resistance of the nonconducting material that causes it to be heated by the passage of electric current, leading to fire and other damage. A lightning rod provides a cone of protection whose ground radius approximately equals its height above the ground.