How Franklin’s Kite Saved The Day: The History of Ordinary Things
Lightning strikes are dangerous when they hit a structure resulting in a fire, or electrocution if they hit a person. Lightning is caused when excess negative electrical charge builds up in the clouds. It is discharged when a lightning bolt jumps from the clouds to the positive charge on the ground. Lightning can strike at one third the speed of light and at temperatures exceeding 50,000 degrees Fahrenheit. It can damage structures made of masonry, wood, concrete and even steel. The strong electrical current can heat materials, especially water, to high temperatures which can cause fire, loss of material strength and explosions from super-heated steam and air.
One day, Benjamin Franklin was flying a kite when it was struck by lightning that burned it up. He wondered if it were possible to attract lightning bolts. He tied a metal key to his kites and continued flying them on stormy days until he was able to capture another bolt. Electricity went down the string of the kite until reaching the key. His kite experiments proved that thunderclouds are electrified, and that lightning is an electrical discharge.
Franklin proposed that a building could be protected from lightning by placing a pointed iron rod at the peak of the building with a wire, often copper, running down the building to another rod buried into the earth. Once on the ground, the electricity of the lightning bolt would be diluted and absorbed. In 1752 he finalized his design of the first lightning rod and the following year the first rods were installed.
In the ensuing years, major buildings added lightning rods with great success. The use of lightning rods on roofs in the United States (and later in the rest of the world) has saved countless lives and has prevented unknown numbers of fires.
Solid glass balls were incorporated into the design of some lightning rods just below the highest point. Glass is an insulator and does not conduct electricity. If struck by lightning, the glass will shatter. This served to confirm an electrical strike and suggest the lighting protection system should be inspected for damage.
The Eiffel Tower was designed as a giant lightning rod. At over 325 meters tall, the Tower receives an average of 5 lightning strikes per year. Another example of a modern lightning collector is a construction crane. Most residential homes do not have lightning rods due to the rare occurrence of strikes on small structures. Long-time firefighters might disagree.
Today, lightning rods are required on structures over 200 feet tall (60 meters). Most are of the traditional Franklin design. A system of lightning protection conductors and lightning rods are installed on the building to intercept any lightning before it strikes the building. A wire enclosure, much like a Faraday cage, surrounds the building to conduct and block electromagnetic fields. The lightning rods are not guaranteed to withstand the electrical voltage of all strikes as they vary in intensity. The lifespan of a protection system is about 30 years. Humm. Who knew?
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