How does a lightning rod avoid electricity? Why does a lightning rod protect against lightning?

The anti-direct lightning device is composed of three parts: a lightning receiver, a down conductor and a grounding device. The lightning receiver is commonly referred to as the "lightning rod", which is connected to the earth by down conductors and grounding devices. The lightning rod stands tall, higher than all the objects being protected, and is closest to the thundercloud, so it is the most likely to attract lightning, which is discharged into the ground through the discharge channel provided by the lightning rod.

In this way, although the lightning strike still happens, it always hits the lightning rod instead of the object to be protected, so the lightning rod is actually a lightning rod. It attracts lightning to itself, causing it to be discharged to the ground through the downwire to the grounding device, thus protecting the object from lightning strikes.

The modern lightning rod was invented by the American scientist Benjamin Franklin. Franklin believed that lightning was a phenomenon of electrical discharge. To prove it, on a thunderstorm day in July 1752, he risked being struck by lightning, and together with his son, he did a very famous experiment with lightning.

He flew a kite with a long metal wire tied to it into a thundercloud and tied a string of copper keys to the end of the wire. When the lightning occurred, Franklin's hand approached the key, and a string of electric sparks burst out from the key, and there was a numbness in his hand. Fortunately, the lightning that came down this time was relatively weak, and Franklin was not injured.

The experiment was so successful that Franklin became aware of the nature of lightning and invented the lightning rod, first installing it in his Philadelphia home. Since then, lightning rods have become popular all over the world.

According to historical records, during the Han Dynasty in China, there was a fire caused by a lightning strike in a large hall, and a sorcerer later put a copper tile in the shape of a fish's tail on the top of the floor, thus preventing the recurrence of lightning strikes. Experts believe that this copper tile is the prototype of a modern lightning rod.

How does a lightning rod protect against lightning? How much can a simple iron rod protect?

In thunderstorms, when there are charged clouds over high-rise buildings, a large number of charges are induced on the lightning rod and the top of the high-rise building.

In this way, the lightning rod gathers most of the charge. The lightning rod forms a capacitor with these charged clouds, and because it is sharp, that is, the area of the two poles of the capacitor is very small, and the capacitance is also very small, which means that it can hold very little charge.

And it collects most of the charge, so when there is more charge on the clouds, the air between the lightning rod and the clouds can easily break down and become a conductor.

In this way, the charged cloud layer forms a path with the lightning rod, and the lightning rod is grounded, and the lightning rod can channel the charge on the cloud layer into the earth, so that it does not pose a danger to the high-rise building and ensures its safety.

According to China's statistics, the maximum amplitude of lightning current is about 300 ka, and the corresponding lightning strike height is 408 m. The lightning strike positioning height is 400m, and the surface radius of the protection area and scattered strike area of the lightning rod at different heights can be obtained as shown in Table 1.

The height of the old-style houses in China is generally below 10 m, the height of the lightning protection belt and lightning protection net is the same as the height of the house, and the height of the installed short needle lightning protection is 1 2 m, and the scattering phenomenon caused by them is not obvious. Lightning strikes from tall buildings and elevated lightning rods lead to increased lightning strike rates and scattered strike zones.

China's lightning defense scholars have never advocated the use of elevated lightning rods to protect buildings, and advocated the use of short roof needles and lightning belts for lightning protection, which is to consider that it can not only play its role in lightning guidance, but also avoid increasing the number of scattered mines.