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I was always under the impression that a Lightning rod worked to protect a building by being the tallest part of the building and having a low path of resistance to the ground, thus being a good target for lightning to hit, reducing the chances of lightning strikes elsewhere on the building.

However when reading an Ars technica article, one comment (from user "malor") said that theory is incorrect, and explained that a lightning rod actually prevents lightning strikes in the first place:

Lightning happens when a really huge negative charge builds up in the ground, corresponding to a positive charge overhead, and the differential becomes sufficient to jump the gap. (and, as others are pointing out repeatedly, air is an excellent insulator, so it takes a whacking huge voltage differential to make the arc.)

A lightning rod doesn't exist to provide a spot for the lightning to strike. Rather, it exists to dissipate the charge so that the strike never happens at all. This comes from early experiments with the Leyden jar; if a pointed metal rod was attached to the jar, it wouldn't charge. The electrons are able to leap off a pointed tip, and into the air, dissipating charge as fast as it accumulates. The lightning rod does the same thing, on a larger scale; it spits out electrons into the air like crazy, so that the charge won't build up sufficiently, and the lightning never hits at all.

Now, if the charge accumulates faster than the rod can dissipate it, there can still be a lightning strike, and the rod typically IS the best route to ground. But everything around the rod is going to take a hell of a jolt anyway, including, most likely, your electronics. A lightning rod getting hit means it failed to work adequately; ideally, it should never be hit at all.

How does a lightning rod actually work? Does a lightning rod actually prevent lightning strikes?

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migrated from skeptics.stackexchange.com May 25 '13 at 2:14

This question came from our site for scientific skepticism.

    
xkcd has a nice article on it –  ratchet freak May 24 '13 at 23:31
    
I've seen that before @ratchet. It's a great article! Still leaves the question of how a lightning rod works unanswered, however. –  Josh May 25 '13 at 13:22

2 Answers 2

This entry that describes the build up of charge is contradictory to the statement you post, free electrons do not escape to the air but go down in the ground leaving positive charges .

In the end, a storm cloud becomes polarized with positive charges carried to the upper portions of the clouds and negative portions gravitating towards the bottom of the clouds. The polarization of the clouds has an equally important affect on the surface of the Earth. The cloud's electric field stretches through the space surrounding it and induces movement of electrons upon Earth. Electrons on Earth's outer surface are repelled by the negatively charged cloud's bottom surface. This creates an opposite charge on the Earth's surface. Buildings, trees and even people can experience a buildup of static charge as electrons are repelled by the cloud's bottom. With the cloud polarized into opposites and with a positive charge induced upon Earth's surface, the stage is set for Act 2 in the drama of a lightning strike.

And further into the narrative:

As the electrons of the step leader approach the Earth, there is an additional repulsion of electrons downward from Earth's surface. The quantity of positive charge residing on the Earth's surface becomes even greater. This charge begins to migrate upward through buildings, trees and people into the air. This upward rising positive charge - known as a streamer - approaches the step leader in the air above the surface of the Earth.

lightning

The entry concludes with:

Lightning researchers are now generally convinced that the lightning dissipation theory provides an inaccurate model of how lightning rods work. It is indeed true that the tip of a lightning rod is capable of ionizing the surrounding air and making it more conductive. However, this affect only extends for a few meters above the tip of the lightning rod. A few meters of enhanced conductivity above the tip of the rod is not capable of discharging a large cloud that stretches over several kilometers of distance. Unfortunately, there are currently no scientifically verified methods of lightning prevention. Furthermore, recent field studies have further shown that the tip of the lightning rod does not need to be sharply pointed as Ben Franklin suggested. Blunt-tipped lightning rods have been found to be more receptive to lightning strikes and thus provide a more likely path of discharge of a charged cloud. When installing a lightning rod on a building as a lightning protection measure, it is imperative that the rod be elevated above the building and connected by a low resistance wire to the ground.

In conclusion the function of the lightning rod is to provide an easy grounding with minimal damage to the building.

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+1, Thanks! This is useful information. However, unfortunately physicsclassroom.com doesn't cite any sources. –  Josh May 25 '13 at 13:28
    
en.wikipedia.org/wiki/Lightning_rod and en.wikipedia.org/wiki/Lightning have a lot of references . The one I linked is succinct . –  anna v May 25 '13 at 13:49

The lightning rod is based on two principles theorized by Benjamin Franklin. Lightning dissipation theory, and lightning diversion theory.

Lightning Dissipation Theory

This theory says that if you point a pointy metal object toward a polarized cloud, the metal object will be able to bleed off some of the energy from the cloud. Thus preventing a lightning strike.

This theory can actually be demonstrated, using a Van de Graaff generator and a nail. This YouTube video demonstrates the theory.

While this theory holds up on the small scale, it's been shown not to be effective at dissipating the large amount of energy built up in a storm. Fortunately, the design of the dissipation device (lightning rod) is also a great diversion device.

Lightning Diversion Theory

The lightning diversion theory says that if you provide a preferable path for the energy to travel along, there's a high probability the energy will follow that path.

Lightning rods are designed to be the highest objects around. This puts them closer to the polarized cloud, and reduces the distance the lightning must travel through the air. They are also made from conductive materials, and are connected to the earth through highly conductive materials. This provides a low resistance path to ground, making it a preferable path for lightning to follow.


While both theories hold up in the laboratory, only diversion theory seems to offer a viable lightning protection system.

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protected by Qmechanic Nov 5 '13 at 14:35

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