How do lightning rods actually work?

[robby]

In an earlier thread (http://boards.straightdope.com/sdmb/showthread.php?postid=692215#post692215), Chronos wrote:

Originally posted by Chronos
...Another common misconception about lightning: the purpose of lightning rods is not to provide a path to ground for lightning. It's actually to prevent lightning from striking in the first place. The idea is to allow the built-up charge to dissipate slowly and gently, rather than all at once in a bolt.

However, at the National Lightning Safety Institute's website, they have a letter (http://www.lightningsafety.com/nlsi_lhm/charge_transfer.html) written about a proposed "Charge Transfer System" for lightning protection. Here is an excerpt:

The present consensus held by members of the lightning and thunderstorm electricity community in both the American Geophysical Union and in the American Meteorological Society about these devices is that they are replays of Benjamin Franklin's original, failed ideas about lightning rods. In the course of some parlor experiments, he and his associates discovered that they could discharge electrified objects silently, without sparks, by approaching them while holding a sharp-tipped needle directed at the object. This discovery of the "point discharge" or "corona current" phenomenon led Franklin to suggest that, perhaps, thunderclouds could similarly be discharged, thus preventing lightning, by taking away its electricity. However, after Franklin erected a sharp-tipped iron rod for this purpose, instead of discharging a thundercloud passing above, his rod was struck by lightning. Thereafter, Franklin recognized that a primary function for an elevated rod was to be a lightning receptor and to carry the lightning to Earth, around structures that were to be protected.

Despite this new and unexpected function that his rods appeared to serve, Franklin (1767) remained enamored of the "power of a point" and recommended that the tips of lightning rods be sharp, a configuration that is still widely used today although the virtue of having sharp tips on lightning rods has never been established. Our assessment of the experience gained since Franklin's time is that sharp rods and "dissipation arrays" exposed in isolation on high towers are often struck by lightning but there is no credible evidence that they prevent lightning, which usually initiates high up in thunderclouds...

...The undeniable facts are that "dissipation" devices do not prevent the occurrence of cloud-to- ground lightning strikes and that they are not designed nor intended to be the preferential receptors of the lightning strikes in their vicinity. Accordingly, such devices serve no useful protective purpose in the prevention nor in the reception and conveyance of lightning to Earth.

Chronos seems to be talking about corona discharge to prevent lightning strikes. The NLSI is insistant that this does not work. Then they add that "the virtue of having sharp tips on lightning rods has never been established"?!

So how do lightning rods actually work?

[astro]

Small part below of a *very* long article/lecture series.


http://www.fau.edu/divdept/physics/pw/jordanrg/LLS/lecture14/LLS_lecture_14.htm - Chapter 14 The Atmosphere

Actually it looks pretty intersting I think I'll read the whole thing.


"In 1794 Benjamin Franklin wrote the following, in a letter to Gentleman's Magazine that was published in May 1750:

"There is something however in the experiments of points, sending off or drawing on the electrical fire ... I am of the opinion that houses, ships and even towers and churches may be eventually secured from the strokes of lightning by their means; for if instead of the round balls of wood or metal which are commonly placed on the tops of weathercocks, vanes, or spindles of churches, spires or masts, there should be a rod of iron eight or ten feet in length, sharpened gradually to a point like a needle, and gilt to prevent rusting, or divided into a number of points, which would be better, the electrical fire would, I think, be drawn out of a cloud silently, before it could come near enough to strike."

Initially, Franklin believed that lightning rods discharge clouds. However, they do not; the charge that flows normally between a lightning rod and a thundercloud is much too small to discharge the cloud. Rather, lightning rods serve to route the current in a lightning strike harmlessly to the ground by diverting it through a very low resistance path as Franklin realized in 1755. Lightning rods were first used for protection in 1752 in France and later that year in the U.S. So, how do lightning rods work? The rod actually diverts the stroke towards itself only at the final stage of the downward path of the leader, see figure 43. As we saw earlier, when the leader is a few 10's of meters from the ground, its large negative charge attracts large amounts of positive charge on the Earth near it and especially on sharp objects like lightning rods that are 'well-grounded'. An upward traveling discharge is initiated from the very high density of positive charges on the end of the lightning rod, and when the two leaders connect the cloud is now 'wired' to the ground through the lightning rod, rather than the house, or tree, etc.

Any high object can initiate an upward moving discharge; it is important that the lightning rod be the highest object near the structure to be protected. A single rod will almost certainly protect a single building if it is tall enough, see figure 44, although to protect larger buildings several strategically placed rods would likely prove more practicable. However, no lightning rod can offer complete protection; for example, the Empire State Building was struck at a point 50 feet below the top! It is also common practice to protect overhead high voltage transmission lines from lightning strikes by stringing a well-grounded wire above the lines to intercept strikes that would otherwise have hit the power lines."

[casdave]

As far as I'm aware the shape of the tip is very important, a blunted point seems to be the best at dealing with lightening arcs.

The problem here is to determine if there are more strikes when such devices are present and if they are more damaging than if they were not present.

It may be that there are more strikes of less severe intensity or there may be fewer strikes(because some potential strikes are effectively dissipated) in the whole area.

I would mention that the terms gentle and slow dissipation might well be relative, say 10mS rather than .1mS-same amount of energy but much shorter time, by providing a point of ready discharge the potential electrical energy does not reach of the much higher levels that would be required if such a point were not available.

Without doubt lightening rods provide a path to earth that offers a lower resistance path to earth thereby bypassing structures(such as roof timbers) that might be vulnerable to the huge heating effects caused by their electrical resistance and subsequent fire hazards.

All the installation work to code I have been involved with is light on explanation and heavy on requirement.

[DaveRaver]

I remember hearing when I was a kid (in the 70's) that homeowners didn't need lightning rods anymore because the TV antenna on the roof did the same job. But now most people have cable TV and have, by and large, taken down their old TV antennas. I think the current generation has forgotten that they now need to install lightning rods again.

[CalMeacham]

As far as the shape of the tip -- I believe that the shape is NOT important. The lightning rod, according to my understanding, provides a path for conduction (regardless of what is cited in the OP), and it is his proximity rather than the shape of the tip that is important.

Some people claim that a sharp tip means a high field at that point, which breaks down he air more easily. Interestingly enough, it turns out that the sharp tip does NOT ead to a higher electric field at that point. Thi was addressed in an article in the American Journal of Physics entitled -- appropriately -- The Lightning Rod Fallacy. It appeared circa 1987, and I'm too lazy to dig out my copy right now. (Nevertheless, from m experience with HV it is clear that breakdown DOES occur NEAR, if mot AT, the tip. The AJP article is wonderful, and true, but it cannot be the entre story.)

[DrMatrix]

I would think the shape is important. According to my physics text, if you have two connected metal spheres, the one with the smaller radius will have a more intense electric field.

[weapons grade bullonium]

One thing to watch for if you have them on a building is to check the glass after a storm. If the rod has been hit, the heat would have broken the glass indicator. This means the wire to ground might also have melted. If that's the case, the next bolt will ground through your building and start a fire. Since the next bolt might be any minute, the only safe rod systems have 2 or more rods.