Talk:Lightning rod

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Contents 1 Not a How-To Article 2 Why rounded ends? 3 "Non-functional" glass balls 4 They are not for the purpose described 5 Inefficiency of the sharp tip 6 Lightning is not DC 7 Disputed 8 Single greatest philanthropist?

[edit] Not a How-To Article

This article needs to be reworded so it does not sound like a primitive how-to article on building lightning rods. It should describe lightning rod techniques, not what 'should' be done. Also, we need to verify the accuracy of these statements -- references would be helpful.

Help. I have discussed this issue with many people but I'm never satisfied with the answer/explaination.

I understand that a lightning rod neutralizes the static charge that is carried in by clouds in a storm.

It is my understanding that if everything is working right, the static charge is neutralized slow enough that there is no bright flash and thunder.

By everything working I mean:

    1: The charge does not move in to quickly (nature cooperates).
    2: The lightning rod is in good condition (resistence to ground is
       low, the rod is high enough, nothing is damaged, etc.)

I'm not sure if this controled discharge is not also refered to as a "strike" by many of the articles.

I understand the lightning (the flash and crash) do occur sometimes and the rod is struck, but isn't that a worse case/not supposed to happen event?

In other words, I understand that a lightning rod function is more to prevent a strike (the flash and crash)by premitting a controlled discharge of current the attractting the high voltage/high current flash.

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The original idea by Franklin was that the lightning rod would indeed discharge the thundercloud and there would not accure lightning. In one of his first experiments, the lightning rod was struck by lightning. This experiments, and many experiment later, proves that you can not prevent lightning by a lightning rod, you can only make a safe path for it to the ground: the path from the lightning rod to the ground.

Because Franklin first thought that lightning could be prevented, he claimed that you should always use a lightning rod with a sharp point. It's strange that he kept convincing people that sharp point were the best, because his own experiment showed that lightning cannot be prevented, so there's no direct reason why pointed lightning rods would be better. In fact, they're less effective in attracting the lightning as blunt ones, because they weaken the electrical field at the tip.

---

The idea of a lightning rod is twofold - to reduce the probability of a strike and to reduce the damage caused by a strike when it happens.

The reduction of the chance of a strike occurs due to electrostatic induction and charge bleed-off. When a charged cloud passes over the landscape, the cloud's charge induces an opposing charge in the ground - so if the cloud is positively charged, the ground develops a negative charge. This increases the electrostatic field between the cloud and the ground. When the E-field exceeds the ionisation breakdown threshold of the air, you get a lightning strike.

The sharp tip of a lightning rod will concentrate the E-field in the region of the tip past the breakdown threshold of the air, without allowing the breakdown to occur all the way to the cloud. The result will be a localize bleed-off of the induced charge on the ground, reducing the overall E-field of the area (and reducing the chances of a strike).

The linked article in USA Today is correct but misleading - the idea of a lightning rod is NOT to "attract" lightning, but to discourage it. *IF* your goal is to make lightning strike - e.g. you are doing lightning research - then yes, use a blunt rod. If if your goal is to protect equipment from a strike - use a sharp rod.

The ARRL has a good link on lightning protection: http://www.arrl.org/tis/info/lightning.html - and they have been dealing with lightning for decades. N0YKG 17:02, 11 July 2005 (UTC)

[edit] Why rounded ends?

It says in the article that rounded ends are better, but doesn't explain why. It would seem that pointed ends are better because they would create a slow discharge before the actual strike. - Omegatron 16:13, Jun 19, 2004 (UTC)

It seems no one has enough interest to answer the question. If we have no sources at all for the assertion (see the preceding section; neither the granting of a patent nor the say-so of Tesla is really conclusive), then we need to hedge this non-authoritative information. One could start with some "According to Tesla" wording; but first, maybe someone can come up with some solid information. Dandrake 19:59, Aug 8, 2004 (UTC)

And by the way, the sentence just before this section asserts that pointed rods work less well because they weaken the electrical field around the tip. This needs explanation. The strong field around a sharp point produces ionization, which ought to make a locally lower resistance between cloud and ground. Intuitively, this would seem to promote discharge (whether gradual or violent) through this path. Why is this argument wrong? If no one can or will explain this, see the preceding. Dandrake 17:28, Aug 24, 2004 (UTC)

There are sources for the assertion ... inparticular the patent or associated research on such topics. Hedge this authoritative information? "According to field research" wording could be good.

As to pointed rods work less well because they weaken the electrical field around the tip? This link may help.

As to the whole "strong field around a sharp point produces ionization" Why is this argument wrong? See the previous link. Other info is available.

I'm gonna add this link to the article. JDR

I doubt that the sharp tip makes the rods work less well than rounded-tip rods. Instead, the ionization would tend to make the sharp-tip rods BEHAVE AS round-tip rods. (But maybe the gas cloud makes the sharp-tip rod behave as a taller rod than it actually is, or maybe it gives the rod a random and changing electrical shape.) Why would sharp tips and ionization tend to weaken the field at the tip? It's because ionized air sent out by the sharp metal tip is a conductive gas, and this gas becomes electrically part of the lightning rod. With this extra region of "fluid conductor" present, the sharp metal tip of the rod is electrically no longer the "tip" of the rod instead the top of the conductive gas cloud acts as the "tip." In other words, the conductive gas cloud acts as a Faraday cage which shields the metal tip. The outside of this "Faraday cage" then becomes the lightning rod. Electrically the rod acts as if it has a conductive fuzzy shape enclosing the tip. Another issue: if the wind is blowing, the conductive gas cloud would be removed. All the above discussion applies only to still air. --Wjbeaty 19:16, Mar 2, 2005 (UTC)

[edit] "Non-functional" glass balls

I notice in this article that it mentions lightning rods with a "non-functional" glass ball on it meant only as an ornament. However, living in storm-active "tornado alley" farmland I have heard that the glass balls actually do serve a function other than just being ornamental.

Supposedly, when the lightning strikes one of these glass-ball lightning rods, it heats up the glass ball so rapidly that it shatters, falling off the rod and serving as visual sign of a strike. This then allows the owner to see which rod took the strike without having to climb onto the roof, and also alerts the owner that they should check the building and lightning protection system for damage. Apparently this is a relatively unknown lightning rod tidbit, so I added it to the article. Torin Darkflight 06:11, Oct 30, 2004 (UTC)

[edit] They are not for the purpose described

They are not there for lightning to hit them; they serve a greater purpose. It is true that smaller objects hold less charge - therefore, the tip of a lightning rod is sharpened to a point. This makes it so that lightning will not hit the building which it is attached to because all the charge induced in the building will leak out through the rod. However, lightning DOES hit the rod sometimes but its main purpose is to prevent a strike.

--Nivedh, March 29, 2005

According to the National Ag safety database (NASD) article on Boating-Lightning protection (which I assume to be public domain),

"Lightning protection systems do not prevent lightning strikes. They may, in fact, increase the possibilities of the boat being struck. The purpose of lightning protection is to reduce the damage to the boat and the possibility of injuries or death to the passengers from a lightning strike."

This seems to be the consensus, at least based upon the results of a Google search using the terms "lightning rod" "prevent lightning". I would agree that a lightning protection system is meant to protect, not to prevent, lightning. There is no way to equalize the massive difference in potential between a cloud, miles up in the sky, and the ground. The only thing you can do is hope to intercept it with an air terminal and send it to your grounding system. --Dual Freq 16:16, 10 December 2005 (UTC)

[edit] Inefficiency of the sharp tip

My understanding was that the sharp tip causes a significant increase of the electrical field directly around the tip. This induces early ionisation that partly and locally discharges the gradually increasing electric field caused by an approaching stepped leader. This reduces the chance of the ligtning rod being the source of the upward leader that makes the (safe) connection leading to the lightning strike.

Eric, June 6, 2005

The article conflicts on this point.

• In the modern research section, it says that a correctly blunted lightning rod is better at conducting current to the ground since the field strength is stronger above it. This would mean that the blunt lightning rod is better because it attracts more lightning. This makes sense to me.
• In the Ben Franklin section, it says that a pointed lightning rod is worse for the exact same reason; that it has a greater field strength and so attracts more lightning.

So from what I understand, Franklin knew about the ability of sharp objects to attract arcs (since the field strength is stronger around them), and he thought that sharp poles would dissipate the clouds' charge continuously through a corona discharge, preventing any lightning from striking the area. Instead, I suppose (original research time) the corona region just ends up looking like a very tall conductor that the lightning is much more prone to striking, making the rods attract lightning instead of preventing it. - Omegatron 01:09, August 8, 2005 (UTC)

As was explained in my US Navy electronics "A" (providing entry level knowledge & skills) school, since a (round) basketball, after being immersed in water, takes longer to shed water than a pointed object of an equivilent area, a pointed object is more efficient for dissipating an electrical charge. - Perry, JAN 20, 2006(CST)

NFPA 780 (Standard for the Installation of Lightning Protection Systems} 2000 edition says the air terminal can be sharp or blunt. According to the references listed on the main page, tests of lightning rods indicate that blunt is more efficient at intercepting a lightning strike. Since the point of a lightning protection system is to prevent lightning from striking a protected structure, you want the rod to be as attractive as possible to the lightning. (Read some of my other refs on this page if you doubt this). It is a common misconception that lightning rods prevent lightning strikes. --Dual Freq 15:49, 21 January 2006 (UTC)

[edit] Lightning is not DC

Maybe I'm mistaken, but DC does not create a changing waveform, a requirement to producing a radio signal. It is well documented that lightning does produce a radio frequency signal, hence lightning must be an alternating current and not DC. User:66.222.126.249 02:29, 26 September 2005

Please sign your posts with 4 tildes (˜˜˜˜). As for lightning being "AC" because it is not DC - there are more types of current than "DC" and "AC". Alternating current specifically means a current in which the direction of flow reverses. Lightning does not have a current flow reversal. Yes, it IS a changing current, but then again, ALL currents are changing - there are no currents that started at time=0, and will continue until time=&infinity; without change. All DC means is that the current flows in the same direction for some subset of time that "makes sense" for the current in question. N0YKG 13:43, 26 September 2005 (UTC) (← that is what 4 tildes looks like, BTW).

DC current through a light filament (or electric arc, for that matter) produces light - a form of high frequency radio. Yet the current involved is indeed DC. On the other hand, are the different directions of the strike due to changes in the polarity of the charge (there are certainly positive and negative clouds) or merely in the direction of the plasma construction? njh 00:22, 27 September 2005 (UTC)

Lightning stikes both ways. Ground to cloud and cloud to ground. The ground usually seen as - and the cloud is seen as usually +, but I thinnk there are virations (and anomolies). The overall system is AC, an individual strike could be seen as DC. I believe it's the "direction of the plasma construction" (unless there is something that you mean that I am missing there). The system exchanges energy from the atmosphere to the earth and from the earth to the atmosphere all the time, but a lightning strike is the "dielectric breakdown" of the insulative medium ... causing the "boom". JDR 00:30, 27 September 2005 (UTC)

If it is the direction of formation then that is no proof that the current direction alternates. For it to alternate, a strike would have to carry more charge than required to neutralise the cloud. Now it is quite plausible that this happens, as it appears to be an LC circuit, but to prove the case I think we should find a reference that this happens. (So I don't consider this case closed just yet :) njh 00:44, 27 September 2005 (UTC)

According to Vince Calder at the DOEs ask a scientist web site (which I assume to be public domain):

"Question - Is lightning stroke DC current or AC current?"

• Answer - Lightning is DC -- well almost. ... Usually the charge carrier is negative (presumably electrons) but apparently some discharges appear to be positive! Often there is a "leader" stroke that on high speed photos approach the surface of the Earth, but "do not quite make it" and then there is a Earth -- Cloud return stroke (presumably the path is "set up" by the leader stroke. So this would be AC but with a single cycle."

That seems to be a vote for DC current. As for the radio frequency component, I think that is part of the Electromagnetic pulse related to the high levels of current involved with a lightning strike. If you looked at this pulse it would probably cover a large chunk of RF spectrum from DC up to VHF. See also Page 4 of Electromagnetic pulse and the radio amateur --Dual Freq 16:16, 10 December 2005 (UTC)

[edit] Disputed

This statement seems ridiculous to me and it should be removed from the page:

Lightning rod dissipaters make a structure less attractive by which charges can flow to the air around it. This then reduces the voltage between the point and the storm cloud, making a strike less likely. The most common charge dissipaters appear as slightly-blunted metal spikes sticking out in all directions from a metal ball. These are mounted on short metal arms at the very top of a radio antenna or tower, the area by far most likely to be struck. These devices reduce, but do not eliminate, the risk of lightning strikes.

The whole purpose of an air terminal and associated lightning protection system, is to give lightning something to hit that is not important and direct it to the grounding system. It's meant to intercept the lightning and stop it from hitting a protected structure like a tower or building. You could look at it this way, but maybe it's an over simplification: Lightning originates from miles up in the atmosphere, where a difference in potential exists. This difference is so large that it jumps thousands of feet to ground. If someone were to tell me that I could stick a ball with metal spikes 10 feet above my house and that would safely equalize the potential difference between the ground and the sky 20,000 feet above it, thus preventing lightning, I would say that I have a bridge to sell them. I need to see a credible source that says this, and not some company web site that sells these things or some snake oil patent. You will not find any dissipater in a UL standard or NFPA document because they are not certified to prevent lightning. They may be approved as air terminals (for lightning to strike instead of a protected structure), but not as lightning prevention. They may reduce static and might lower noise levels on an antenna structure vs a pointed air terminal, but they will not prevent or stop lightning. See also: [1] and [2] --Dual Freq 17:15, 10 December 2005 (UTC)

Yep, I agree, this is a weak line and I was tempted to remove it when I rewrote this article - Be Bold and delete it, someone can always add it again when they've got some good references. It sounds to me a lot like one of those things that gets passed around as fact without any good basis (like the common explanation of lift). njh 10:33, 11 December 2005 (UTC)

I reworded and moved the section to a separate part at the bottom of the article. I attempted to cite my edits appropriately. If anyone feels they have better information, please cite your sources and make the appropriate changes. I'm only trying to help dispel a common misconception about lightning rods. I also think we need a picture of a 'real' lightning rod / air terminal, not one of these misleading lightning-protectors. I think a nice close up of a blunt-tipped air terminal would be best since that seems to be the standard one currently backed by NFPA, FAA (See page 31 of 100) etc. --Dual Freq 23:42, 11 December 2005 (UTC)

Thanks for this! If you feel like cleaning up the rest of the article we may be able to remove the refs contention. njh 10:14, 12 December 2005 (UTC)

Added image of a pointed-tip air terminal and relocated fictional "protector". --Dual Freq 23:50, 7 March 2006 (UTC)

Do you really think the 1/2 inch cable connecting lightning rods to earth could carry the temendous current from a lightning strike? Not likely. Immediate vaporization would eliminate the wire, and the strike would then travel through the structure causing damage. The "lightning preventer" idea has more credibility than this. You are right that it cannot completely neutralize the charge difference, but that is not necessary. As long as the lightning rod makes the structure a less attractive target than other nearby objects, it has done its job.63.85.214.60 14:49, 20 January 2006 (UTC)

Yes, I really think that the NFPA 780 recommended down conductor can and does carry the current to ground, but it doesn't really matter what I think, check the sources I cited. Those sources are the organizations that keep homes, business and government safe from fires caused by lightning and other lightning damage, groups like UL, NFPA, NLSI and Lightning Protection Institute. No one certifies that any equipment will prevent a strike from occurring. All the articles cited above on this page should answer this question. If you disagree, please cite a source. If Daktronics or somebody else up there in Brookings, South Dakota sells something that prevents lightning, please post the website location I'd love to read about it, but I couldn't find any information on your company's website pertaining to lightning protection or prevention. I suspect that you do have lightning protection equipment installed with your signs, so I'm a bit surprised that you would make this kind of statement. Lightning rod types has some photos of rods that were struck by lightning. The 19mm diameter blunt tip lightning rods have small pits on them but they did not melt or 'vaporize'. --Dual Freq 20:31, 20 January 2006 (UTC)

The lightning page states that each discharge produces 30kA. The conductor proposed (.25inch*.25inch*pi = 126mm^2) has a resistance of 1.6mR cm * 1m/126mm^2 = 0.13mR/m, each m weighs 126*9g = 1.14kg giving a heat capacity of 438 J/K per metre. Each m will drop 3V, i.e. 90kW of power. The flash lasts about 100ms so each meter of the bar has to dissipate 9kJ by heating by 20K. I think the mistake is to assume that because there is a lot of power dissipated in a strike, the conductor must dissipate a lot. In fact, as the conductor is probably 1 thousandth the total resistance of the strike it gets only 1 thousandth the power. njh 02:29, 21 January 2006 (UTC)

To follow up on this (from a discussion on slashdot). Some people have proposed that a lightning rod is just an initiator rather than the sole conductor of the discharge. A parallel path ionisation seems very unlikely considering the available conductor density. If there were a parallel discharge one would expect melting or charring around conductors, either of which I've not seen.

The resistance of the lightning rod might be 1000th the resistance of the whole strike (from cloud to ground), so although a lightning bolt has a high power, most of that power is being dissipated elsewhere. Lets work it out:

From Lightning: An average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts and dissipates 500 megajoules (enough to light a 100 watt lightbulb for 2 months).

100MV/30kA = 3.3 kohm

So a lightning discharge has a total resistance of 3K3 over say a distance of 3.3km, for a plasma resistance of 1ohm/m, nearly ten thousand times greater than that of the lightning rod, so it is unlikely that the lightning would choose to make its own path, and if it did, we would expect only 1/10000th the current to flow (a measly 3A).--njh 06:13, 4 March 2006 (UTC)

I don't think it is disputed that lightning rods work, but how they work is quite a matter of contention. The organizations cited could very well have the effect desired (that is, protection of structures) while having the reasoning wrong. As industry organizations, the effect of their rules is what is important to them, not the scientific basis for those rules.205.241.238.3 22:28, 9 March 2006 (UTC)

Oops, I edited the article quite a bit without looking here first. Anyway, I came here to say that I removed the latter part of "Lightning rod dissipaters (known as Early Streamer Emission, Dissipation Array Systems, and Charge Transfer Systems) claim to make a structure less attractive by which charges can flow to the Earth's atmosphere around it.", replacing it with "attractive to lightning." I don't understand what the original is trying to say, I don't even think it's grammatical. I'm pasting it here in case something was lost in doing this. -- Coffee2theorems | Talk 16:11, 18 September 2006 (UTC)

[edit] Single greatest philanthropist?

I think that calling Franklin the single greatest philanthropist ever not only a big POV but alot people help without expecting reward 70.36.255.222 12:31, 6 January 2006 (UTC)