In today’s column (http://www.straightdope.com/columns/010824.html), Cecil says “A lightning bolt strikes a Franklin rod and is carried harmlessly to earth by the grounding apparatus, sparing lives and property.”
I was always taught that the whole point of lightning rods was to prevent the lightning from striking in the first place. I was taught that lighning is basically an arc created when there is a substantial difference in the charge of the air and the ground below it. Once this differential is great enough to overcome the resistance of the air, an arc strikes between the earth and the ground.
My understanding of the role of a lightning rod is that it slowly equalizes the charge between the earth and the air above it, so that the electrical charge differential is never enough to allow an arc to strike. The key difference here is that if the lighning rod works, you’ll never know it. If a lightning rod were designed to attract lightning and safely ground, I think it would be necessary to have a much more substantial route to the ground to carry the charge.
I was raised being told that lightning rods attracted lightning. By attracting any forming lightning to the well-grounded rod the force of the lightning was kept safely away from the building. The building where I work has been stuck by lightning at least twice while I was here. The lightning rods obviously did their job because all the lights flickered but all of the computers and lights stayed on. By directing the force of the strike away from the core of the building the rods seem to protect the building and the electronics within.
I’m the guy who asked the original question that Cecil answered, and I was taught both versions when young, that lightning rods both prevented lightning, and handled it safely if it did manage to strike. If you read my question carefully, this is the debate I was really hoping to resolve.
I just sent the following e-mail to Cecil & Staff:
Cecil, in your infinite wisdom (bolstered by the practically-infinite wisdom of your research staff), you shed much light on the curious world of lightning rod research and standards, but there is yet another issue: the idea that lightning rods might actually prevent lightning from happening, or reduce it.
For example, the respected BBC tells us ( http://www.bbc.co.uk/history/programmes/local_heroes/diy/lcd.shtml ):
Franklin found that sharp, pointed metal objects, connected to the ground, could actually discharge the lightning without a huge and damaging lightning strike. This is because the point has such a tiny area that it is easy to build up enough charge density for a discharge. But the amount of electric charge is small, so it is not a full lightning strike.
The earliest literature available that proposes protection from lightning starts in 1752 with
Benjamin Franklin. Franklin’s original idea was to use a sharp point to draw charges from the
cloud to discharge it and thus prevent lightning. Early experiments by Franklin in electrostatics had him arrive at this conclusion. By placing objects with different geometries (sharp and blunt) near a charged object, Franklin found that differing amounts of charge were drawn from the original charged object.
…
Faced with empirical results, Franklin published further on the topic in 1767, refining some of the design notes regards grounding in particular. He notes that that his lightning protection system may not work by “silently discharging the cloud.”
Boy there are a lot of misconceptions about lightning rods, and to my astonishment, Cecil ADDED to the misconceptions. Even worse, he didn’t even answer the question. In case you don’t recall, it was “Do houses with lightning rods actually get hit less often than houses without?”
The answer is NO. Statistically speaking, your average building with rods will get struck just as often as a building without rods. The difference being, buildings with rods have a much reduced rate of catching on fire. Buildings without rods have no fixed path for the lightning to go to ground, it could easily flow through the interior plumbing or electrical wiring (both are grounded) and cause serious building damage to everything in its path. I’ve seen houses that were struck by lightning, and the ones without rods are usually damaged and scorched (at a minimum) but on buildings with LRs you can’t even tell it got hit.
Cecil really should take another crack at this question. He went off on some weird tangent about which type of rod worked best. That wasn’t the question.
And no supernelson, rods do not dissipate the charge. Yes, rods have a substantial route, my old house had 1 inch braided copper cables running from the rods down the side walls directly to the ground. My current house has no rods because it doesn’t need any, it is not the highest point in the neighborhood, I have a row of wonderful lightning rods right next to my house, some high tension power poles about 80 feet high. During intense storms, I can see lightning strike them. Lightning always strikes the highest point in an area because that is the path of least resistance (shortest gap through highly resistive air). That is why lightning rods work, the copper cable is the path of least resistance to the ground.
You were probably just being brief, and you probably are aware that lightning does not always hit the tallest structure in the neighborhood. There are various theories and formulas for calculating the “cone of protection,” a phrase that reminds me of Get Smart, and which isn’t always cone-shaped.
There is a nice illustration in Encyclopedia Britannica that basically says that your house has to be well within thirty meters from the towers in your neighborhood:
see http://www.britannica.com/eb/article?idxref=121910
“On structures less than 30 metres (about 100 feet) in height, a lightning rod provides a cone of protection whose ground radius approximately equals its height above the ground. On taller structures, the area of protection extends only about 30 metres from the base of the structure.”
Uh, Arnold. Why would there need to be a statistical analysis somewhere. It is simple logic.
Except for the fact that “House A” has a lightning rod sticking up above the eves about 12 inches, and “House B” doesn’t have such a rod, they are both the same. There is no significant measurable difference in the houses. Why would lightning strike one more than the other?? hhmmm?
Well, that cite is nice but it’s a paid link so I can’t see anything but the thumbnail. Fortunately, I am well within 30 meters of the huge poles in my backyard.
Yeah, I was being brief, lightning is a very complex phenomenon.
Robby, while he was invisible, also answered your issue of how a lightning rod could theoretically affect how often a house is hit (making it more likely, perhaps):
"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. "
Thanks, jglenn for pointing out something I should have read more closely before. I went back to the old thread and read the quote in astro’s post and link, but his link doesn’t seem to work any more. So This one should. It was a great series of lectures and drawings.
But here’s my answer to “Statistically speaking, your average building with rods will get struck just as often as a building without rods.”
Assume there are two identical houses, side by side, say, 20 meters apart. House A has rods, House B doesn’t. If a lightning stroke/leader were headed toward a spot inbetween the two houses, YES, the bolt would probably hit House A with the rods. But…I doubt that MOST houses have lightning rods. So bolts of lightning coming down in random locations are probably MORE likely to hit a house which DOESN"T have a rod. After all, that bolt coming down isn’t gonna change direction in the last 50 meters and travel up a BLOCK just to hit the house with the lightning rod. If every second house in the world had rods, then yes, lightning would probably be more likely to strike those houses.
First, I think the best bet is to put your house inside a Faraday cage. That way any electricity on the outside won’t get into the inside. From my loose understanding of Faraday cages, everything on the conductive surface of the cage is at the same potential so there’s no potential difference between any two points on that surface–hence, no current caused by the cage is possible inside the cage, irrelevant of any current going through the cage. The size of the “holes” in the cage dictate some multiple of the wavelength of radiation that can enter so given that lightning has a large DC coefficient (although since it’s sort-of a square wave it has a lot of fairly strong high frequency components too) you can make the holes pretty big and still have the thing work. I’m guessing that’s why lightning rods sort-of work [at reducing damage caused by an actual strike] because they act like a very bad Faraday cage.
Second, pointy things in the presence of a source of high potential difference (more than several thousand volts) have a corona effect–the air around the pointy thing gets ionized pretty good. By making a lightning rod pointy, you end up getting it to ionize the air around it when it’s in the presence of a potential strike (i.e. a “leader”.) This ionizing effect may tend to make the valid strike area for the lightning to be much larger than the pointy tip. If you use a plate, you’d need to make the plate pretty big compared to the pointy tip to get the same conductive surface area.
I don’t know, I just work here. That’s what you smart posters are supposed to figure out!
But I misread Chas.E’s post. I thought he meant “a house without a lightning rod would get hit just as often as the same house with a lightning rod.” Your interpretation seems to be “since there are more houses without lightning rods, the number of strikes on houses without is probably the same as the number of strikes on houses with.” I don’t have any sources to dispute either of those statements, but I just like saying “Cite, please?” when I’m feeling peevish.
What?! ightning rods not work? I’ve got a ibrary of books on lightnin protection, including a couple of books published Dover. People have run a lot of tests using artificial lightning bolts, and the verdict, I was always told, was that proper use of lightning rods minimized damaging strikes on structures. It didn’t prevent lightning from striking, but the rod with its able provided a safe path for the dicharge to find its way to ground, without cause explosive heating and subsequent destruction of buildings. There’s lots of pretty pictures in these books, showing the paucity of lightning strikes in the vicinity o a rod.
I’ve never hearde a eliable report that lightning rods “bled” the clouds of excess eectricity and thus prevented strikes from occurring. The point of a lightning rod is to provide an alternative path.
Strictly speaking, the electric fiel is not highest at the sharpst radius of crvature. For one thing, radius of curvature a local condition (the radius here doesn’t depend on what the rdius over there is), hile the electric potential and the electric field depend on global conditions (the electric field here certainly des deped on what the radius of curvature is both here an there). This was pointed out in a wonderful, eye-opening article in the American Journal of Physics circa 1986 entitled “The Lightning Rd Fallacy”. Ca’t recall the authors right now, but well worth digging up and reading. It might even be on-line by now.
Yes, let me rephrase this. All other factors being equal, any given house has the same odds of being struck by lighting, whether or not it has lightning rods. However, the odds of the house being damaged are extremely diminished by the use of lightning rods.
And a Faraday Cage has nothing to do with any of this (BTW, I used to work inside a Faraday Cage). The cage might provide a nice path to the ground terminal for a few nanoseconds before the copper mesh vaporized, and then the grounding path would be disrupted. Faraday Cages are only intended to block stray RF noise. Fat copper cables would be more effective at directing lightning to ground. Big surprise, that’s what they use with lightning rods.
First - Nelson: Yes, you were way off, at least with traditional passive rods. Active rods produce ions to either attract or deflect lightning, but are not dependable because lightning can be either positively or negatively charged. Thus, the result may be the complete opposite of that intended.
Second - This was the first Cecil response I recall being dissappointed in, not because it is incorrect but, because it doesn’t attempt to provide the common sense answer. Worse, it overly focuses on codes. Bleah!
If properly installed, lightning rods attract lightning within a range of around 100’ to 200’ and discharge the current harmlessly to ground (most of the time). They save many millions of dollars per year for the DoD alone. The real pros in this field are at Sandia labs. They use lightning rods to attract lighning strikes for measurement. Theses guys are brilliant and will actually be glad to talk to you if you can find their number (I have).
As Cecil states, though, “…rods merely improve your chances rather than offering guaranteed protection.” As such, lightning has been known to strike the side instead of tip of lightning rods and sometimes strikes the area intended to be protected. Still, I feel much better with valuable trees and property protected with a lightning rod and know the rods occasionally take strikes. What I don’t know is whether the lightning would have struck the property without the rods or if it only struck because the rod was there in the first place.
Wait, I’m confused. Cecil talks about the two types - the old Franklin type and the new dissapator (ESE) type. He says that the new dissapator type does not work, but there is question on whether the old Franklin type works, too.
Subsequent discussion talks about Franklin’s original intent being the dissapative effect, and that he later found he was wrong. However, the lightning rods did work by providing a better ground path. So lightning rods were instituted for that purpose.
Cecil seems to be saying that the Franklin style lightning rods don’t work, either, in that they don’t provide protection from the hits. As he puts it, it’s not a guarantee, merely improves your chances (of not getting damaged?).
But the link by jglenn ( http://www.lightningsafety.com/nlsi_lhm/charge_transfer.html ) seems to be saying that it was Franklin’s original idea, i.e. the dissapative effect, that was wrong but that lightning rods do work by channeling the strike.
It seems Cecil says there isn’t conclusive proof.
This is all so confusing.
NeedAHobby said:
Chas.E said:
It seems to me that Chas.E is referring to a Faraday Cage as a specific device, whereas NeedAHobby is referring to any device that functions in that manner. I’m pretty sure the effect in question is not limited to any one particular device, but rather any metal enclosure will work that way. (Isn’t that what protects you from lightning in a car?) So I don’t think your rebuttal is particularly addressing the point. Though I don’t think that lightning rods are working in any way like a Faraday Cage.
Just because nobody seems to be able to find a properly done, properly scientific, conclusive study, doesn’t mean that lightning rods don’t work. Cecil (“They probably do work”), and apparently just about everyone else, seem to feel that the circumstantial evidence, the anecdotal evidence, and the preponderance of not-quite-scientific-enough data are enough to assume that lightning rods work, on a gut-instinct basis.
However, that gut-instinct has been horribly, terribly wrong many times before, which is why history has taught us to do things scientifically.
For example, people used bee-sting allergy shots for about 40 years before this scientist decided that the original science proving that the shots work was flawed. (I’m reporting this from memory from an article in, perhaps, Scientific American.) the original study showed that the shots work on beekeepers, but when the tests were redone with normal citizens, it showed that they had no effect. It turns out that beekeepers are allergic to a different part of the bee, which they develop through frequent contact.
So it’s always important to do good science, even when you think you know the answer, and sometimes you come up with a very surprising answer.
(Bee allergy shots have been reformulated and now work wonderfully.)
Incidentally, Faraday cages are for electromagnetic radiation (waves/photons), which lightning isn’t. Lightning is electrical current (electrons) which travels along a path of ionized air. It’s a whole different ball of wax.
Actually, Faraday cages protect you against lightning srikes, too. Lightning certainy is elecromagnetic radiation – look into hose books on lightning, or Jackson’s book “Classical Electromagnetism”. In an case, you can see visual proof that it works. The Museum of Science here in Boston has a Van de Graaf lightning demonstrator in its Theater of Electricity. In fact, what they have is van de Graaf’s own van de Graaf generator, and it’s a wonking BIG hunk of metal. The lecturer does his/her spiel from within a Faraday cage for protection. He/she even raises the cage as part of the show, letting the bolts of artifical lightning strike the cage and displaying their safety (while ostentatiously holding onto the cage from the inside).
Seems to be a li’l missing of the boat, here, IMHO.
The whole point was that Cecil went into this thinking, this is silly because of course lightning rods “work” – they get hit and the lightning is grounded, rather than the house being burned (what blaine calls the “common sense answer” and accuses Cecil of not giving.) There is lots of literature on this – hence, Meacham’s “library” of such works.
But Cecil’s point was that, when looked at carefully, there are no scientific studies that prove the “common sense” answer that appears in all the books.
Does the absence of scientific evidence mean that “common sense” is wrong? Not necessarily, as per jglenn. Does it mean “common sense” is right? Not necessarily. The fact is that despite a body of literature and common sense and housing codes, we don’t (to date) have valid evidence that lightning rods really do work.