How Do Lightning Rods Work, Exactly?

Factual Questions
1

Hi all-

I know I could just google this, but I’m curious as to your responses.

I just bought a home, and the previous owner was showing me about the place, and I noticed four pointy lightning rods in a row across the roof of the house. The rods also have cables that run off the roof and into the ground along the sides of the house.

So I say “Lightning rods, huh? So, do they allow the lightning to be funneled into the ground via those cables?”

He said “No, actually the rods create a static field over the roof so that lightning physically cannot strike the house”.

Now I know I’ve seen pictures of lightning striking a lightning rod. If it creates a field of static electricity over the house as Mr Previous Owner claims, then how is that possible? Or is there more than one kind of lightning rod setup?

So which is correct? Does lightning in fact contact the rods and get the electric charge funneled into the earth? If not, then what are the cables for? Do they “ground” the house, or something?

How do these things actually work?

2

I thought I heard recently that there’s some doubt that they work at all. Someone was trying to do some kind of building code reform, tried to find scientific studies showing which type of lightning rod works best, and discovered a general lack of evidence for the efficacy of any type.

Am I imagining this?

3

Cecil has an interesting column on lightning rods here: Do lightning rods really work?

As you suspect, the general idea is to provide a conductive, high-up point to receive the lightning strike and conduct the current to the ground. The guy may be confusing his lightning rods with the ESE systems described in the column.

In any case, nobody seems particularly sure if the damn things work at all.

ETA:

Nope.

4

The question posed by the OP was actually my first question on the SDMB, back in December 2000. I quoted some of the conflicting standards and lack of evidence for the effectiveness of lightning rods.

The question was posed again a few months later in 2001, which led to Cecil’s column.

Unfortunately, those threads have been “disappeared.” :frowning:

5

Thanks for the link to the article, I had no idea that there was skepticism regarding their efficiency, I assumed that they just worked.

So does anyone have any first-hand eyewitness accounts that they worked (eg, “That thing saved my house!” “I saw it hit the rod and it did what it was supposed to”, etc)?

6

How “big” is lightening anyway? The bolts appear to have thickness, but that’s deceiving. Is a bolt of lightening’s thickness approach zero? Or is its diameter more like meters, or 10’s of meters (decameters for metric nerds)?

If I create an electric arc (say, from an arc welding power supply) without delivering the piece or the wire to the target surface, that arc damage is significantly larger than a point-size. That would typically be in the low voltage (15 or so) range. What’s a bolt of lightening? 200,000 volts or so?

When people are “struck by lightening” are they typically hit by the visible bolt, or are they just kind of generally in the range of the bolt. For example, don’t get under a tree in a storm. People get hit there all the time, even though the tree is taller.

7

It is my understanding that what you are seeing is the superheated ionized oxygen and nitrogen gas (actually, a plasma) that is conducting the current. Depending on the voltage of the bolt of lightning, the width of ionized gas may be wider or narrower. It is in the range of centimeters wide, I would guess.

You can’t see the actual current any more than you can see current traveling through any other conductor.

8

This seems going a bit far. There’s a LOT of work with artifical lightning bolts and models on the effect of “traditional” lightning rods. If you look in Peter Vieneister’s The Lightning Book you can see a plot of lightning strikes near a model protected by lightning rods. The rods are undoubtedly doing something, because there’s a conspicuous lack of strikes near them. The book has references to primary works on this sort of thing. I don’t know anything about the newfangled active lightning rods mentioned in Cecil’s column, which lie at the root of the controversy. But I get the distinct impression that the argument isn’t over whether lightning rods work, but if the newer type work, and ought to be subject to the same regs as the traditional kind.
Lightning rods don’t create a “field of static electricity” . They do provide a safer and more desirable route to ground for any strikes occurring nearby. It’s an easier conduction path than going along the side of a water-coated building or through conducting sap in a tree, and more likely to be taken. Note the “likely” – there’s still a chance that the lightning will take all or part of its path through something else.
There’s another issue about lightning rods – their tips on’t have to be sharply pointed in ordeer to be useful. In fact, Richard Price’s AJP article about this point (snerk!) is entitled “The Lightning Rod Fallacy”, and I highly recommend it

**
R.H. Price and R.J. Crowley
The Lightning Rod Fallacy
American Journal of Physics
Vol. 53, pp. 843 - 848 (1985)**

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000053000009000843000001&idtype=cvips&gifs=yes

9

My understanding is that if you stand near an object that’s struck by lightning, you are subject to electrocution from the ground as the charge bleeds off into the surrounding earth. So you don’t have to be struck directly. I gather this is why you’re told to crouch rather than sit or lie down if you suspect you or something nearby might be struck.

When I was a kid once I was walking in the street not far from a power pole when lightning struck it. I wasn’t really electrocuted, but I could taste the electricity (like licking a 9 volt battery). That might have been sheer terror, though.

It occurs to me that one could compare the incidence of direct house-strikes by lightning in neighborhoods with overhead wiring to those with underground wiring (being careful to compare areas with similar ground conductivity). I always assumed lightning was more likely to strike the power pole than the house, so I never worried about lightning rods.

I remember reading that Charles Steinmetz had a mirror in his summer house that was struck by lightning, and he painstakingly reassembled the mirror’s shards to trace the pattern that the lightning burnt into the silver. Unfortunately I don’t recall whether the house had a lightning rod or not, but I’m pretty sure it wouldn’t have been on any electric grid in that day and age.

I have also read that there is a vast difference between run-of-the-mill lightning bolts and so-called “superbolts,” which pack a lot more energy. I have little doubt that a lightning rod system provides some protection, at least against lighter discharges; just as an interior bathroom or hallway provides protection against smaller tornados.

What I’m sure of is that there’s a lot of room for controversy about what constitutes “enough” protection.

10

I wouldn’t be surprised if some of the apparent width of the bolt comes from it being slightly out of focus when seen - the eye is most likely resting upon closer objects.

11

I recall reading once (sorry, no cite) that the coronal glow might be 20 feet across.

12

When I was taking physics back in college, my professor showed how a pointy rod will actually stop lightning strikes (simulated by a van der Graf machine). I would need a person better versed in electricity to explain it, but as I remember it had something to do with the concentration of the em field (hence the sharper the point the better). So yes, a lightning rod may prevent a discharge.

13

It’s been reported that lightning rods were useful in protecting gunpowder stores from exploding due to lightning strikes. After inventing the lightning rod, Ben Franklin later came up with an improved system in which the powder store was enclosed in lead shielding, forming a sort of Faraday cage.

14

When my husband put up the 60 foot ham radio antennae, which pole also had the TV antenna and probably other stuff I’ve forgotten about by this point, we also bought a big-ass lightning rod, attached it to said pole, and ran the conductive cable down to the ground.

A couple years late during a storm there was an amazing **>BANG!< **

In fact, along with the eye-watering flash, the bang shook the whole house. Well, we both thought lightning strike and went outside with flashlights to see if the truck or the building was on fire. (Yeah, it was that alarming). While we’re tromping around the flower beds I noticed something.

“Honey… where’s the lightning rod?”

“What do you mean, where’s the lightning rod? It’s right over there, you help put it up.”

“Well… it’s not there anymore!”

Indeed, it was not. The mounting brackets were still there (sort of), but the rod and cable were gone. Vanished.

Digging into the ground beneath the area we found some lumps of slagged metal and a fulgurite. We couldn’t get it all out in one piece, but it was sort of interesting.

Seeing as the lightning rod seemed to have functioned as intended - taking the hit instead of the building - we bought another one just like that afternoon and installed it the same day (after the storm had passed, of course). It’s still there. No more direct hits, which is fine by me.

That was several years ago. Every so often when cultivating the rose bushes I find another lump of slag. Um… no, I don’t fool with the roses during inclement weather.

15

The issue is potential differences. The air and the earth have impedance, so the massive instantaneous current of the strike moves through the media around the strike at different rates. And if you are within the areas of difference, you may have a considerable potential difference across your body, and be more conductive than the other media (like air or dirt). Crouching down reduces your vertical cross-section, but keeps your feet together and your horizontal cross-section small. This reduces the likelihood of large potential differences across parts of your body. Sitting or lying on the ground will give you points of contact that are further apart, and thus more risk of a big potential difference across your body. Standing means that your head will be at a different potential difference to your feet, and feet apart on the ground means that your feet could be at completely different potentials. All these could be lethal.

Si

16

These ‘superbolts’ I think are reference to positive lightnings which are indeed more powerful (and more unpredictable) than standard lightnings.

17

A local electrical substation is surrounded by a set of tall poles with lightning rods at their tops. I’ve seen similar installations at places where hazardous (explosive) materials are handled. They may not stop the lightning, but they keep it away from the equipment that is being protected.

18

Saint Cad writes:

This is precisely the issue addressed by the AJP article I cite above. It turns out that, as usually stated, the proposition that the electriv field muast be greatest where the curvature of the conductor is tightest, is simply incorrect. Curvature, for one thing, is a local variable, while the shape of the em potentials is governed by the global shape of the conductor (but there’s a lot more to it; read the article). Nevertheless, as a practical matter, electric discharges do tend to concentrate near sharp points. A modified version of the “lightning rod fallacy” must be correct, but I’ve never been able to formulate such a form. Neither has anyone else, as far as I know. But I believe that Corona Dope is a useful product, not in the same class as Ear Wax Candles.
And, in any event, lightning rods need not be razor-sharp to be useful.

19

OK, it’s taken eight years, but I found that old thread I started here. For some reason, it wasn’t appearing using a keyword search with the board search function.

TroutMan actually found it first using a Google search here.

20

Are lightning rods a regional thing? I know about them only from movies and such. I can’t recall them being used at all when growing up in West Texas, and we had a lot of thunderstorms. Not sure they’re used here in Thailand. In fact, I’m not sure I’ve ever seen one in person.