When I took high-school physics, my teacher explained (rightly or wrongly) that a lightning rod does not attract lightning. A lightning rod is pointy on top, which concentrates its electric (or was it magnetic?) field-lines at one point, which makes it possible for it to discharge electrons steadily and harmlessly, from the ground into the air, thus preventing the charge buildup which leads to a stroke of lightning. A van der Graaf generator, by contrast, has a globe on top because that shape holds the charge on the surface and prevents it from dispersing, thus it builds up a charge that we see discharged in sparks when you put your finger close to it.
I have always believed this to be true, and in consequence I have always been annoyed whenever I hear or see the phrase “lightning rod” used as a metaphor for something that draws trouble to itself. I recently mentioned this in a nitpick in a GD thread – http://boards.straightdope.com/sdmb/showthread.php?t=273839 – which quickly turned into a hijack. Several Dopers joined in and insisted a lightning rod does attract lightning, therefore the metaphor is an apt one. They cited several physics sources. What’s the Straight Dope?
The action of a lightning rod is complex as this cite from HowStuffWorks shows. I believe the first action is to raise the electric field intensity around the end of the rod and so hasten the discharge. However, the charges can be huge and the increased field can lead to a breakdown of the air in the vicinity of the point and that can propogate upward in a lightning strike. In short, the rod tends to concentrate around itself the leaders that inititate the strike.
Here’s what Franklin said: “May not the knowledge of this power of points be of use to mankind, in preserving houses, churches, ships, etc., from the stroke of lightning, by directing us to fix, on the highest parts of those edifices, upright rods of iron made sharp as a needle…Would not these pointed rods probably draw the electrical fire silently out of a cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible mischief!”
Franklin was both right and maybe wrong. I think there is some evidence that a blunt point is just as good as a sharp one, if not better. However for someone so early on in the the electricity game Franklin had a good grasp of the idea of the rod.
I was going to ask this here also after it was brought up in GD, but then I saw that Cecil already answered the question. According to the Master, no one is sure that they work at all.
Plus, I always thought that the point was to make a path of least resistance that didn’t include the actual structure. Electricity will always take the easiest route to ground, so this should be the easiest in the area.
Think of electrical force as water pressure for a moment. Imagine building a valve that needs lower pressure to open it and put the water somewhere safe, thereby preventing a catastrophic blowout.
That’s what the HowStuffWorks cite says. It also says that the rods MERELY increase the probablity that the strike will be on the rod rather than the structure.
QUOTE: “As you can see, the purpose of the lightning rod is not to attract lightning – it merely provides a safe option for the lightning strike to choose.”
Yeah, I can’t find any sites that say they repel. Even if they do work by discharging static slowly, the sites I’ve found, like this one, say the static goes from air to ground, not ground to air.
Cecil was a little too accepting of the “throw out the lightning rods” crowd. I’m a little surprised that we don’t have more hard data (assuming we don’t) that rods give a statistical advantage over no-rods. But we’ve all seen lightning hit trees, metal towers, and buildings, and quite clearly has a preference for tall, conductive things.
About the Van de Graaf sphere versus the shart point - a sharp point will conduct more current through corona discharge, and you can even completely discharge a Van de Graaf ball if you use a sharp point, without a spark. But for clouds, there are thousands, if not millions, of coulombs of charge up there, and there’s no way that your puny little pointed rod a half-mile below is going to conduct all that without lightning.
When I googled earlier today, I found a few sites that said that, although their credibility was questionable (they said things like rods have to repel, because a single strike has so much power that it would vaporize the rod and burn down the building). About the direction of the stroke, that site you cite describes it correctly. A small channel begins at the cloud, taking multiple steps toward the ground. When it gets close to the ground (10-100 yards), the taller things on the ground will distort the electric field at their tops and a small discharge will go from there towards the tip of that stepped leader. This happens because the tip of the stepped leader is basically at the same potential as the clouds, but it’s much closer to the ground. When the leader from the ground and the one from the cloud meet, there is suddenly a conductive path between clould and ground, and that’s when the heavy current starts, and you see the lightning. The leader itself is very faint.