My understanding of lightning rods is that they are more of a preventative measure than a conductive element.
When a cloud starts to generate a static charge, the ground builds up an opposing charge underneath it. When the charge potential exceeds the resistance of the air, an arc occurs (lightning).
A funny thing happens, though, when charges are building up. The very shape of the surface can help or hinder the amount of a static charge building. Think of an egg. If the egg were perfectly sperical (like an ostrich egg), the charges would be evenly distributed across the shell. However, a chicken egg has a pointed end. The pointed end attracts more charges, and will therefore reach that level where an arc (lightning) can occur.
Apply this to a geographic formation. A hilltop has this "pointed" top, and it also closer to the cloud, than the surrounding ground. Those two factors combined will make the hilltop a frequent site of lightning strikes. A mountain is even better, as it's higher. Rooftops are common strike points, as the "point" is usually a sharp 90° angle, and attracts a great charge.
Something else funny happens when you start going from a gently-rounded surface to a "sharp point". Not only do the discharges (Lightning or sparks) become more frequent, but since they don't have as long to build up, they become less powerful. Instead of one, massive, strike, you get lots and lots of little ones that don't do much.
Take this to extremes. Instead of a small, tightly curved surface, image a point. Like the point on the top of a weather vane, lightning rod, needle, etc. This point, as the theory was explained to me, cannot build up enough of a strong charge to discharge with any great force. What happens is that the charge is "bled" into the air at an almost continuous rate, but with such a low voltage that it's virtually undetectable. Therefore, a lightning rod doesn't so much "conduct" a lightning bolt from a dangerous material, but actually prevents one from striking in the first place, as the necessary opposing charge cannot build up in the ground during a storm, as the lightning rod is "bleeding" the charge into the air.
This was all explained to me back in HS Physics. Whether or not I have remembered it accurately, I don't know.
Another interesting tidbit is that when lightning strikes the ground, it usually fuses the dirt into a hollow glass tube that extends deep into the earth. This is called a "fulgerite". Sometimes people find them washed up on a beach, especially after a t-storm.
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