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Lightning rod ball – EDN

I had been noticing a whole series of lightning rods mounted on roof peaks at a nearby shopping mall. They all looked like this:

Figure 1 Lightning rod mounted on roof peak.

I wondered what purpose that ball in the center of the vertical rod might serve. That was not an easy answer to find though. Most websites talked about the decorative allure of colored spheres and I didn’t think esthetics would have much to do with what I wanted to know.

Finally, I did a Google search on “lightning rod orb” and in the results found the following:

Figure 2 Google “lightning rod ball” search result excerpt.

I imagine that sudden heating of the vertical metal rod at the moment of a lightning strike would cause rod expansion against the glass, however slight. That expansion would then fracture the glass which would not survive the event.

Looking around the mall, there were several such roof peaks, each equipped with the same kind of lightning rod and ball and none of them showed any signs of damage.

I guess that means so far, so good.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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