How did lighting rods work?

nyboySeptember 7, 2012

Was looking at some antique lighting rods on ebay. How where they supose to protect the house? What did the glass balls do? Is there a more desirable glass coloer? Can a waethervane also be a lighting rod? Thanks

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calliope

Lighting rods should still be used. They protect a structure the same way ground wires protect a structure now for electrical circuits. The rod is connected to ground. I won't go into details, but since barns and farmhouses were often the tallest objects during storms, the rods were attached to 'attract' the lighting to them so the charge could be safely carried to ground, instead of the structure itself, since it will travel along the course of least resistance. My son's barn still has the old rods on it. The balls, many people say are just for decoraton, but they probably blow when a current as strong as a bolt of lighting travels the rod, and indicates the structure has indeed been hit. I am not a collector, so I imagine there are certain things making a particular one desireable over another.

    Bookmark   September 7, 2012 at 7:58PM
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lindac

yeah what Calliope said.
However I don't think the balls blew in a strike, I think they were often the target for kids with a sling shot.
Definitely decorative.
Yeah...a weather vane can be a lightening rod....but the lightening strike isn't pretty!! LOL! They are not grounded !

    Bookmark   September 7, 2012 at 9:39PM
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bulldinkie

Yes the lightening rods are connected to a wire that runs down the side of building into the ground,that way if it would strike it goes down the wire into ground.We have them on our buildings..I saw lightening strike oh my.

    Bookmark   September 7, 2012 at 10:07PM
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jemdandy

The working part of a lightening rod is its sharp tip. The rest of the stuff and details along its length including balls are decroative. The smaller the radius of the point, the easier it is to break down the air surroundoing the tip and pass current. (Consult your physics book on this.) Therefore, lightening rods were given fairly sharp points. This also encourgaged a bolt of lightening to strike the rod in preference to the large available roof area. To be effective, the tip of the rod must be above anything else on the roof, hence these rods were srtung along the roof peak. If a chimney stood above the roof, it should have a rod on top of it. A common mistake was to forget the chimney.

The rod protects only a small cone if airspace below the tip, typically a 30 deg cone. Thus, for full protection, rods should be spaced fairly close to each other. This was rarely done, Typically, rods were spaced several feet apart.

The rods were connected by a wire cable to a grounding rod. (A rod driven into the ground to a depth of 4 ft or more.)

The grounding wire was of a special non-inductive braid. A soild wire has a small amount of inductance which in normal useage is ignored. However, the rise time of the current in a lightening bolt is very short and the very small inductance of the grounding wire presents a significant inductive impedance. The special braiding of the wire reduced this inductance and reduced the chance that the lightneing bolt might jump from the wire to other parts of the structure.

Lightening rods were assumed to work by attracting nearby charges and provide a low impedance path to ground. Hopefully, a local charged atmosphere above the building would prefer the lightening rod and wire path to ground rather than the building.

    Bookmark   September 8, 2012 at 1:52AM
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nyboy

Thank You Did not know about wire to ground, learning a lot here!!

    Bookmark   September 8, 2012 at 4:16PM
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