A metal rod sticking up into the air is both a silly thing and nearly indestructible. By adding apical decoration it is transformed into an asethetic statement, the price goes up, and there is something to be damaged and repaired/replaced.
-- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz.pdf
Hello, I was wondering why there is a ball on lighting rods? example
It seems there is a metal one in this picture which has some arcing under it, Its hard to believe they serve no purpose.
Decoration. Most of these balls are colored glass.
-- Bert --
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Oh the irony--you're standing in the front yard when the lightning strikes the lightning rod... potentially saving your life. The thermal shock of the strike fractures the glass ball, half hurled striking your head killing you! Oh the irony.
purpose
field
Still decoration. At one time (early 30's or late 20's ) there were rods made with a ball at the top based on some half baked idea that came into vogue (to raise the price, probably) that it would provide better shielding. To sum up- it didn't work. There have been, are, and will be a variety of devices made and sold to provide lightning shielding. Typically they don't do as well as simply sticking a bunch of old bedspring on the roof and grounding them well - but they are shinier and more expensive.
-- Don Kelly dhky@peeshaw.ca remove the urine to answer
If you get killed by this sequence than it was definitely your time to go!
Hi,
The ball is usefull in spreading heat from a direct strike. A #8 wire can carry any known strike but it will bern at the point of the strike without more thermal mass.
Ray
of
at
wouldn't the head produced be caused by the current through the wire, or is there another method of heating in lightning strikes i'm not aware of? Wouldn't this spread the heat through the region of the ball but not farther along the path of the wire? I hadn't considered the heat involved before, intresting.
The heat generated is much more in the air above the wire. The power generated causes the vapor in the air to turn to steam and the rapid expansion of the steam creates the thunder.
Ray
is
farther
can
without
purpose
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What's lightning, life just gets darker with age.
Not exactly... thunder does not require the presence of water in any form. Like all sparks, the rapid expansion of the spark channel creates a shock wave that ultimately creates thunder. For lightning, the exact mechanisms are subject to some debate, but thunder simply does not require the presence of water in either vapor or liquid form.
-- Bert --
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Lichtenberg Figures ("Captured Lightning" in acrylic), & Out-of-Print
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That would appear to be an "early streamer" type lightning rod. i.e. it is supposed to attract lightning better than a Franklin rod, by some means of ionizing the air around it.
There is a lot of debate as to the effectiveness of these devices, in my opinion they are no better than snake oil and the Australian standard at least is unsupportive of them (despite appreciable pressure from manufacturers of these devices).
V.
I didn't catch the original post but I thought that this may be a good time to mention that lightning rods are/were not designed for direct hits. The purpose is to keep the accumulated charge below a safe level. Tom
can
without
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--------------- Actually this is extremely questionable. Design that I know of is based on the probability of the stroke striking the rod rather than the protected area. Granted, in some situations, particularly with tall structures, such draining does occur (possibly increasing the chance of side flashes-another problem) but this is a bonus, not the basis for design. I am aware of a purveyor of lightning protection that claimed that his protection works on the basis of charge dissipation. I have seen no evidence that it does. I have no idea if he is still is in business.
Generally the source of the charge is several miles overhead -the rod doesn't get seen by it. When a leader gets near a rod then it may be a preferred target for the next step and if it is, then the main stroke will be to the rod-if not something else gets hit. Design is based on it being such a target for higher current strokes (but not necessarily lower current strokes). This is true for protective systems for transmission lines as well as structures. Catch the damaging strokes know that some of the little ones will get by.
References: Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct Lightning Strokes" Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, 1988 Energy Systems Journal, Vol. 11, N0.1 1991 EPRI Transmission Reference Book, 345KV and Above. I believe that IEEE changed its standards to reflect the concepts involved in these references. Sorry that I do not have more recent references but I have been retired for some time.
I agree with Vermin on the lack of usefulness of the ball. There appears to be no rational basis for its use. Note that its thermal mass is generally pretty small and by the time it starts to dissipate heat, any damage is done.
-- Don Kelly dhky@peeshaw.ca remove the urine to answer
A best source of information in this discussion are both posts from Don Kelly. Especially where he defines what makes a lightning rod effective: earth ground. We tend to rationalize only upon what we see. We see the lightning rod and assume IT is the protection. We then get all hyped over blunt rods verses pointed rods verses one with a ball. All irrelevant once the facts are considered. Since we don't see earth ground, then we tend to forget the most important component of a lightning protection system.
Don also cites another well regarded expert on the subject - Dr Abdul Mousa. Dr Mousa's IEEE reviewed papers contain good, scientific language. But to summarize into laymen's terms - Early Streamer Emission (ESE) protectors that are suppose to discharge the air are scams. Air terminals provide the best or more conductive path from cloud to earth. Which again demonstrates what makes lightning rods effective - quality of that earth ground system.
D> Thanks for the info, I'll see what else I can find. Probably the
Thanks for the info, I'll see what else I can find. Probably the foremost expert on lightning in this group is Mark Kinsler. I believe he did his doctorate on the subject. I have not seen posts from him in a long while. Tom
I'm no expert on this, but I do recall my undergrad Electromagnetic Fields instructor coming down on the draining side of this issue. His examples were opposite to yours: He pointed out that the size of the ground wires on the typical barn or home lightning rod was far too small to withstand a direct hit. He noted that tall buildings, which do sustain repeated hits, have massive ground conductors to handle the current.
That was probably the only meaningful thing I recalled from that class. All the rest was curls, dels, and other funny symbols. (That was over 35 years ago. A lot of synapses could have drained to ground since then!)
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
I agree with the #8 wire not seeming to be adequate. The coast guard / power squadron use the #8 wire and 60 degree protection cone as there rules but for my sail boat I use a welding cable size ? connected from the aluminum mast directly( no curves ) to the lead keel. I have observed a boat being directly hit by lighting about 100 feet away from me. The vhf antenna made a baby carriage size puff of orange smoke. No one was injured but the electronics were fried. I would like some explanation of ball lighting. I have known of a ball ( corona ) in the cockpit of a sail boat.
Ray
on
such
flashes-another
Not true.
Even some reference books claim the above. They've fallen for a type of physics myth; a "science urban legend."
Yes, in 1790 they assumed that a little bitty lightning rod could discharge a miles-wide thunderstorm across the miles of space above the rod. The experts reasoned that, after all, a needle could discharge the main terminal of an electrostatic generator even if the needle was many inches away. Researchers later figured out how silly this was.
Check out Dr. Martin Uman's book "LIGHTNING" for some ACCURATE lightning-rod discussion. (It's probably unwise to listen to any of the emotional and dishonest "belief systems" espoused in this thread, including my own! Go see what lightning physicists actually say. Experiments and mathematical reasoning will win over politics and "beliefs.")
Still reading?
Well then it's your own fault.
Why can't a lightning rod discharge the storm? The scale is wrong. While a needle can discharge an electrostatic generator via ion-leakage across a large gap, a lightning rod is not like a needle. A lightning rod is tiny: like one fiber in a piece of felt if we obey the scaling. If one fiber is slightly taller than the others, well, the high voltage electrode doesn't care. If you erect a lightning rod, the storm won't even notice.
Also, the scale is wrong for "charged wind" to transport any charge upwards. If you hold a needle near a high voltage terminal, the needle "emits" charged wind which travels at many cm per second, and there is a significant electric current in the air; on the order of microamps or tens of microamps. This easily shorts out an electrostatic generator.
But if we scale things up and use a lightning rod and thunderstorm, the "charged wind" coming from the tip of the rod STILL TRAVELS AT CM/SEC SPEEDS AND THE CURRENT IS STILL MICROAMPS. To have a significant effect on the charged storm cloud, this velocity would have to scale up too. The lightning rod would have to "emit" an electric wind that traveled at tens or hundreds of KM per hour, and the electric current would have to be thousands of times higher than 10uA.
Note that the above is one of several "lightning rod controversies." Emotions run high in such controversies, so you cannot trust what either side tells you. You can't even trust many reference books!
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