I’m specifically thinking about those dome shaped metal jungle gyms such as this. Would it act as a Faraday cage and keep me from harm? Or would I be toast if I took shelter in one?
Not really. An ideal Faraday cage is a solid skin of conductor which blocks any traveling electric charge by redistributing the charge evenly around it in a Gaussian surface (a surface of equal electrical potential). A real-world Faraday cage is usually a fine mesh which blocks “low” frequency EM signals (la large wavelength relative to the openings in the mesh) and transfers electrical energy to ground, which is fine when dealing with relatively low energy impulses, but once you get to a saturation current (where the current across the surface causes the relative permittivity to be less than the atmosphere, or the material starts breaking down) you can no longer count on the field being in a quasi-static equipotential distribution, especially if your Faraday cage has any geometric singularities (sharp points or edges).
Lightning is a result of conduction of static electricity from elevation to ground via dielectric breakdown of the atmosphere. By definition, it has enough energy to cause the atmosphere to break down (ionize), and in the process of this, it radiates in a wide spectrum of frequencies, but most of the energy is concentrated in the high frequencies. When it contacts a grid, instead of spreading evenly around, it’ll tend to focus at any singularities until the material breaks down, and then conduct around and away.
The problem with using a jungle gym to like the one you link to conduct away lightning to is two-fold; one is that the openings are large enough to permit much of the high frequency energy to pass through them; the other is that it does not have sufficient permeability to conduct all of the energy to ground itself, which means it will break down and conduct energy via other paths, including the back of salty water inside (that would be you). The metal structure might help protect you some, insofar as it will at least conduct away some energy, but only until it becomes a twisted molten mass of metal, after which you become a charred marshmallow.
Lightning rods and towers that are designed to protect structures and vehicles are designed with a number of intentional singularities (sharp point or small radius tip) that are directly connected to a highly permeable conduit (small gauge pure copper wire) that goes to a mass that is literally buried under ground. Instead of trying to surround the object being protected, they just provide a highly preferable path. They do not protect against ionization of the nearby atmosphere, so delicate electronics still need to be either shielded or robust against plasma discharge, but they’ll conduct the bulk of the energy directly to ground.
Stranger
Even if the structure successfully channeled the charge away from you, I’m sure the blast from thunder generated just a few feet away would at the very least knock you off your feet and smash your eardrums. That close and “thunder” would be an explosive shockwave, not just a really loud bang.
You would be better off sitting or laying flat on the playground nearby, rather than inside the jungle gym.
That metal structure sticking up from the ground could be a likely place for lightening to strike, so you don’t want to be inside it, even if it does help conduct some of the lightening into the ground. But if you are nearby, you may be safer, because lightening is more likely to strike the jungle gym than you.
They say that cars are sufficiently enclosed (and with rubber tires) to offer protection during a lightning storm.
But they also say that standing under a tree is one of the worst things you can do because it’s enough to attract the lightning bolt and not enough to actually protect you.
Most jungle gyms seem more like trees than cars in my mind. I’d stay away. Lying down really is your best defense.
I may be mistaken, but my understanding is that it is dangerous to stand under a tree during a lightning storm because if lightning strikes the tree, the sap will heat and expand violently, exploding splinters of wood at you. Additionally, this can weaken the structure of the tree, allowing branches to fall on you. I’m sure there are other considerations as well.
Despite what others say in this thread, I think it isp possible to derive protection from a “jungle gym” structure.
The Theater of Electricity at the Museum of Science has an Operator Area that looks like a giant birdcage (you can see it in some of the shots here: http://www.youtube.com/watch?v=IJSNiDbob6Q ) which protects the operator from the “lightning” discharges of the Van de Graaf Generator*. In fact, he demonstrates this by raising the platform up towards the globes atop the generator and letting it Zap the cage. The operator will even touch the bars while this is going on.
There are similar wire strung between the generator and the audience to intercept lightning strikes. The operator points out that the exterior wires do not constitute a Faraday Cage. The operator cage isn’t a really good Faraday Cage, either. (I’ve built and repaired Faraday cages, and I agree – you could easily listen to a radio or a cell phone inside that birdcage). But it is sufficient to protect him from the strikes of artificial lightning the van de graaf produces.
A completely enclosed “Jungle Gym” well embedded in the Earth thus might protect you, if the bars were close enough. No jungle gym I’ve seen had bars as close together as this operator cage has, however.
the Bottom Line is that, although it might work., I wouldn’t use it. Unless I had no other resort.
*The Van de Graaf generator at the MOS is actually Van de Graaf’s own Van de Graaf generator, built and operated by him before being obtained by the MOS and installed there. That’s pretty neat.
Given the voltages and currents generated in a lightning stroke sequence, the presence of rubber tires is irrelevant. Lightning will merely laugh at a wet rubber tire as it arcs around and through it if it hits your car directly. The protection is solely in the Faraday cage effect.
That is an issue, but the potential (as in voltage) gradient in the ground will kill you first.
things close to the ground do not attract lightning. lightning has just traveled miles through nonconductive air, what there is close to the surface doesn’t matter much. lightning also has lots of energy to get rid of so it will make multiple current paths especially as it nears its target.
if you are unsheltered in a lightning storm it is best to squat down. this leaves a smaller surface for multiple split strikes to hit. lightning can travel on the ground surface and squatting leaves a single point to be in contact with the ground. providing the shortest paths across your body minimizes harm if struck.
inside a jungle jump the lightning bolt may split and conduct across and into it. lots of closer spaces bars may help.
Not only that, but at least part of the lightning bolt can also jump from the tree and go through you as it finds a path to the ground. At the high voltages of a lightning bolt, things that aren’t usually a conductive electrical path (like open air) suddenly become paths for the lightning to follow.
The voltage gradient that VunderBob mentioned can also kill you. The lightning bolt is tens or hundreds of millions of volts. This large voltage spreads out as the bolt strikes the ground, so right at the tree you have millions of volts, and twenty feet away you have nothing. As the voltage level rapidly changes across the ground surface, you can easily have several thousand volts develop between one foot and the other, which can easily result in a fatal shock going through you.
7 out of the 27 people killed by lightning so far this year died while taking shelter under a tree.
This is by far the best thing to do. Squat, and keep your feet together to minimize the risk of shock from a voltage gradient that might develop from a nearby strike.
A jungle gym is not an enclosed surface, so it lightning does happen to hit it, the bars aren’t going to provide any protection. The bolt may hit the bars and then jump to you.
Open pavilions are not safe, even though you have a roof over your head. Standing under an awning or in an open doorway is also not safe. You can be killed inside of your garage if you leave the garage door open.
2010 is the first year that I’ve seen “getting better cell phone reception” listed in the lightning statistics. Anyway, if you are curious, all of the statistics are here. You can get detailed lists for 2006 through 2010, and more generic statistics dating back to 1959.
It might not be safe, as I say, but an open structure can give more than adequate protection. See the link in my post above.
I wouldn’t trust it, though.
As you noted in the above post, the bars in the cage are much closer together than the bars in a jungle gym. To high frequency electricity, if the holes in the bars are significantly smaller than the wavelength of the electricity, the metal might as well be solid. This is why Faraday cages can be made out of screen instead of solid copper, and why radar antennas are often screen meshes instead of solid metal as well. The screen mesh works as well as a solid piece of metal and doesn’t catch the wind as much as a solid antenna would.
The bars in a jungle gym are too far apart to offer any meaningful protection. A T-top car will also not protect you, for the same reason. The lightning may be slightly more likely to strike the jungle gym bars or the T-top, but there is still a very good chance that it will continue to arc over to you.
Step away from the jungle gym and squat. It’s safer.
it depends on the size of the jungle gym. Have you looked at the linked site? The bars on that cage are much farther apart than the mesh on any Faraday cage I’ve worked with, and they are just about as far apart as the bars on the Jungle Gym I played in as a kid. And the ends of that gym were buried in the earth. I thin it would’ve made a credible lightning-safe cage.
Not that I would’ve trusted it, especially with all those homes and safe structures around.
Agreed that rubber tires do nothing for you in terms of isolating the occupants from electrical strikes. However, it should be pointed out (and I’m as much at fault at not clarifying as anyone in this thread) that the protection offered by a metal shell or fine mesh against an electrical arc or lightning strike isn’t specifically because of the Faraday effect (which applies to a static charge being evenly distributed around a Gaussian surface) but because of the skin effect, which causes transient high frequency electricity to flow along the outside of the conductor, and will distribute evenly enough that you create what is essentially a static field that can’t create a potential inside the body. In the case of a car with a continuous, enclosed thin sheet metal body, the vast majority of the current will remain on the outside of the shell because that is the easiest path to ground. For a convertible or fiberglass top, this isn’t the case. With the jungle gym of the o.p., the skin effect will cause the current to flow along the outside of the individual members, and the field will be localized enough (because of the large gaps between the members) that it will not create a constant potential field along the “inside surface” of the gym. This, combined with the previously mentioned singularities (protruding bolt heads or shafts at the joint) can readily cause arcing to occupants inside the dome.
While it is true that a man standing in a field will not attract lightning that is initiated a few miles away, there can be enough of a difference in resistivity that the current preferentially conducts through him. This is why high frequency fields generated by Tesla coils will arc toward an outstretched hand rather than directly to ground.
Stranger
Another YouTube Video of the MOS Theater of Electricity, this one with actual moving pictures!
Note that the operator inside the cage is actually touching the bars as the discharge strikes the cage. They point this out during demonstrations, and even will hold onto two different bars at the same time. Note also that the cage is surprisingly open – far more, as i say above, than your typical laboratory Faraday Cage, which usually has a single or double wall of copper cloth or close mesh.
My point here isn’t to advocate doing this. I don’t know what the safety limits are for such a cage – but minimum safe weave is clearly a LOT more coarse than my direct experience with Faraday cages would suggest. My gut feeling would be that this is too open, and that I would NEVER put my fingers on two different bars while the lightning is striking. But the MOS demonstrators do this every day. I haven’t been able, in quick internet searches, to find any guidelines about minimum safety requirements. But it’s not at all obvious to me that Jungle Gyms of the sort I played on as a kid would be unsafe. (Note that “Jungle Gym” covers quite a bit of playground equipment variation. What you played on might not be at all safe, especially if it wasn’t made of metal). I’d certainly feel safer in that Jungle Gym than in my car, which has big open window areas larger than the interbar spacing I had.
and, again, I’m not advocating actually using a Jungle Gym as protection, since i don’t know what the protection limitations are. But I DO want to point out that a safe environment can have an amazingly open appearance.
The change in resistance is local, there is a close distance where you would have an effect as you approached the Tesla coil. Actual distance will depend on distance and voltage for both lightning and the coil.