"lightning ball" gizmos

Cecil's Columns/Staff Reports
1

In http://www.straightdope.com/mailbag/mplasmasphere.html, Chronos gives a pretty good explanation of the plasma sphere device. But he leaves out one important part (possibly so as not to over-complicate the answer). The high voltage is AC at a pretty high frequency (any idea how high?).

It works like this. Let’s say that the central post is charged with high negative voltage, much higher than the surrounding air. The electrons in the post repel each other and are also attracted to the neutrally-charged air outside. When the voltage is high enough, the electrons manage to leap across the argon gas to the glass, just as Chronos said.

Unfortunately, glass is an insulator. The electrons basically pile up on the glass surface. They still repel each other, and the resulting negatively charged “pool” tends to resist the continued flow of electrons outward. In a very short time, equilibrium is reached and the current flow stops. If this were the whole story, then you would get one momentary “lightning stroke”, and the show would be over. The plasma pillar would cool and disappear.

Ah, but this is AC (alternating current)! The charge of the central post changes from negative to positive. The pool of electrons is suddenly attracted back to the post, and they once again leap across the argon.

If you alternate the charge fast enough, the back-and-forth leaps of the electrons manage to keep the plasma pillar going.

When you put your hand against the glass, you’re basically doing what Cronos said - providing a better attractor, which allows more electrons to pile up against the glass, causing a greater current during each back-and-forth cycle.

By the way, this is pretty much a description of a capasitor, an electronic device that appears to pass alternating through it. In reality, no electrons manage to get through the device, but the back-and-forth flow of electrons manages to pass the energy of the current.

(Sorry to be long-winded…)

2

It seems unlikely to me that “one or more of the electrons on each gas atom is stripped away” in these devices. I’m not an expert on such things, but when I was in graduate school, I used inductively coupled plasma torches as part of a course on instrumental techniques. The ICP is a much more intense device than a plasma sphere and yet I recall reading that only a fraction of the gas atoms in the torch were ionized at any given moment. Am I mistaken?

3

What sford said. Thank you for clearing that up, that was the last little piece of the puzzle I was missing.

As for those inductively-coupled plasma torches, do they operate in open air? If so, then you’re dealing with pressures much higher than inside a plasma sphere, so it would take more energy to ionize any given fraction of the atoms.

4

Chronos may not have intended his use of “each” to be taken literally. Although I don’t know much about the physics of a plasma, my understanding is that you only need a high enough density of free electrons to make a current flow freely. In something like a metal, you don’t need ions because electrons can already flow relatively freely across bonds. In a gas, you need ions to have enough free electrons. But I’m not sure what the critical density is.

Also, I’m not sure how much the temperature of the plasma pillar contributes to the light emission. My understanding is that most of the light is caused by electrons being stripped and recombined with their electrons. As a free electron gets captured by an ion, the electron gives off photons to shed its energy.

I once made a “Jacob’s pendulum” (my own term). Two long wires hung from free-swinging hooks. When a high voltage is supplied, the wires attract and swing towards each other. When they get close enough, the high voltage breaks down and an arc jumps across. This arc is relatively low resistance, and the voltage drop across it decreases drastically. The wires no longer attract nearly as much, and they start swinging away. When the arc gets too long, it can’t hold together any longer and it breaks. When that happens, the voltage drop across the wires goes back up.

MAN I wish I still had my 15KV transformer. There are all kinds of interesting experiments I would like to conduct.

5

In his answer, Chronos writes:

Then goes on to describe the phenomenon as:

Could someone tell me in what way that description of the phenomenon doesn’t apply to lightning? That’s EXACTLY what I have stored in the “what is lightning” box of my brain…

6

I do believe that ICP torches work at atmospheric pressures. It’s a really cool (not literally as we are talking 7000 K here) technology which can be used in any number of hyphenated techniques such as mass spec and atomic absorbtion.

I too don’t see the difference, other than intensity, between lightning and the plasma filament in the plasma globe…

7

The author pointed out that it might be unwise to touch your mouse to the “lightening ball”.

He said “trust me on this one.”

Based on the mega voltages and necessarily resulting mega currents involved here, I tend to believe him.

But the initial poster here stressed the insulative properties of the glass, and those lightening balls do have pretty thick glass.

Anybody else try to ruin their 'puter by putting their mouse on a lightening ball?

C’mon, you know who you are. Stand up and be counted.

8

Great column, Chronos. Thanks for the detailed explanation of how a Tesla globe works.

But this reminds me of a recent experience at the local Discovery Channel store. I was looking at one of those plasma cylindars - four feet high, 5 inches diameter, plasma snaking all along the inside of the glass. A little girl came in and started watching it with me. When she saw me touch my finger to the glass and draw the plasma to it, she wanted to try it, too. Something interesting happened when she did.

She was wearing those kids sneakers that have flashing lights in the heel, the kind where the lights flash with each step. When she touched the cylindar, while standing still, the lights in her shoes started flashing quickly, almost strobing.

She asked why it was doing that and I, NOT being a physicist, fumbled about and came up with the explanation that the electricity must be going through her and lighting up her shoes. I was surprised, though that there’d be enough charge going through her to do this.

So, what do YOU all think is the reason?
BTW, while looking around just now (doing some slight research before asking this question), I stumbled across some REALLY cool pictures of Tesla coils in action here (middle of the page).

I particularly like the pictures of the CD resting upright on a coil with the plasma shooting around it.

9

Ok, ok. So this isn’t exactly science, but it’s still fun…
The things you will need:
1 Plasma Sphere
1 Penny (preferrably one with Abe on it…)
1 Finger (preferrably on attached to your hand)

What to do:
Place the penny on top of the plasma sphere then slowly bring your finger to the edge of the penny. You have to get pretty close (we’re talking about 1 millimeter) to get the reaction we’re looking for. What are we looking for?
Little sparks of electricity will jump from the penny to your finger. If you have a steady hand and can keep your finger within the desired range, you can get a continuous bolt that will quickly burn a tiny little black hole onto your finger. Be careful, it does sting a little.

10

Points, roughly in order:

1: Lightning. The initial arc (when the center globe reaches breakdown voltage) is a lot like lightning, but lightning isn’t sustained. You get one near-instantaneous arc, and after that, it doesn’t matter if the air is ionized, because the charge is already spent. With a plasma sphere, you’re actually using that nifty low-resistance path you’ve just gone to the trouble of making.

2: The mouse. As I mentioned in the article, we’re not talking about mega currents here, since that would be dangerous to humans, as well as computers. Apparently, though, mega voltages by themselves are enough to be bad for electronics. My knowledge of this is empirical, not theoretical, since I don’t know much about high-tech electronics, so don’t ask me for further explanations.

2.71828182845904523536: Hi, Anthracite!

4: The shoes. I have no friggin’ idea. Even for a kid of the age likely to be wearing such shoes, it’s a long way from the hands to the feet. Maybe she was so excited about seeing physics in action that she was wiggling her heels really fast?

5: The penny. I have, indeed, fooled around with pennies atop the sphere (I keep a supply on hand, for use in my coin vortex right next to it), but I’ve never managed to produce arcs from pennies to finger. I’ll try again next time I’m up at my office.

11

[brushing cobwebs from ancient grey matter]

I’ll try not to butcher too badly some stuff I learned about lightning.

If I’m not mistaken, the part of lightning that you see has almost nothing to do with the plasma globe. It’s the part of lightning that you don’t see that is similar.

When lightning is getting ready to flash, the voltage between two areas (cloud and ground, cloud A and cloud B) becomes high enough to start ionizing the air between the two areas. You get “leaders” - small tubes of ionized air that start from the more negative area (?) and build towards the more positive areas. These leaders build “realtively” slowly (a few tenths of a second) and can branch in different directions depending on local charges. These leaders do actually glow faintly, and I have seen a photograph of a leader just before it reached the ground.

These leaders do not involve a high current flow. They are not extremely hot either. They just conduct electricity from the area of high negative charge to … well … nowhere while the leader is still growing. Finally though, the leader reaches an area of high positive charge.

At this point, the similarity with plasma globes disappears. You now have a low-resistance highway between two charged areas, and WHOOSH! Several zillion electrons merge on and emigrate to the promised land (where, I’m sure, the established electrons protest their jobs being taken by the newcomers). This rush of current heats the air to incandescance, resulting in a bright flash and that wonderfully satisfying shock wave.

Once the two areas of charge have equalized, the current stops flowing. BUT! The ionization tube still exists! It won’t last for long - just a few tenths of a second before the wind disipates it - but it is usually long enough for the clouds to move far enough for more charged areas to reach the tube. And you get another flash. And another. The flickering quality of many lightning strokes is caused by the movement of the charged areas of clouds.

In the plasma globe, all you get are the leaders. The glass prevents the connection from being completed. But the high-frequency alternating current combined with the capacitive effect maintains enough current flow to keep the ion tube relatively intact.

As for electronics being destroyed by high-voltage, low-current discharges, it is because of the micro-miniaturization of the devices. In a high-density integrated circuit, there are paths of conductive (or semi-conductive) doped silicon (or whatever) separated by very thin insulative areas. It’s these thin insulative areas that make the devices sensative to static electricity. All insulators will break down given sufficient voltage, and the insulative paths in ICs are thin enough to be broken down by the static charge you generate by walking across a carpet.

When air breaks down as an insulator, it will recover its insulative properties once the ionization path disapates. Alas, the silicon insulative paths are damaged by a breakdown and don’t recover, even if the amount of current that flowed is negligable. The circuits can become flakey, erratic. Or they can just refuse to work at all. (Gee, sounds like me.)

Most consumer computer equipment is protected against the kind of static charge that people get from carpets, hair brushes, cats, wool sweaters, etc. Using a combination of sheilding and high-voltage insulating covers (plastic), your computer should even survive a brush with a cat wearing a wool sweater.

But those plasma globes are another matter.

Again, it is the high-frequency alternating current that is the culprit. With AC, you don’t need a full connection to get a current flow. The capacitive effect can allow the energy to pass through even the best insulators, even though not one electron actually gets through. When you brought your mouse up to the globe, the back-and-forth rush of electrons between the central post and the glass caused a similar back-and-forth rush of electrons in the mouse circuitry and cable. The voltage from this induced current was high enough to break down the micro-insulators inside the ICs.

No blue smoke (not enough current), but plenty of flakey behavior. The plasma globe effectivly bypassed all the protective insulation and injected energy directly into the wiring. High-tech main-lining I guess.

Doesn’t make quite as good of a “this is your brain on drugs” visual though…

12

Sford’s description of damages to electronics from low-voltage, high-current discharges is a little off. True, the thin insulators in electronics can be damaged by Electro-Static Discharges (ESD). In fact, microelectronics for years have had built-in protection devices to prevent such damage. Now, the point of failure tends to be those protection devices themselves, which usually fail not due to insulator damage, but due to the literal melting/fusing of the silicon and various metals used as conductors. (This causes a short circuit, or high leakage, rendering the circuit inoperative.) This is pretty neat, since silicon melts at about 1500 degrees C. The area of melting is very small, so the energy required is also.

Prior to putting these protection circuits in place (back in the 60s and 70s), the thin insulators could be damaged by a sub-1000 volt pulse. Today’s microelectronics, with much thinner insulators, would be damaged by just 10’s of volts, but with their built-in protection, can typically withstand about 2000 volts. It’s worth noting, however, that this is near the level of what you can perceive as a static “shock”. Rub your hands across the cat and then touch an integrated circuit, and you will stand a very good chance of sending it to silicon heaven, since you will easily generate upwards of 4000 volts, and up to 10,000 or more on a dry day.

13

Heh. Toldja that the cobwebs needed a good brushing. Thanks for the update.

I learned most of that during a company-required training class on the dangers of ESD. This was about three years ago, so it’s pretty sad that they couldn’t get their technology right.

A minor nit - I was talking about high-voltage, low-current discharges. Since you were too, I suspect your statement to the contrary was just a typo.

14

Duh. Yes, I wrote it backwards.

Interestingly (to geeks like us, at least), it’s more accurate to talk about high voltage, high current, low power events. The standard test setup for human body model ESD (where the human is charged up, and discharges through the device under test) is for the voltage source to go through a 1500 ohm resistor in series. So, if you have a 3000 V charge built up and discharge it through a 1500 ohm resistor, you get a 2 ampere current (Ohm’s law). Of course, you don’t get it for long…if I remember right, the rise time of the pulse is about 10-20 nanoseconds, and the decay time is about 100-150 nanoseconds. (1 ns = 1 billionth of a second.) So even though the voltage is high, and the current is high, the total power (due to the short time duration) is still pretty small. Still big enough to melt silicon, aluminum, tungsten, etc. in a very tiny area, though.

In my experience, most company training (and textbook material) on ESD is of marginal accuracy. It appears that most have been drawn from source materials that were out of date over a decade ago, but there’s continual inbreeding that keeps the old beliefs alive, plus a lack of informed dissent. And probably a huge lack of interest as well.

15

You mean low-energy, not low-power, right? A high voltage, high current event will have a high power, regardless of the duration of the event. Unless, of course, you’re talking about averaging the power over some extended period of time?

16

You’re right. I always get those two mixed up. The energy is pretty low, but since it is dissipated over such a small time period, the power is rather high. P=E/T.

17

You are a naughty boy, Count Chronos. :slight_smile:

18

HEY HEY HEY!!!

I just bought a gas plasma globe for TWENTY FREAKIN BUCKS! It’s not as big and powerful as the ones I’ve seen in science museums, but it does the trick.

I tried the penny trick and it worked. In fact, by holding a nickle in my hand and bringing it up close to the penny, the spark didn’t burn a hole in my finger. It just sustained between the penny and the nickle. The penny all by itself tends to attract the beams.

Another cool thing I used was a metal chain (the kind made out of little balls). When I brought the chain close to the side of the globe, it attracted the chain. It actually held the tip of the chain against the globe.

Next experiment - I held a needle up to the glass. It attracted a small streamer. After holding it there for a moment, I smelled a VERY strong odor of ozone. Then I held a metal ball (about the size of a marble) to the glass. No ozone smell.

Say hello to my buddy Benjamine Franklin who figured out that a grounded metal rod sharpened to a point would disapate an electrical charge slowly and prevent a large charge buildup. Thus the lightning rod was invented. In my little experiment, the sharp tip of the needle disapated some of the energy into the air, which formed the ozone. The metal marble didn’t disapate the charge, so no ozone.

I’m gonna have a blast with this thing! (Although I’m a little dizzy now from the ozone - time to turn it off.)