by Ted Beyer
Nikola Tesla. Amazing guy. He came up with a huge number of inventions, but outside the scientific community he is largely overshadowed by his better known contemporary, Thomas Edison. Tesla developed a stream of innovations that we use every day—things like AC power, fluorescent lighting, on and on.
What you might not know is that Tesla, when working on electric light in February of 1894, came up with the concept for what we now call the Plasma Globe.
Now—credit where credit is due—even though Tesla conceived of the idea in the first place, it was not until scientist/artist James Falk came along in the 1970s that we had the plasma ball to enjoy. I remember seeing some of Falk’s originals at an art exhibit in the late ‘70s with prices well over $2,500 (about $11,000 today). They were jaw-droppingly cool, beautiful, and endless fun to play with. Well, at least until the exhibit people chased me and my friends off.
There are many variations on the Plasma Globe, but they all share some basics. First and foremost there is a glass enclosure: usually a sphere, filled with noble gasses—usually neon, but manufacturers will add traces of argon, xenon, and krypton as well—all at nearly atmospheric pressure. In the center of the enclosure, there is an electrode being fed by a high-frequency electronic oscillator circuit powered by a high-voltage transformer. Combined, they output high frequency and high voltage AC to the electrode, most often around 35 kHz, 2-5 kV. Don’t let that scare you; the voltage may be high, but it’s really the amperage that is dangerous. The spark you get from walking across a carpet and touching a doorknob is around 4 kV.
I should point out at this juncture that this whole high voltage, high frequency, low amperage generating thing is also known by most people as a Tesla Coil. See—I told you he had a lot to do with this thing!
Anyhow, all that high frequency/high voltage power (actually it’s up in the area of radio waves at that wavelength) is sent through the gasses in the globe from the electrode in the center. Fingers of plasma emanate from the electrode to the outer glass. The fingers are actively competing for space in the globe—they all share the same polarity, and of course that means they all repel each other (remember, like charges always repel!)
Let’s stop here for a moment since we have to figure out just what the heck the glass is doing here. It’s not a wall—it’s acting like an interesting kind of insulator called a dielectric.
Wait—what the heck does that mean and why do we care?
Stay with me here—it gets a little weird.
The plasma, the glass, and your finger all create a thing called a capacitor.
Okay. A capacitor consists of two conductors, separated by an insulator that is referred to as a dielectric. It’s a gadget that stores electric charges, kind of like a battery, but it operates much faster both when charging and discharging. Electronics folks use them for all kinds of things, especially when they need a lot of energy for a very short period of time. When a capacitor charges, electrons leave one conductor (the plasma in this case) and accumulate on the other. A negative charge builds up on one side of the dielectric (the inside of the glass), and a positive charge on the other (the outside of the glass), creating an electric field that stores up the energy until something comes along on the other side of the glass that allows it to restore balance.
When you touch the outside of the glass with your finger, you are creating a lovely spot for the negative charge stored up on the inside of the glass (remember, it’s actually a dielectric busily collecting a charge from all the electrons trying to escape) to push away the negative charges in your finger while leaving the positive ones—to which the negative charge inside of the glass is attracted—behind. That is, you are a far better friend to the electrons than the air around you—so all that energy will flow towards you.
If someone else also touches the globe at the same time, more often than not, one of you will have a brighter, thinner finger of light heading for your finger—or perhaps you will take all of it! Just goes to show that you are a better ground than they are. The plasma finger is brighter because there is more current flowing through it, and it’s thinner because its own electro-magnetic fields create a force acting to compress the size of the plasma finger itself.
A great deal of the movement of the plasma fingers is due to the plasma itself heating the gasses around it. As the gasses are heated, they naturally become more buoyant, and that in turn causes the plasma finger to rise as well. When the plasma finger discharges into a fixed object touching the globe (like your hand), it will begin to deform, usually into a curved path between the electrode and that fixed object. When the distance between the electrode and your hand is too great to sustain, the plasma finger will break and a new one will form between the electrode and the object.
Of course, it’s also pretty, and really cool to look at. I should have said before that the colors are determined by the mixture of gasses in the glass enclosure. The exact mix used in any given Plasma Globe is a closely-guarded secret of each manufacturer, but as I mentioned, it’s most often a mix of neon, argon, xenon, and/or krypton.
Wait a second. What just happened?
Here at Educational Innovations we love to play with things. We are fascinated to discover unexpected behaviors in objects, and often we hear from customers with questions about things we never even considered.
We had read about an effect that can be observed under high voltage power lines—namely, that fluorescent tubes will actually light up when held in your hand. That same high voltage effect can be shown using a Plasma Globe. In our Plasma Globe Experiment Kit (PLS-110), we provide you with a small 9” fluorescent tube. If you hold one end of the tube and bring it close to the Plasma Globe, it will light up.
Look Ma, no wires!
How is that happening?
Well, a fluorescent tube is a glass enclosure containing a mercury vapor and—get ready for this—a noble gas such as argon, xenon, neon, or krypton (sound familiar?). The inside of the tube is coated with a phosphor (most often a mixture of barium, strontium, and calcium oxides, but that’s not important). Excite the mercury vapor and gas mixture with an electric current (somewhat differently than the Plasma Globe, but let’s just avoid that conversation), and it gives off a large amount of very short wavelength ultraviolet light (known as UV-C) that, in turn, excites the phosphors which glow with visible light.
Okay—remember the charge on the inner surface of the Plasma Globe that was looking to push away the negative charges in anything that comes close to it? It’s still looking for that path to ground well beyond the barrier of the glass enclosure. So when you bring the tube near the globe, the charge finds that gas/vapor mixture and again goes on pushing away anything with a negative charge, and thus passes enough energy to cause the mix to emit their UV-C, and that—as you might expect—lights up the phosphors. Magic! Electric light with no wires!
The Plasma Globe Experiment Kit also has several neon bulbs in it, which, predictably, light up when brought near the globe. The same principle is involved, but far more simply. The negative charges in the neon gas are getting pushed away—after all, it’s a far more popular destination than neutrally charged air, and again, the neon contained in the bulb gets excited and lights up.
If you approach the Plasma Globe with one of our Energy Balls (SS-30)—or better yet, the Energy Tube (OHM-350)—you will discover that without touching the two metallic contacts that usually serve to activate them, they will light up and make their sounds as though the contacts were being touched.
So what’s up with that?
In the case of the Energy Ball or Energy Tube, there are no noble gasses to excite to glowing, and individual LEDs don’t light up by themselves when brought close to the globe. So what’s causing them to get all turned on?
These devices rely on the use of some batteries controlled by a couple of transistors (FETs if you are into that kind of thing) to complete the circuits to the sound and light producing gizmos. The wave of negative charge seeking a home from the Plasma Globe desperately searching for a better path to ground causes these transistors (really just a kind of switch) to close. Okay, actually the electrons are jumping over the gap, and completing the circuit without the ‘switch’ actually needing to close at all.
The more powerful the flow is (the closer you get to the Plasma Globe), the more electrons manage to bridge the gap. So, when far away, the lights and sounds are dim and low. The closer you get, the more charge flows, and the brighter the light. In the case of the Energy Tube, the LEDs light in sequence as you get closer to the globe because of the way the LEDs are wired into the circuit—that and the fact that red LEDs are the more efficient and require the least amount of electricity to light, followed by the green and then the blue.
So there you have it—all kinds of high energy physics in there. Most people don’t have a clue as to how the Plasma Globe does what it does, they just think it’s pretty and fun to play with… And of course it is!
I want to thank my good friend Dr. Rebecca Thompson from the American Physical Society for helping me through the more complicated bits of physics, and the folks at Aviation Week & Space Technology magazine for just happening to have an article that also helped me out in their August 1, 2016 issue.