Issue Date: October 27, 2008
What's that stuff? Plasma Globes
As is often the case in the annals of invention, Bill Parker dreamed up the product for which he is famous after a fortuitous mishap in the lab. An undergraduate at the time, Parker was in his physics lab at Massachusetts Institute of Technology late one night in 1971 experimenting with gaseous fuels for electrical rocket engines when he developed the concept of the plasma globe. He accidentally left a valve open, filling his test chamber with neon and argon—which he had been ionizing in his experiments to create rocket thrust—to a higher pressure than usual. This time, when he applied a voltage, visible plasma formed. "I had these colored streamers climbing all over the surface of the engine," Parker remembers. "That same night, I took a glass ball, put a little of the gases inside," hooked it up to a power supply, "and took it to my girlfriend's house for a party she was having."
It wasn't until three years later, when he was an artist-in-residence at the Exploratorium, a museum of science and art in San Francisco, that Parker put the first serious plasma globe on display. The museum "wanted me to do exhibits to help people understand electricity," he says. With its electrode-generated streams of dancing light, the glass sphere Parker had previously used for party entertainment fit the bill.
But plasma, an ionized gas often referred to as the fourth state of matter, was used to create a light fixture long before Parker's time. Nikola Tesla, prolific inventor and electrical engineer, developed and patented the "inert gas discharge tube" in the 1890s to compete with Thomas Edison's light bulb. Using high-voltage, high-frequency current in an evacuated glass bulb, Tesla generated plasma—and light.
Light streamers in plasma globes are generated by high-frequency alternating current, in which electron flow changes direction cyclically. When a high-voltage electrical current is applied to gaseous atoms, the electrons in the outer atomic orbitals become excited and jump to higher energy orbitals. And when the excited electrons relax, they emit photons.
The color of the light streamers is a property of the gas inside the globe: It is dictated by the energy spacing between the gas's electronic orbitals. Plasma ball makers rely on inert gases because such gases won't react with the globe's central electrode. Small commercial plasma globes, mainly produced in China, are usually filled with neon, which produces reddish orange light streamers. "As far as noble gases go, you get more bang for your buck" with neon, says Scott Bogard, a plasma globe hobbyist and undergraduate at Pennsylvania State University, Harrisburg. Bogard builds plasma globes in his spare time, chronicling the trials and tribulations of construction on his own website. Neon is inexpensive and produces some of the brightest light, so you can use a lower voltage power supply to generate plasma, Bogard says.
Larger professional plasma globes found in museums or classrooms typically contain some combination of gases, such as helium, xenon, and krypton, which produce a multicolored streamer display. Parker, who now runs the Bill Parker Studio & Laboratory, in Vermont, uses a minimum of three gases and as many as a dozen in his globes. But the exact mixtures are his trade secret.
In addition to preventing him from buying some of these expensive gases, Bogard's modest student budget creates other problems, among them having to rely on affordable glass globes that then sometimes require innovative ways of creating a vacuum seal and maintaining adequate pressures.
Commercial globes, Bogard says, use working pressures in the range of 2 to 10 Torr. If the gas pressure is too low, the density of the plasma won't be high enough to support streamers. But if the pressure is too high, the globe's electrode most likely will be unable to ionize the gas sufficiently to produce plasma at all.
Small novelty plasma balls need only a few thousand volts at a low safe amperage for operation. But larger, thick-walled globes used in museum displays often can take up to 30,000 V for generating quality streamers. Despite this high voltage, the spheres are safe to the touch because the glass acts as a dielectric. And the way that the globes react to human touch has mesmerized people since Parker put the first one on display; plasma streamers migrate to any fingertips placed on the glass surface.
A lot of people mistakenly think that this happens because their hands are warm, Parker says. In fact, the return path for the current flowing from the single-electrode plasma globe is through the surrounding air. "When you touch it, the electricity is looking for a ground path," Parker explains. "You are fairly conductive—your body is mostly water.
"The first people to see my original sculpture" at the Exploratorium "were a bunch of kids. They immediately started touching it and interacting with it," Parker continues. The adult staff at the museum "didn't get it," he says. "Kids get the concept instinctively."
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