From Tesla with love - plasma ball science
These light sculptures are still pretty amazing even today, and unlike its mod counterpart from the groovy '60s, the lava lamp, Plasma balls aren't likely to burn you if you just bump up against it.
The little dancing rays of light are caused by electricity moving from a high-voltage center to the insulated glass outer wall through a mixture of non-reactive gases.
(These gases are known on the Periodic Table as the Noble Gases, because their completed electron shells prevent them from bonding with other elements in most situations. In the words of one chemistry professor, remembered from way back, "They don't lower themselves to get into scuffles with other elements.")
Most commonly, a mixture of Neon, Argon and Xenon is used. The other gases are either too expensive, or radioactive, in the case of Radon. As the electricity moves through these gases, trying to find a way to discharge to the ground, it ionizes the gases, converting them into little tendrils of matter known as plasma. As the energy is expended, the plasma glows.
Some electricity actually leaves the glass ball in the form of radio-frequency energy, and can do some pretty neat tricks, such as causing an led light bulb to glow just by placing it against the glass (and possibly burn out, so DO NOT do this with anything you want to keep). Since the human body is slightly electrically charged -- the same force that allows you to use a touch screen on a tablet or smart phone, but keeps you from using it while wearing gloves -- when you put a finger, or your whole hand, on the glass, the electricity seeks to ground against that spot rather than discharge through the air. The effect seems dramatic, and can be on a much larger scale, as demonstrated by these amazing Tesla Coil photos from Australia, but in a plasma ball it's very small. Over several minutes your hand will become slightly warm from this effect, and someone touching you while you touch the glass may receive a small shock.
Nikola Tesla (1856-1943) patented a light he later called the Inert Gas Discharge Tube on Feb. 6, 1894. He displayed his early models of them among the electrical exhibits that thrilled visitors to Chicago's World's Columbian Exposition in 1893.
The library has a fantastic video about Tesla that can shed some more light on this genius inventor who is just now starting to come into greater recognition for his work, or check out Empires of light : Edison, Tesla, Westinghouse, and the race to electrify the world by Jill Jonnes.