The plasma globe was invented by Bill Parker who was playing with neon and argon as an undergraduate at MIT. Later as an artist at the Exploratorium in San Francisco, he made a larger one for display. See more in C&E News for Oct 27, 2008. The site is down this weekend, so I can't put in a link. Of course, the original invention of the glowing plasma was by the great Nicoli Tesla in 1894. It seems Tesla never gets proper credit for anything.
Electrodes ionize the gases, which flow inside the globe and form a current. It is important to have AC current so the gases & current actually flows inside the ball. The current flows from the center electrode to the glass, and then through whatever path to ground. (Some of the current flows out into the globe, and then back to the center electrode when the field reverses.) Voltages vary from 3000 to 30000V depending on the size of the ball and plasma. The gases are between 2-10 torr, or about 1% of atmospheric pressure.
The plasma is attracted to your finger when you touch the ball, and that is the fascinating thing about them. Your finger provides a better path to ground than the surrounding air. Wikipedia has more details.
There are also some interesting non-globlular plasmas at: http://www.plasmarama.com/
The above caught my eye. It is by Morgan Crook, and is called God City.
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