Dear Straight Dope:
Could you possibly explain an object that I see sold at some stores? It is a clear glass sphere that appears to have lightning emanating from a shaft inside the sphere. If you touch the glass shell, the lightning (or whatever) seems to be attracted to your hand.
The name for such a device is a "plasma sphere," and in this case, at least, the name fits. What you’re seeing inside the ball isn’t lightning, but strands of plasma. When you add enough energy to a gas (usually by heating it), one or more of the electrons on each gas atom is stripped away, leaving a positively-charged ion. One of the interesting properties of plasmas is that, since the positively-charged ions and the negatively-charged electrons are free to move independently, they’re pretty good conductors of electricity. In a plasma sphere, you’ve got the glass globe, filled with a low-pressure gas, usually argon, and a pillar in the center with a metal ball on top at very high voltage. When the voltage gets high enough, it reaches what’s called the breakdown voltage of the argon, and arcs of current jump to the low-voltage glass on the outside and dissipate to the air. These arcs heat the gas, turning it into a plasma, which is suddenly a much better conductor, so current continues to flow along that same path. Because these paths of plasma are hotter than the surrounding gas, they glow, and also because of their temperature, they rise due to convection until they’re pointing straight up. Eventually the bottom parts lose contact with the center ball, interrupting the current, at which point new streamers form. The same principle is at work in the Jacob’s ladder, that prop often seen in horror movies, where an electrical arc climbs up between two vertical wires. The main difference is that since the gas involved is air (which is much harder to ionize) at relatively high pressures, it takes a lot more juice to get a Jacob’s ladder going.
When you put your hand onto a plasma sphere, your body, being a much better conductor than air, brings the voltage down even further (that is, closer to ground potential) at that point on the glass, making it a preferred point for the streamers to connect to. The sphere is producing a very high voltage, but the current is low enough that it doesn’t have any noticeable effect on your body, so don’t worry that it might be dangerous. However, it can be damaging to electronic equipment, so it’s probably not a good idea, for instance, to hold your mouse up against the glass, just to see what would happen. Trust me on this last point.
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