The plasma globe is a modern electronic sculpture that uses an electric current flowing under a high potential to ionise a chamber of gases. The resulting plasma streamers are characterised in form by the nature of the electrical signal (usually a high frequency AC signal) and in colour by the type of gas used. Plasma globes can now be bought in the high street for under 100 pounds.
The diagram below shows the basic features of a globe. The high voltage signal is generated by a fly-back transformer, similar to those used in television tubes. This produces between 5000V and 10000V at a frequency of around 20kHz. The globe is filled with inert gases - for example, neon and argon are commonly used.
The glass is sealed when the gases are at low pressure, around 1/10th atmosphere or so. This reduces the mean free path of the gas mixture (the average distance a charge carrier will travel before colliding with another carrier or atom). If the mean free path is long, then the charge carriers can accelerate to greater kinetic energies before collisions, but more importantly, can do so with a lower applied electric field. Thus the effects of discharge in the gas can be seen with a far lower applied voltage than would be possible at atmospheric pressure. Ionisation of the gases occurs at the middle electrode and the discharge runs out to the glass globe, which is effectively at ground potential. The strands are numerous and have no particular preferred direction since the distance from the central electrode to any part of the glass globe is the same. When an external ground is brought near the glass, such as a person's hand, the electric field increases between the central electrode and the improved ground where the person's hand is. Discharge will then preferentially occur in this region giving an intensification of streamers in this region.
The use of high frequency makes the device safe, since the electric currents escaping to ground through the glass, and ultimately down via somebody's hand and body, will do so by travelling over the surface of the skin, not through the body. This skin effect will protect the user from electric shock (which would be small anyway given the tiny currents involved). Although plasma globes will not deliver an electric shock, they can inflict small skin burns.
If someone places their hand over the top of a plasma globe, the stands of plasma will be attracted to it, and the current will discharge to ground over the surface of the skin of the person's hand. If now another person comes along and lightly strokes the back of the first person's hand (that is on the globe) then both persons will feel small stinging at the point of contact between the two hands.
To see what is happening, turn the globe off and place a piece of tin foil, around 4cm square on the top of the globe. Switch the globe back on and hold a key tightly in your hand, bringing it slowly toward a corner of the tin foil. It should be possible to strike a small spark, which when started, should be able to be drawn out to a few mm in length. This type of spark is responsible for the small skin burns experienced above.
To emphasise the size and extent of the electric field surrounding these globes, a household fluorescent tube is a good tool. If such a tube is gripped at one end and the other end brought near a working globe, the entire tube should light. If the tube is held in its middle, then only the section between the person's hand and the globe will light, showing clearly the path of the electric current.