The plasma sphere


The plasma sphere is composed of a spherical glass shell containing a low pressure gas (1-10 mm mercury) and a central electrode. The base where the ball is leaned contains an oscillating circuit that feeds the central electrode with a voltage of about 10 000 Volt and a reversing polarity at about 35 000 Hertz of frequency.

After having connected the device to the electric grid, many "bright filament" form inside the sphere. Some interesting phenomena can be observed, such as: a) the filaments slowly move up; b) approaching a finger to the sphere, the filaments thicken towards it and follow the finger if it moves; c) approaching a low consumption electric bulb to the sphere, you can see its ignition.

The sphere in action
The name of this object comes from the fact that actually a plasma (so it is called by the physicists) forms in the sphere. A plasma is an high ionized gas; ions, which are normally present in all kind of gas due to natural radio-activity phenomena, are put in motion by the electric field in the sphere and, if the average distance (mean free path) between the molecules is not too short, they get enough energy to ionize and excite the other molecules through a collision. The excited molecules lose energy giving it out as electromagnetic waves, in the field of the visible too.
The filaments tend to move up because their temperature is quite high and their density is lower than the rest of the gas inside the sphere. They thicken towards the finger because it is at ground potential, that is to say 0 Volt: thus the difference of potential between central electrode and finger is much bigger than the difference between central electrode and whatsoever other point of the sphere. Therefore it comes out that in this point the electric field is more intense and as a consequence there are a larger number of ionization phenomena.
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