Plasma Sphere

The sphere is full of moving lightnings, which follow the finger. The gas in the sphere is simply air, just a little rarefied, and the dancing fibres are plasma strings, electric current which flows in the gas.

The plasma-sphere is made up by a glass spheric bubble full of rarefied gas (1 - 10 mm Hg) and of a central electrode. This circuit feeds the internal electrode with high voltage, at about 10 000 V with variable polarization of 35 000 Hz. The electric field spreads in all the directions and the plasma fibres follow it nearly so.

The plasma-sphere is similar to a neon pipe. This would shine of a blue gleam too, if it hadn't got on its walls a protective screen, which shines a white light.