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.

The gas emitting light in the plasma sphere is simply air, being only under lower pressure than in the atmosphere. An electrical discharge under atmospheric pressure takes the form of a spark or plasma torch. The dancing wires inside our sphere are plasma streams - the electrical current flowing though the gas medium. They are similar to atmospheric lightning during a tempest, but they develop under a lower pressure.

Moreover, this electric current (or more precisely the electric field which drives this current) propagates also outside the ball. Observe how a "neon" lamp illuminates if placed near the working plasma ball. The electromagnetic waves propagate across a paper sheet as well, but not through an aluminium foil or your hand. They do not propagate also through wet materials, like soil - you cell phone does not work inside tunnels.

The name of this toy comes from real plasma which is formed inside. Plasma is a highly ionized gas: ions (i.e. atoms without usually one electron) and free electrons inside the ball are moved by the high frequency variable electrical field, created by an external electrical circuit and emitted from the central black ball. If the pressure is low, electrons find enough free path to be accelerated and they ionize more atoms in collisions. The excited atoms (or molecules) lose their energy by electromagnetic radiation, also in the visible range - the gas emits light.

The plasma wires tend to rise, having a higher temperature than the rest of the gas and therefore a smaller density. The wires follow your finger, because it is at ground potential: the difference of the electrical potential between the centre of the lamp and your finger is higher than between the centre and any other point of the glass ball. The wires concentrate around your finger, as the electrical field there is higher than elsewhere. This leads to the more efficient ionization.

A lot of different phenomena, in a simple plasma ball!