Most people remember being told at school that there are only three states of matter; solids, liquids, and gases. They might be surprised to hear that there is a fourth state of matter called Plasma and think that the list ends there. This isn’t true and there are several more and the number stated varies from seven to fifteen depending on who is counting! Some examples of others are; degenerate matter, quark-gluon plasma, amorphous solid and Bose-Einstein condensate!
We are digressing, let’s get back to the original question. What actually is a plasma? A plasma is an ionised cloud. This means that it is a gas cloud that has had electrons stripped from its atoms leaving behind positively charged ions.
Now let’s actually talk about plasma balls. A plasma ball has a very high alternating voltage in the centre called a Tesla coil. This generates a large electric field and increases the temperature of the gas. This strips electrons from the gas molecules held inside by the outer globe. A plasma is formed. Currents flow through the plasma as arcs of colourful light. The light is caused by excitations of atoms that then shed further electrons. The process goes on and on until it is switched off.
If you touch the outside of the dome the current actually flows harmlessly through your body into the ground. The tendrils of light shift from the coil to the outer casing as currents are always pushed from a high to a low potential.