This coil is 12volt or 24volt DC powered with plasma output.
A pipe from a hole in the top of the sphere and down the inside of the secondary coil is used to supply gas to form a type of plasma electrode. Using Butane gas and air, a blue flame can be used as an interesting discharge terminal. The heated CO2 emissions provide a low pressure channel to conduct the electricity more easily than air. This produces a large plasma column above the flame. At certain spark gap discharge rates the plasma column can be made to resemble a stable double helix formation. Small quantities of other gasses such as neon or helium can be mixed with the butane to produce slightly different colours and effects.
http://www.aaroncake.net/circuits/hvgen.htm
