The Concepts of Spark, Corona, and Breakdown
Spark breakdown and spark discharge soon became a subject of interest for fundamental physicists and engineers. One can observe a leader-free breakdown, when an extremely fast streamer overlaps a gap to transform to an arc. The leader channel arises from the stem of a corona flash, where the gas is heated to a maximum value by the total current flowing in from all streamer branches. A leader is formed against the background of a corona discharge, as the voltage is slowly raised further. In its 'pure' form, this phenomenon can be best observed at pulsed voltage. Modern laboratory equipment permits the study of structural details of a corona and a spark discharge. Some of them can be observed visually; in any case, one can see an external manifestation of these forms of discharge. A high voltage electrode can be suspended a few meters above a grounded metallic plane.