Birkeland-Eyde style plasma disc. Two water-cooled copper tube electrodes are connected to a high voltage alternating current supply (connections and power supply are not shown) to produce a spark gap. Water circulates through the tubes to keep them cool as shown by the arrows. Direct current is applied to the electromagnet.

The arc starts between the electrodes and moves outward forming a semicircle due to the Lorentz force applied by the static magnetic field. Once the arc length becomes too long, a new one forms in the center between the electrodes and the process begins again. The direction the arc moves depends on the polarity. A high voltage alternating current results in a plasma disc, but pulsed DC results in a half disc.

Based on diagrams and description from the book "The Fixation of Atmospheric Nitrogen"^[1] and a photo of the electrodes:

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1. ↑ Knox, Joseph (1914年) 《 The Fixation of Atmospheric Nitrogen》、​D. Van Nostrand Company、​pp. 45−50