Ken Sikes for their support with equipment and space, to Taylor Chapman for photography, and to Mickey McDonald, Ben Olsen, and Steve Ward for reading the manuscript. This work was performed before the author joined Facebook.

![ Spark discharge photographs and video. (a) Larger version of inset in Fig. 1. A vertical, thin metal wire acted as a breakout point for several sparks that do not appear to connect to the output terminal. (b) Larger version of inset in Fig. 3, with a larger terminal placed above the terminal shown in (a). A piece of metal foil tape acted as a breakout point for the longest discharges. (c) Still image from supplementary video. The terminal arrangement is the same as in (a).[]{data-label="figSI-banjo"}](FigSI5_McGuyer_MarxCoil){width="8.6cm"}

Supplementary Material
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Spark discharges
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Fig. 5 provides larger versions of the insets showing spark discharge in Figs. 1 and 3. These pictures and the supplemental video show that the Marx coil apparatus is able to re-breakdown previous discharge channels from past bursts, as is known to occur with Tesla transformers. This phenomenon is visible in photographs and videos because of convection during the time between bursts (about 8.3 ms here), similar to how convection produces a 