Hi, Stuff! I was thinking of just a flat spiral, to minimize the possibility of a "hotspot" of concentrated electric field, as Ode alluded to. But maybe this is not significant for low voltages. Jan's idea of a conical spiral may have some advantage, as you allude to. Only experience can decide, and this is all untrodden territory as yet.
The cathode could be made into a spiral too, though it would be wasteful of good silver. Copper could serve the purpose, as Ode points out. This whole thing is starting to resemble Yin and Yang! Matthew
