Jerry Avins wrote:

Rob Judd wrote:

...

Open-air triode?? Do tell, you have me fascinated...

Rob

I wrote of it before, so I'll be brief and hope I'm not boring. An
open-coil hot plate with a ceramic holder to support and contain the
nichrome element as filament*. Topped with galvanized window screening,
the zinc removed with muriatic acid so as not to noxiously sublime,
resting on the ceramic as (literally!) grid. Small glass beads -- around
1/16" -- scattered on it to create a gap and hold the screening flat.
Atop that, a pie tin weighted with cooling water to serve as
(literally!) plate. With 700 volts on the plate, I got measurable
current on my 1000 ohm/volt meter. At -90 volts on the grid, the current
was too small to read. The gain was enough to sustain oscillation.

Très bizarre!

Rob