Nobody's mentioned thyratrons (gas-filled triodes/tetrodes) either. I suspect the thyratrons used in ring counters, etc, were always inert gas (and not mercury vapour) ones, does anyone know for sure? -tony I know we used mercury vapor thyratrons in the Army (Signal Corps) on some of the triggering circuits on early radars. One of the NCOs made a beautiful chrome chassis and mounted 8 of them on it. Then used them to drive the spark plugs on an old straight 8 Buick. Was gorgeous to watch running at night. I still regret not getting some photographs of it. (This was in 1960 at Fort Monmouth, New Jersey.) Billy