On May 11, 2:46 pm, herbertglaz...@webtv.net (G=EMC^2 Glazier) wrote:

Bert, it works by what's called thermionic emission. Electrons are
'boiled' off a hot cathode into the vacuum. Edison noticed this in his
light bulbs, and it became known as the 'Edison effect'. Later a guy
named Fleming found that by putting a 'plate' inside the bulb near the
filament, and putting a positive voltage on the plate, making it the
anode, the boiled-off electrons would be accelerated across the vacuum
into the anode/plate, and current would flow. BUT it would only flow
in one direction, from cathode-to-anode, which made the device a one-
way valve. It was called the Fleming valve.
It was a diode ('di' since it contained only two elements). Diodes
serve as rectifiers, turning AC into DC.
Then a guy named De Forrest found that by putting a
third element, a grid, between the cathode and plate, the plate
current could be controlled with a small voltage on the grid. A small
change in grid voltage caused a large change in plate current, making
the device an amplifier. Since it had three elements, it was a triode.
The first was called the De Forrest Audion and was the predecessor of
all vacuum tubes. The Brits still call them valves.
So you see a hard vacuum is necessary for them to
work.