[...] the two wires that are a filliment (heater to
some).
As I understand it, a filament and heater are not just two different
words for the same thing (for a long time I thought they were...).
A filament is what this is: the electron emitter is the same thing that
is generating the heat through ohmic losses. A heater is a case where
the heat generator is not the same thing that is emitting electrons, as
in many vacuum tubes: the heater generates heat, which then warms up
the (separate) cathode electrode.
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