[qppo]

The story I've heard is that the original lightbulbs filled with soot and darkened well before they burnt out. Edison guessed the soot was coming from the filament and was charged, so he placed a charged plate across from the filament to attract the soot before it could stick to the glass. It worked, but the bulb was much darker than before as the charged plate siphoned a lot of current off the filament.

That device would have three conductors, which you can see in the photos. Two for the filament, one for the "plate." Later, a fourth conductor called the "cathode" was added to be heated by the filament (rather than pulling charge directly from the filament) to make the first diodes.

There's something funny about how modern active components are designed to reduce how much heat they consume, but the first active devices required an active heating element to function at all.

[Johnythree]

The first thermonic diodes (or triodes) didn't need an indirectly heated Cathode.

They operated just fine with just a bare (or coated) filament.