What is inside the envelope of a triode tube?

The glass or metal envelope of a triode tube, like most standard radio vacuum tubes, contains a vacuum. Removing the air is strictly necessary because the heated filament or cathode would quickly oxidize and burn out if oxygen were present. Furthermore, a vacuum allows electrons to travel freely from the cathode to the plate without colliding with gas molecules. If gases like argon, neon, or normal air were inside, they would ionize and interfere with the controlled flow of electrons, ruining the tube's amplification properties.