I think the filament stays on and you switch what cathode is being used. edit, I just read the wikipedia page on nixie tubes, there is no filament, despite loo

I think the filament stays on and you switch what cathode is being used. edit, I just read the wikipedia page on nixie tubes, there is no filament, despite looking like one it is not a thermionic vacuum tube. it is a cold cathode gas discharge lamp. so one anode with many switched cathodes.