Lab demonstration of the operation of a triode valve.
It has a very open wire grid, and anode is a plate that fluoresces apple green under electron impact.
As the grid is made more negative wrt the cathode, the area of fluorescence shrinks to striped areas between the grid wires.
The valve can demonstrate also the three triode parameters (amplification factor, transconductance and anode resistance) by measuring and recording anode and grid volts and anode current, as they are varied.