The following topics can be discussed with this highly evacuated tube: 
•  Quantitative analysis of inelastic collisions between electrons and Helium atoms 
•  Determination of the ionization energy 
•  Resolution of the energy states of various primary and orbital angular-momentum quantum numbers
 

The critical potential tube features an electron gun with a directly heated tungsten cathode and a cylindrical anode in a glass bulb, evacuated and then filled with Helium. The inner wall of the bulb is covered with a conductive coating connected to the anode. The collector ring is situated inside the bulb in such a way that diverging electron beams cannot directly collide with it.
If the collector voltage is positive, then those electrons which have lost almost all their kinetic energy due to an inelastic collision with a Helium atom will be deflected towards the collector where they accumulate. The maxima of the resulting curve for the electron current (collector current against anode current) will correspond to the excitation energies of the Helium atoms.
If the collector voltage is negative, the positively charged Helium ions will be deflected towards the collector and accumlate there. The ionization energies of Helium atoms can be read from the plot of the resulting ion current curve (collector current against anode current).