Pressure display

I bought this educational set-up blind from an auction as it didn’t say what the tubes were. I was hoping they were spectral lamps for various gasses. However, they have turned out they demonstrate different pressures of, I think, air. There is no information on them other than one sticky tape label with 0.01mm Hg on it. Other than this all the pressures labels have been lost. The picture shows the label on the tube with the hardest vacuum and I think I have the rest in pressure ascending order towards the left.

They are all shown illuminated here by capacitively coupling the tubes to an HF HV supply. The coupling is achieved by running an insulated wire through the eyelet electrode connections. The resulting corona discharged can be seen where the wire insulation is in contact with the electrode, both top and bottom.

Each tube presents a very different resistance and breakdown voltage to the supply. By capacitively coupling them, this way, prevents the tube with the lowest resistance pulling the supply down and preventing other tubes from conducting.

Although all tubes appear to be from the same setup, I can’t explain the brightest tube, 3rd from the left having two restricted ring indented sections or the different colour in the discharge.

All the tubes are of a good age and clipped onto a nice teak board for display.