I asked the firm who sells them here in Sweden, but they didn't know. I asked sellers on eBay, same answer. Is it different with say neon and hydrogen? Or is it the same pressure in all? I'm particularly interested in hydrogen and deuterium.

BTW, why is there a capillary in the middle of these tubes? Somehow this increases the luminosity of the gas, but why?

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  • $\begingroup$ Re, "why is there a capillary?" I don't know the actual answer, but if you were planning to look at the tube through a primitive spectroscope (e.g., as in a high-school physics classroom or, in a museum display case), then the width of the lines as seen in the spectroscope would be the same as the width of the glowing plasma as seen with your naked eye. The thinner you can make the glowing plasma, the better the resolution will be of the lines that you see in the spectroscope. $\endgroup$ – Solomon Slow Mar 7 at 18:15
  • $\begingroup$ At a guess, I assume the pressure is in the usual range for neon lights, 3 to 20 torr. $\endgroup$ – PM 2Ring Mar 7 at 18:41
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    $\begingroup$ I believe that the pressures are relatively low because higher pressures mean that the spectral lines become broader and that's not a good thing if the tubes are being used for spectroscopy calibration. As an added bonus, lower pressures mean that the manufacturer cost per tube goes down. Any particular reason why you want to know what the precise pressure in the tube is? As for the narrow capillary part, that's probably just to increase the luminosity by increasing the electrical current density there. $\endgroup$ – Samuel Weir Mar 7 at 19:07
  • $\begingroup$ @SamuelWeir - plus, you'd like to be on the portion of the Paschen curve that actually allows for breakdown in the tube at reasonable voltages. Actually somewhat related to spectral broadening as well through the mean free path. $\endgroup$ – Jon Custer Mar 7 at 20:51

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