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Glowing glass In the present case I evacuated the discharge tube to the fullest extent possible using my new (rotary vane) vacuum pump, whose lowest residual pressure is rated at 2 Pa (0.02 mbar). The resulting plasma excited at 15 kV is so tenuous that it is not even clearly visible, but its energetic electrons and photons do excite the glass vessel, which radiates a nice blue glow. Like in other tubes shown in this series, the gas pressure and composition changes gradually due to a plasma-driven cleanup process: molecules are dissociated by energetic electrons and the resulting atoms react with the tube materials. As a result, the glass fluorescence diminishes over time as there are less and less molecules present in the volume. I should be able to fix that when I'll switch to argon and helium fills later this year (or early next). Keywords: Lamps

Glowing glass


In the present case I evacuated the discharge tube to the fullest extent possible using my new (rotary vane) vacuum pump, whose lowest residual pressure is rated at 2 Pa (0.02 mbar). The resulting plasma excited at 15 kV is so tenuous that it is not even clearly visible, but its energetic electrons and photons do excite the glass vessel, which radiates a nice blue glow. Like in other tubes shown in this series, the gas pressure and composition changes gradually due to a plasma-driven cleanup process: molecules are dissociated by energetic electrons and the resulting atoms react with the tube materials. As a result, the glass fluorescence diminishes over time as there are less and less molecules present in the volume. I should be able to fix that when I'll switch to argon and helium fills later this year (or early next).
DSCF0141m.jpg DSCF0104m.jpg DSCF0109m.jpg DSC_1510m.jpg DSC_1394m.jpg
Lamp/Fixture Information
Manufacturer:Me
Lamp
Lamp Type:Crookes-type discharge tube
Filament/Radiator Type:Nonthermal discharge in extremely low pressure air (less than 0.05 mbar)
File information
Filename:DSCF0109m.jpg
Album name:Max / Misc lamps and lighting
Keywords:Lamps
Filesize:858 KiB
Date added:23 Jan 2026
Dimensions:1500 x 1000 pixels
Displayed:333 times
DateTime Original:2026:01:22 19:11:27
Exposure Time:13 sec
FNumber:f/4
File Source:Digital Still Camera
Flash:No Flash
Focal length:55 mm
ISO:250
Model:X-E4
Software:Adobe Photoshop 25.7 (Windows)
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=1096
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Comment 1 to 5 of 5
Page: 1

Ria   [Fri 23 Jan 2026 at 11:59]
That is one very weird shaped arc tube Mr. Green
Max   [Sat 24 Jan 2026 at 05:01]
I'm not sure this qualifies as an "arc" tube. "Vacuum" comes more naturally to mind in the present case, I think Wink
Sammi   [Sat 24 Jan 2026 at 20:07]
Whatever it is, it's awesome.! Love
Ria   [Sun 25 Jan 2026 at 00:44]
Yeah, I said arc tube as a default really.
Max   [Sun 25 Jan 2026 at 09:51]
This is a very nice phenomenon indeed, Sammi! However, such glow should be a cause for concern when multiple tens of kilovolts are applied across the electrodes. Electrons in such a tenuous gas can acquire enough energy to produce soft X rays (cf. Crookes X-ray tubes). Nevertheless, here the peak voltage did not exceed 21 kV and the current was extremely low, the glass fluorescence was very weak and it required a 13-second exposure to get the picture shown above. So, it's most likely that the amount of generated X-rays was extremely low and was of such low energy that it was mostly absorbed by the glass wall of the tube.

I should not take such a chance anyway, next time I'll use a Geiger counter to check this tube (better be sure than sorry). I've once seen the effect of an accidental X-ray emission from a vacuum valve (it occurred for a brief moment when the incandescent cathode was switched off while the HV was still applied) and I remember seeing the glass glowing a eerie greenish color, not the blue light see here, hence my conclusion that the glass excitation comes primarily from the tenuous plasma, but I better check that to be sure.

Comment 1 to 5 of 5
Page: 1