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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 0.05 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 0.05 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 _IMG79187B87D.jpg _IMG77827B47D.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:656 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 12:59]
That is one very weird shaped arc tube Mr. Green
Max   [Sat 24 Jan 2026 at 06:01]
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 21:07]
Whatever it is, it's awesome.! Love
Ria   [Sun 25 Jan 2026 at 01:44]
Yeah, I said arc tube as a default really.
Max   [Sun 25 Jan 2026 at 10:51]
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