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Andrews Rapid Here’s one to think about. I wanted to take a quick picture of this cold cathode Crookes tube operating and to check the vacuum condition. This tube is likely to date somewhere between 1915 -1930. Cold cathode tubes of this type have imperfect vacuum and the gas pressure, or vacuum hardness, is controlled by passing a current through a small reservoir made with mica covered electrodes attached at the top of the tube. I’ve done something wrong here, any ideas? I’ve shown an image of it off and operating to help. Keywords: Miscellaneous

Andrews Rapid


Here’s one to think about. I wanted to take a quick picture of this cold cathode Crookes tube operating and to check the vacuum condition. This tube is likely to date somewhere between 1915 -1930. Cold cathode tubes of this type have imperfect vacuum and the gas pressure, or vacuum hardness, is controlled by passing a current through a small reservoir made with mica covered electrodes attached at the top of the tube. I’ve done something wrong here, any ideas? I’ve shown an image of it off and operating to help.

QL_85.jpg half_coated_Hg.jpg crookes.jpg SOX180.jpg CDO_PHIL_250W.jpg
Lamp/Fixture Information
Manufacturer:Cuthbert Andrews
Physical/Production
Factory Location:England
Application/Use:medical
File information
Filename:crookes.jpg
Album name:Tuopeek / Misc
Keywords:Miscellaneous
Filesize:694 KiB
Date added:10 Feb 2026
Dimensions:2500 x 2844 pixels
Displayed:88 times
DateTime Original:2026:02:09 21:20:49
Exposure Time:1 sec
FNumber:f/4
File Source:Digital Still Camera
Flash:No Flash
Focal length:26 mm
ISO:1600
Model:NIKON D3400
Software:Adobe Photoshop Elements 2.0
White Balance:0
URL:https://trad-lighting.net/gallery/displayimage.php?pid=1108
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Comment 1 to 6 of 6
Page: 1

Sammi   [Wed 11 Feb 2026 at 01:08]
Ermm... Over to you Max.! Mind Blown
Max   [Thu 12 Feb 2026 at 07:20]
Interesting picture and peculiar luminescence pattern! About your question: isn't the left electrode supposed to be operated as a cathode only? Here we can see the (faint) electron projection from the central (tilted) electrode towards the glass bulb, which is certainly not supposed to happen under normal conditions. At what voltage did you run it?
Tuopeek   [Thu 12 Feb 2026 at 11:17]
Well spotted Max, I think your pretty much on the money there. The left electrode IS supposed to be the cathode only. In my haste, I have connected the tube in reverse bias. In the 'off' picture you can see the heavy insulated wire, which is positive, connected to the wrong side. I was wanting to grab a very quick picture and didn't take note of the voltage as its from a variable HV supply. Just turned it up and remotely grabbed the picture when emission started. Even with the gas regulator operated for a short time this tube will not start to conduct until close to 30kV so I am guessing it will be just above 30kV here.
Sammi   [Fri 13 Feb 2026 at 18:14]
I knew that we could count on Max for coming up with the answer.! Very Happy
Max   [Tue 24 Feb 2026 at 06:47]
Your mishap certainly resulted in a spectacular effect. I'm really curious as to why the glass has an "uranium glow" in the top right corner. The whole sphere should be of the same glass material, so this greenish glow looks like something was deposited onto the glass when the four right feedthroughs were made. Or is this the result of an intense and strongly divergent electron "beam" emitted at the back of the central electrode? That's very peculiar indeed. In any case I hope you protected yourself from X-ray emissions.
Tuopeek   [Tue 24 Feb 2026 at 21:01]
Very observant Max, I hadn't actually spotted the Uranium glass content there. I checked the tube out under UV and indeed the whole sphere is uranium glass. Being quite thin I hadn't noticed this under normal lighting. There are only two small areas that don't seem to have a uranium content and these are the short cylinders behind the anodes (although one is also glowing green here). All glass-metal seals are uranium loaded. It would appear the glow is the result of electron bombardment here. I wouldn't think the UV content from the remaining gas would create this effect.

Comment 1 to 6 of 6
Page: 1