On our last Atomic physics lecture, we experimented with gas discharge tubes, and the dark-light zones. During the experiment, bright, white flashes of light appeared on the cathode of the tube. The number of them increased with the anode-cathode voltage, and didn't seem to be related to the pressure inside the tube. I asked the professor and the TA presenting the experiment about them, but at the end we could not come up with a statisfying explanation.

The phenomena is observable on the following video I took after the lecture:
Flashes on the anode of a gas discharge tube (YouTube)

A frame from the video (though the video gives a much better insight):

Flashes on the anode

The flash rate seems to be consistently 3-4 flash per frame at 29.7fps, giving a rate of approx. 104 flashes per second.

The tube was an approximately 1m long, 5cm diameter cylinder filled with air, connected to a vacuum pump and an adjustable valve to dynamically sustain an adjustable degree of vacuum. The anode and the cathode are both approx. 1cm wide aluminium cylinders with a small hole in the middle. Their sides are exposed to the environment outside of the tube. The anode-cathode voltage is adjustable between 1-6kV, but the exact voltage was not noted.

What is the explanation of these flashes?


  • They are caused by the dust in the air, turned into plasma (source: the professor)

  • They are sparks on the anode's surface (source: TA)

  • They are caused by the electrons when leaving the surface (my idea)

  • It is some kind of a scintillation effect (my idea)

NOTE: a previous version of this question wrongly stated that the flashes are happening on the anode. Older answers and comments are reflecting this.

  • $\begingroup$ What's the material of the anode or screen? In my experience, Maybe the material of screen has a scintillation effect. $\endgroup$
    – Feynman
    Mar 22, 2017 at 2:44
  • $\begingroup$ @Feynman as I said, both the anode and the cathode are made from aluminium. There is nothing else I know about them. The device is a self-built one, so the source is probably an ordinary aluminium block a technician lathered. So oxidation and others could occur. $\endgroup$
    – Neinstein
    Mar 22, 2017 at 22:31
  • $\begingroup$ I think that is not the anode, but the cathode instead. This judgement is based on the position of the shape of the striation of the "positive column". Just compare your video to the video and photos on en.wikipedia.org/wiki/Glow_discharge . If that is the cathode, the flashes may be caused by some positive ions hitting the cathode at high speed and maybe cause some kind of chemical reaction with aluminum. $\endgroup$
    – verdelite
    Apr 15, 2021 at 1:42
  • $\begingroup$ @verdelite Revisiting this and it's definetly the cathode. The periodic glowing being on the other side of the tube tellls it. $\endgroup$
    – Neinstein
    Apr 15, 2021 at 19:17

1 Answer 1


If you are pretty sure the flashes are on the anode and not distributed within the air/gas or in the air near to the anode it would appear to be a surface phenomena. To check this repeat experiment and look at anode from different angles, perhaps with 2 or 3 persons viewing same flashes. If convinced they are from anode surface then remove anode and examine it under magnifying glass or microscope to see if there is anything unusual about surface. Also try cleaning anode or covering it with fresh aluminium foil (may have to do this v. carefully to avoid sharp edge effects).

My personal guess would be it is a surface phenomena caused by impurities on the anode itself. Most airborne impurities are more likely to become positively charge and attracted to the cathode.

  • $\begingroup$ Thanks. The flashes happen on the surface without a doubt. (They are much more visible in real life than on the video, and they are in the plane of the anode. The system is impossible to dis- and reassemble with reasonable effort (as the glass is glued to the aluminium...), and is fully closed except the small tube connected to the vacuum pump. Impurities could be the reason as the device is a "home-made" one, and most probably built from a random CNCed aluminium block... maybe the oxidation on the surface is the key? $\endgroup$
    – Neinstein
    Mar 22, 2017 at 22:24
  • $\begingroup$ Cleaning the anode may be tricky then ! Perhaps ithe whole assembly can be fitted into an ultrasonic cleaning bath filled with appropriate cleaning fluid, probably something pretty volatile like industrial alcohol. You would probably have to run the vacuum pump for several hours to get the last few molecules of the cleaning fluid out of the tube. $\endgroup$ Mar 23, 2017 at 19:40

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