From where do the electrons making cathode rays come? How is the gas in a discharge tube ionised? Do cosmic radiations affect it?

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    $\begingroup$ Where have you read about this experiment? Does it not tell you where the electrons come from? Have you tried searching the internet for a description of the experiment and how it works? $\endgroup$ Jul 9, 2016 at 20:23

1 Answer 1


Cathode Rays

First, here's a diagram of a cathode ray tube:

cathode ray tube

Cathode rays were named as such because they were emitted from the negative electrode, or cathode, of a high voltage generator. This was done in a vacuum tube. In the diagram, you can see the cathode, from which the rays (really electrons) were emitted. You can also see a tube that went to a vacuum pump. At the other end is the anode of the power supply. So, to answer your first question,

To release electrons into the tube, they first must be detached from the atoms of the cathode. In the early cold cathode vacuum tubes, called Crookes tubes, this was done by using a high electrical potential between the anode and the cathode to ionize the residual gas in the tube; the ions were accelerated by the electric field and released electrons when they collided with the cathode.$^1$

So what little gas was left in the vacuum tube was ionized. The particles of gas were then accelerated by the electric field, hit the cathode, and knocked electrons off of the cathode.

Discharge Tube

Below is a diagram of a discharge tube:

discharge tube

The ions were just naturally present in the air of the tube. The air around us has some ions in it, weakly ionized from cosmic rays or other sources. These ions were then accelerated from the anode to the cathode, creating a glow throughout the tube.

Cosmic Rays

Finally, I'm not sure what you mean by your last question. Cosmic rays weren't really a part of the experiment.

Hope this helps!

You can read more about cathode rays and discharge tubes here.

  • $\begingroup$ Cathode ray tubes apparently can detect cosmic rays (radiation?) en.m.wikipedia.org/wiki/Electric_discharge_in_gases $\endgroup$
    – user108787
    Jul 9, 2016 at 20:38
  • $\begingroup$ @count_to_10: Why would one want to run a gas filled detector in discharge mode? That's usually highly uncalled for by the folks who are building them. Having said that, there are really cool examples of detectors that can visualize particles, I love this one (except for the noise): youtube.com/watch?v=DpW08xV3RI8. As far as I know spark chambers are externally triggered, though. $\endgroup$
    – CuriousOne
    Jul 9, 2016 at 20:43
  • $\begingroup$ @CuriousOne by coincidence I was reading Facts and Mysteries In Particle Physics" by T. Veltman yesterday, and the illustrations are a history lesson in detector technology. Did they record the sparks on film to analyse them later, or just get postgrads to sit in darkened rooms till their eyes burned out? $\endgroup$
    – user108787
    Jul 9, 2016 at 20:55
  • $\begingroup$ While I am "oldish", I am not quite old enough to have ever seen a spark chamber used for anything but as an exhibit piece. :-) I would have to do some research into old experiments to see where and how they were used. The discharge is restricted to a short amount of time (probably microseconds to milliseconds) and actively terminated. A precision analysis would certainly have required recording of some sort, but I don't know how they did it. @anna_v seems to have had direct exposure to pre-electronic detectors, while I missed that era by some time. $\endgroup$
    – CuriousOne
    Jul 9, 2016 at 21:01
  • $\begingroup$ @count_to_10 - I'm not sure about cosmic ray seeking folks, but for Geiger, Marsden, Rutherford, et al., it was generally accepted that you could only count little flashes in the dark for about 10 minutes before your eyeballs went crazy. So, they traded off, ~5 minutes to adjust to the dark, 10 minutes to count, then out of the dark area... $\endgroup$
    – Jon Custer
    Jul 9, 2016 at 23:11

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