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Can a cathode ray tube be made from a light bulb?

  1. May 18, 2017 #1
    Light bulbs and cathode ray tubes are structurally similar in some respects. For example, both contain a filament -- in the light bulb, the filament heats up to produce light, while in a cathode ray tube, the filament emits electrons, which are then steered into a target (in a CRT TV, the phosphor). Furthermore, both the tube and the bulb are often filled with an inert gas.

    I'm wondering if it's possible to make a cheap cathode ray tube from a light bulb. Presumably, there are some electrons emitted from the tungsten filament via thermionic emission. Can those electrons be focused into a beam by external circuitry?
     
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  3. May 18, 2017 #2

    sophiecentaur

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    I was searching around for a reference and I found this link, which describes the use of a twin filament light bulb to examine thermionic emission of tungsten. So you can see that you're part of the way to what you want. However, in addition to electrons being emitted from a hot filament, you need a way of focussing them into a beam and to give them a conducting fluorescent screen to form a spot. So one out of three - close but no cigar, I'm afraid. :frown:
    If you want a pretty display with a light bulb (conventional, of course) you can put it in a microwave oven (for a very few seconds) and you will see bright streaks of the ionised low pressure gas in the bulb. This is like the Plasma Bulbs that you can buy (safer and a lot cheaper than a microwave oven to replace!!!)
     
  4. May 18, 2017 #3
    Awesome. Thanks for the link!

    In their version of the experiment, they used a two filament bulb, burned out one of the filaments, and then sent electrons "boiled off" of the other filament across the remaining gap.

    I'm wondering if the same effect could be achieved using a single filament bulb, if an electrode were placed on top of the bulb. But maybe that's too big of a gap?
     
  5. May 18, 2017 #4
  6. May 18, 2017 #5

    berkeman

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  7. May 18, 2017 #6
    Yeah, I just figured if he needs a cheap 'scope there are easier ways to get there from here than trying to macgyver a lightbulb into one...
     
  8. May 19, 2017 #7

    sophiecentaur

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    The positive electrode and everything else would have to be Inside. That's not a trivial consideration.
     
  9. May 19, 2017 #8

    tech99

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    I have tried making cathode ray tubes as in the attached link with some results, although I did not perfect the fluorescent screen. I think I tried a bit of white copier paper, as it is fluorescent. You need a vacuum pump, but the pressure is not as low as for "proper" tubes - these are soft tubes. I found my school had the necessary 5kV power supply and pump.
    http://www.sparkbangbuzz.com/crt/crt6.htm
     
  10. May 19, 2017 #9

    berkeman

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  11. May 19, 2017 #10
    I wasn't trying to build a cheap scope. I know how to make one from an old TV and I've seen instructions like the ones you linked to before.

    I was interested in building a cheap cathode ray tube because I'm a physics geek and I thought it would be a neat experiment.
     
  12. May 19, 2017 #11
    Very cool!
     
  13. May 19, 2017 #12
    I don't understand why the positive electrode would need to be inside the tube (or bulb). At first, I thought this might be because of boundary conditions -- that maybe the electric field stops at the glass. But then I realized this was all nonsense, since E fields extend through insulators. Boundary conditions would be relevant (I think) only if the bulb were made of conductive material (that would be an odd light bulb!). The other possibility that I thought of is that the electrons might not have enough energy to penetrate the glass. But, if that's right, then if I placed a positive electrode outside the glass, wouldn't the whole thing form a capacitor? Wouldn't the glass acquire some charge? It seems like that charge should be measureable and that, if I connect an ammeter in between the positive electrode and ground, I should see a transient current while the glass becomes charged.
     
  14. May 19, 2017 #13

    sophiecentaur

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    You need the screen (anode) to be inside the glass. You would have to cut it open to achieve that and re-establish the vacuum. Not easy without a good vacuum pump. (Vacu-vin not good enough
    A surface charge would build up on the glass and eventually the Potential on the inner surface would reach the same value as that of the cathode. You could expect a small leakage current through the glass perhaps but any electron beam you could form would defocus due to the surface charge. It is essential to carry the electrons away with an electrode inside the glass.
    Tech99 (post #8) reckons that a simple vacuum pump would suffice. Indeed, I remember, at school (1961ish) using a hand operated (turn the handle) vacuum pump to reduce the pressure in a discharge tube to get a glow and to demonstrate 'Crooke's Dark space - which made us all laugh. The voltage was source was an Induction Coil.
    If you went to all that trouble, though, it would probably be better to look for a more suitable glass container. Perhaps a Demijohn, used for wine making would do and you could always drill a hole in the bottom for the anode connection. You could fashion a simple electron gun which would fit through the stopper hole. Without some focussing, you couldn't get a decent beam. It would be a bit like a 'Ship in a Bottle" exercise.
    Vacuum pumps are all over eBay for a few tens of GBP.
     
  15. May 19, 2017 #14
    By all means! Have fun! Follow your dreams. :smile:
     
  16. May 21, 2017 #15

    tech99

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    By the way, soft CRT tubes can be self focusing.
     
  17. May 21, 2017 #16
    Indeed. It's quite literally "old" news... But I am not the one working on this project and I'm not sure I understand why you replied to my reply, however I am new here on the physics forums and do not know the conventions;
    8XP4CRTSheet1.jpg
     
    Last edited by a moderator: May 22, 2017
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