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Vacuum question

  1. Dec 16, 2007 #1
    This is my first post here so hello to anybody who reads this!

    I just finished calculus based physics II this semester and there are a few things I want to design and make for myself to help better my understanding of what I learned in physics. One is device I have seen that generates an electron beam and uses a magnetic field to give the electrons circular motion. The part of it that I am the least comfortable with is the vacuum. The question I have is this: What sort of pressure do I need in the vacuum so that the tube is evacuated enough to allow the electrons freedom to move?

    Also, I understand that helium gas will fluoresce when struck by the moving electrons. But how much helium per unit volume is needed? Currently I am considering the use of mercury vapor instead because it would be very easy for me to add a bit of mercury before I evacuate the tube/housing. I would appreciate any opinions on this because I intend to do a complete design before I attempt any assembly.

    Thanks in advance.
     
  2. jcsd
  3. Dec 19, 2007 #2
    Mercury vapour is a greater health risk than the Helium and alternatives...

    http://en.wikipedia.org/wiki/Gas-discharge_lamp may give you some ideas or links.

    I assume these are the Teltron ( 3BScientific now own them ) or similar demonstration tubes you are talking about, they normally use Neon (classed as residual amounts)
    http://www.3bscientific.co.uk/shop/u.k./physics/teltron,pg_83_675_0_0.html

    As for the actual pressures I do not know, but I assume the lower the better ;) and then with the gas it'll be enough to make it visible.
    Try it the traditional way... trial and error :D
     
    Last edited: Dec 19, 2007
  4. Dec 19, 2007 #3
    Why would you want an electronic device to cause the electron beam to cirulate? Unless you want them to radiate due to loss of angular momentum.
     
  5. Dec 19, 2007 #4

    russ_watters

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    Staff: Mentor

    Why build one? Couldn't you just buy a used oscilloscope or tv?
     
  6. Dec 19, 2007 #5
    Thanks for the link to the gas-lamp, bsimmo. That did give me some good ideas and I hadn't thought about it before, but different gasses give me different colors so I can go for style too! And I agree with what you said about doing things the traditional way...as far as the pressure goes, all I know is the lower the better and it's looking like I'll just have to put a guage on it and see what works. And yes! that Teltron thing is basically the same thing I am making. I'm going to look at that closer tonight and see if they give any of the specs.

    To Russ, I had thought about doing something like that (at the least getting a heater-cathode/anode set up that way). But I really want to make this all myself...or atleast as much as I can.
     
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