Electron beam spot

Discussion in 'Supporting Physics' started by hadley, Nov 29, 2019.

  1. Okay, intelligent physics folk, can you help me understand this phenomenon? The green spot on the phosphor does not appear unless the beam is swept from the glass upwards. When swept downwards, there is no spot. There appears to be some sort of hysteresis effect, but I cannot explain it at all.

    For information, the anode voltage is around 50V. When it is 200V, this effect is not observed (the spot is always visible on the phosphor). I'm not currently able to (easily) test voltages between 50V and 200V. You can vaguely make out a diffraction pattern on the dark spot when sweeping down.
    Last edited: Nov 29, 2019
  2. Nick Mitchener

    Nick Mitchener COMMITTEE

    ok, I've now been reported to an assistant head for trying to access social media on a school computer. How about posting it on youtube?
  3. Oh really? The video should just be embedded in the post.I'll get HOD to send and i'll upload to youtube
  4. Well the beam itself is quite visible, so the only thing that I can think of is that at 50V the beam only just reaches the glass and will need a certain amount of energy to make the glass fluoresce. Quite how this translates into a directional thing I'm not sure, but I guess it's to do with the change in path length between anode and screen
  5. are you using a EHT supply, how is it wired?

    Run at around 3-4kV , i think you need more than 200V.
    Last edited: Nov 29, 2019
  6. This is its current set up, using multiple ranger units to get the 50v and an HT

    Attached Files:

  7. :eek::eek::eek:
    What kind of EHT do you have? :D
  8. an old one, Philip Harris 6kV one
    Last edited: Nov 29, 2019
  9. I think Paul meant 3 to 4 kV! If I remember correctly, this tube requires 3 to 5kV accelerating voltage, 6V ac heater voltage and 0 to 50 V bias voltage plus 0 to 12 V DC for the coils. To be honest, I've never used the bias voltage as my tube works fine without it, I think it's there to sharpen up the beam if required, so that may have something to do with it. Ignore my last post about the length of the beam as that is obviously controlled by the 0 to 5 kV EHT supply not the 50V bias.
  10. i did :oops::oops:

    tried with our Griffin HT supply, voltage is not high enough.

    Do you have one of these ones. (presumably if you have the tubes you have an EHT supply.)

  11. I know, I was being an unhelpful pedant who is easily amused. :)

    Agreed, there must be an EHT somewhere.
  12. My excuse is that I am a Biologist doing Physics.
    chris vaudin likes this.
  13. PhysicsSimon

    PhysicsSimon COMMITTEE

    Am not sure what's going on here - what type of teltron tube is it and what are you trying to do? If it's the spot you're worried about I wouldn't be - as long as you can see the beam move it looks fine (possibly your phospor coating is a bit iffy in places). I'm not a massive fan of your daisy chained PSU's & would try it without any voltage whatsoever instead of 50 or 200V (we're the same as Chris & never bother with this).

    From my POV, the beam bends when you want, in the direction you want = success! :)
  14. Ha, fair enough, I wouldn't know where to begin with Biology. :)
  15. PhysicsSimon

    PhysicsSimon COMMITTEE

    I do!.....(points) - look, squishy things!
  16. Nick Mitchener

    Nick Mitchener COMMITTEE

    OK, still can't see the video because now I am at home the thing says the youtube account is closed.

    I can see your pic though.

    You talk about a diffraction pattern but is that a diffraction tube? It's incredibly blurred and out of focus but is it a fine beam tube?

    The diffraction tube needs the anode at the same potential as the cathode. It doesn't need helmholtz coils.

    The helmholz coils are run at 6V and with a rheostat. They are used with other tubes.

    Could you confirm the identity of the tube?

    I have the instructions for all the tubes, if you could either identify the tube or take a decent pic
  17. Nick Mitchener

    Nick Mitchener COMMITTEE

    Just had a thought, your tube has grey mounts, which means it is gas filled. The diffraction tube has black mounts because it is an evacuated tube. I think you have a fine beam tube there, but whatever it is I don't think it is a diffraction tube. The fine beam has two guns, one vertical, one horizontal.

    The fine beam tube needs an HT supply, not an EHT supply. It needs an anode supply of 0-300V. A deflection supply of 0-25V. The helmholz are used to make the vertical beam form a circle, they need a variable supply of 0-6V dc (from batteries)

    Although you can turn an EHT down to 300V you probably won't have enough current, but I don't know I have never tried it. They make a special supply for this called an HT supply, it has a dual output of 0-30V, and 0-300V plus a heater supply. I use a strip of 4 cells and a rheostat as a variable resistor to vary the helmholz coils.
    Last edited: Dec 4, 2019
  18. Wow, well spotted, if it's a double beam tube rather than a diffraction tube, you definitely need 300 to 500 volts anode, not 3 to 5 kV
  19. Nick Mitchener

    Nick Mitchener COMMITTEE

    Philip Harris don't seem to make the supply we have any more. 3B do some but they are an eye watering price. I'm sure they are lovely though.

    Timstar do one very cheap but I would miss the meter we have on the philip harris one. As these supplies are quite dangerous I wouldn't want to be adding meters in the circuit If I could avoid it.


    I never understood why people were so excited about EHT supplies when they have such low currents. Our HT supply is far more lethal but nobody ever noticed that before I pointed it out. I have adapted all of our EHT and HT supplies to take shrouded leads and labelled them with additional hazard warnings and that they are only for teachers, not students.