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How did Roentgen rectify current in the first x-ray?

  1. Dec 23, 2016 #1
    Hi, I was studying Roentgen (and Crookes tubes) and realized that they used spark gap generators which sent out bursts of AC current. However, Crookes tubes work with one end being the cathode and one the anode. They did this before diodes so how did they ensure that one side was negative and one positive? Or is a Crookes tube *itself* a diode (meaning does it only work if one end is positive and the other negative)?

    Any thoughts? Thanks, Kathy
  2. jcsd
  3. Dec 23, 2016 #2
    I believe he used an induction coil, which is a step up transformer with an interrupter in the primary. So the pulse in the secondary is uni-directional. It is only if you use a secondary spark gap that high frequency AC is generated. This was only discovered quite late on by observing spark patterns.
  4. Dec 23, 2016 #3
    But I thought most inductor coils at the time had capacitors in them to make a bigger voltage spike + reduce sparking. However that would make the induced current AC. Am I totally confused ??
  5. Dec 23, 2016 #4


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    Staff: Mentor

    Can you post a link to a typical circuit that was used?

    Even if there is a capacitor on the output of the coil and switch, the cap will mainly limit the peak voltage that is reached after the switch is opened. The energy stored in the coil current gets converted to the energy stored in the cap at the peak of the voltage spike (minus any energy lost in generating the x-rays, etc.).

    http://www.schoolphysics.co.uk/age1...ic induction/text/Induction_coil/images/1.png
  6. Dec 23, 2016 #5
    I thought that the capacitor in the figure you gave (right under the interrupter) caused the coil to produce a burst of AC.

    Thanks. Kathy
  7. Dec 23, 2016 #6


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    Staff: Mentor

    I probably should have found a better diagram. Do you have one?
  8. Dec 23, 2016 #7
    Last edited by a moderator: Dec 23, 2016
  9. Dec 23, 2016 #8
    Not sure about it, Kathy, I will investigate. I thought the capacitor was proportioned so it just speed up the switch action and there was a uni-directional pulse on the secondary. Like a motor car ignition, it is DC. Anyway, I think X-ray tubes are sometimes operated on raw AC as they can obviously self-rectify if wanted, so the waveform out of the induction coil does not really matter.
  10. Dec 23, 2016 #9
    Thanks for looking into it. The more I think about it the more I am convinced that the Crookes tube only works one way.
  11. Dec 23, 2016 #10
    Or I guess you would call it self-rectify
  12. Dec 23, 2016 #11


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    Gold Member

    Old rectifier tubes were very similar to old x-ray tubes. Here's a cool collectors site.
    From the "Victor" cold cathode valve description:

    Edit: BTW A couple pics of my Victor x-ray tube; on and off.
    Last edited: Dec 23, 2016
  13. Dec 24, 2016 #12
    Thank you so much!! And your Victor x-ray machine is amazing
  14. Dec 24, 2016 #13
    Later on the Coolidge tube had a hot cathode, so it would have self rectified.
  15. Dec 24, 2016 #14
    Thanks again! I had heard about the hot cathode but I am writing a book about the history of electrical experiments and then got so stuck on this point. I feel much better now. Plus, it helps to introduce vacuum diodes. I really appreciate the help :)
  16. Dec 25, 2016 #15
    By using a
    With the hot cathode, the beam current can be adjusted independently of the HT voltage, so that the X-Ray wavelength and tube dissipation can be separately controlled. Might I also mention that a magnetron is a type of diode.
  17. Dec 26, 2016 #16
    you certainly can
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