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

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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
 
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  • #2
tech99
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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
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.
 
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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 ??
 
  • #4
berkeman
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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 ??
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
1.png
 
  • #5
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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
 
  • #6
berkeman
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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
I probably should have found a better diagram. Do you have one?
 
  • #8
tech99
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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
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.
 
  • #9
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Thanks for looking into it. The more I think about it the more I am convinced that the Crookes tube only works one way.
 
  • #10
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Or I guess you would call it self-rectify
 
  • #11
dlgoff
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Thanks for looking into it. The more I think about it the more I am convinced that the Crookes tube only works one way.
Old rectifier tubes were very similar to old x-ray tubes. Here's a cool collectors site.
From the "Victor" cold cathode valve description:
Because of the important difference in the surface size of the electrodes, the current passes much more easily in one direction than in the opposite one.
... use of two valves, connected in series in opposite directions, between the induction coil and the x-ray tube as shown below (right) was highly recommended.
image011.jpg


Edit: BTW A couple pics of my Victor x-ray tube; on and off.
x-ray-off-jpg.98256.jpg

x-ray-on-jpg.98257.jpg
 
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Thank you so much!! And your Victor x-ray machine is amazing
 
  • #13
tech99
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Thank you so much!! And your Victor x-ray machine is amazing
Later on the Coolidge tube had a hot cathode, so it would have self rectified.
 
  • #14
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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 :)
 
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tech99
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By using a
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 :)
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.
 
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  • #16
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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.
you certainly can
 

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