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B J.J. Thomson's Coaxial Cylinders

  1. May 29, 2017 #1
    As part of an ongoing project I have been working on, I have been reading through J.J. Thomson's Cathode Ray Experiment. As part of his setup Thomson writes, "The arrangement used was as follows: — Two coaxial cylinders (fig. 1) with slits in them are placed in a bulb connected with the discharge–tube; the cathode rays from the cathode A pass into the bulb through a slit in a metal plug fitted into the neck of the tube; tins plug is connected with the anode and is put to earth. The cathode rays thus do not fall upon the cylinders unless they are deflected by a magnet. The outer cylinder is connected with the earth, the inner with the electrometer."

    Could someone please explain to me the setup of this experiment, especially the part about the coaxial cylinders and how they relate to the image that he drew (I attach that with the post).

    Thank You,

    Attached Files:

  2. jcsd
  3. May 30, 2017 #2
    I am ashamed to say I had not come across this one before. The tube can demonstrate the bending of the path of electrons by a magnetic field, which would probably be applied by a pair of Helmholtz coils outside the tube.
    It looks as if the cathode is A and is at Earth potential. The anode is B and is at a high positive potential relative to Earth and to A, so that electrons are drawn off the cathode at right angles to its surface. When the electrons enter the big flask, they will continue straight unless deflected somehow. The electron detector is the inner cylinder, which is connected to a (gold leaf?) electrometer. This probe is surrounded by an earthed cylindrical shield whose job is presumably to prevent the electrons being repelled by the charge accumulating on the electrometer probe. The magnetic field will cause the electrons to follow a circular path, and when increased to a critical value, will cause electrons to enter the cylinders and charge up the electroscope.
    There are a number of tubes available for these sort of experiments, particularly the Perrin Tube, as you will see from the following link:
  4. May 30, 2017 #3
    Follow Up Question: Can you help me visualize the cylinders a bit better because in the original drawing they don't look like cylinders at all.
  5. May 30, 2017 #4
    Now I look more closely, you are right. It looks as if the cylinders he mentions are the little ones with the slits, and they are connected via metal strips. These strips will still provide shielding.
    I am also incorrect about the earthing of the electrodes. Thomson seems to have the cathode at a high negative potential and the anode and the plug between the two flasks are earthed. Also the electrometer shield is earthed.
  6. May 30, 2017 #5
    Okay, thanks for all your help so far!

    Let me just say it back just to make sure, I'm getting all this. The Red circle is the inner cylinder with the electroscope from a top down view. The Blue circle is the outer cylinder which acts as the earthen shield. The black lines are the slits made into the cylinders. If, that's the case, what are the two yellow strips as well as the green box? Are the yellow strips part of the outer cylinder?

    Thank you for your continual help.

    Attached Files:

  7. May 30, 2017 #6
    All correct. The yellow strips are connection strips. These still provide shielding. I think the green box is a cork and does nothing.
  8. May 31, 2017 #7
    Thanks for all your help.

    How should I credit you for your contributions?
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