I understand that Sir William Crookes' Maltese cross tube shows that cathode rays travel in a straight path but I am unsure of why the cathode rays aren't affected by the anode as there is an electric field and therefore bend.

Sorry if this is a silly question but I am relatively new to physic.

  • 3
    $\begingroup$ Electrons in a Crookes tube only travel on straight paths if you squint with both eyes and put a magical spell on the students in the audience that makes them believe that they do. :-) You are, of course, correct. The electrons will be deflected by the peculiarities in the electric field in the tube. In practice it's just "straight enough" to simplify matters for the audience to the point where they don't have to worry about the actual beam geometry. Once you try to engineer a commercially useful cathode ray tube, however, you will be surprised by just how non-trivial the beam dynamics is. $\endgroup$
    – CuriousOne
    Mar 9 '16 at 6:03
  • $\begingroup$ @CuriousOne Thanks!! I thought so!! However why do some of the electrons go over the Maltese cross in the Crookes Tube to produce the glow? $\endgroup$ Mar 9 '16 at 6:39

The left hand image is a schematic of the Crookes' tube arrangement.

enter image description here

The tube had gas at low pressure inside it and electrons which original from the cold cathode were accelerated towards the anode.
A lot of the electrons were travelling so fast that they were unable to turn the corner towards the anode and there went onward towards the Maltese cross along almost straight line trajectories.
Some hit the Maltese cross but a number reached the glass and caused fluorescence and the relatively sharp image of the Maltese cross.
What is interesting is that the electrons which hit the Maltese Cross leak away and do not affect other electrons coming towards the Maltese Cross. The voltages used were very high but even so the flux of electrons was not.

enter image description here

A modern tube uses an electron gun and has a screen with a phosphor on it so much lower accelerating voltages can be used.
The Maltese cross is also at the same potential as the final anode of the electron gun to try an ensure that the trajectories of the electrons are relative straight to produce a sharp shadow.

An interesting effect can be obtained by removing the high potential lead from the Maltese Cross and this is shown on the right hand image.
Because of the high electron beam currents significant numbers of electrons are stored on the Maltese cross which the repel the electron beam.
The sharp shadow of the cross being produced by visible light emitted from the electron gun.


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