5H30.10 Cathode Ray Tube (Crookes Tube)


Electricity, force on moving charge, right hand rule


A beam of electrons is deflected by a magnet and/or charged plates.



  • [1] Cathode ray tube (Crookes tube)
  • [1] Crookes tube power supply or [1] Heathkit IP-32 power supply with (optional) [1] Xantrex XT 60-1 power supply
  • [6] Banana cable
  • [1] Bar magnet
  • [1] Power bar with long cord
  • [1] Camera (lecture hall document camera may work)

Classroom Assembly

  1. Dim lights in order to make the electron beam more visible.
  2. Wire up the heater and accelerator contacts (and also deflector contacts, if desired) on the tube to the power supply (or supplies).
  3. If using the Heathkit supply, let it warm up by leaving it on standby for a few minutes.
  4. If using an external camera, position and focus it to see the tube end-on. Leave the camera on.
  5. Turn off automatic brightness adjustments on the camera, if applicable.

Important Notes

  • Do not exceed 300 V on the accelerating voltage from the Heathkit power supply.
  • A common error is to use the left-side knob on the Heathkit supply.


  1. Project the camera view to the class.
  2. Dim the lights.
  3. Turn on the power supply or supplies.
  4. Increase the accelerating voltage.
  5. Deflect electrons using magnet.
  6. (Optional) Deflect electrons with the deflector plates.
  7. When done, turn down all voltages.
  8. Turn off the power supply or supplies.

Additional Resources



  • Don't attempt this at home!

Last revised

  • 2020


  • The gas inside the tube is probably argon.
  • We should probably not exceed 100 V on the deflecting plates. The Xantrex power supply is only rated to 60 V, so it should be fine.
  • A more user-friendly power supply for this demo is available.
  • The supplier for the cathode ray tube (Crookes tube in the catalogue) was Nada Scientific.
  • The full-HD gooseneck camera works well with this demo.

Related AV

Related demos

  • Magnetic force on a current-carrying wire
  • CRT TV and magnets


If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.