5H30.22-Magnetic deflection of cathode rays

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Video: Watch this demo

Instructor’s video

Description: A beam of electrons is directed through an evacuated glass tube. A current is applied to the adjacent Helmholtz coils, creating a magnetic field that deflects the electrons.


  • DC power supply
  • E/M Tube with matching high voltage power supply
  • Black cloth sheet
  • Video camera able to capture in low light
  • 2 Banana plugs

Setup Procedure:

  1. Move the E/M Tube and high voltage power supply as one unit (don’t take apart).
  2. Connect the E/M tube to the DC power supply.
  3. Test to make sure it works correctly.
  4. Cover it with a cloth and align the camera at it. Connect the camera to the AV equipment.

Demonstration Procedure:

  1. Turn on the high voltage power supply and wait a few minutes for the filament to heat up.
  2. Turn on the DC power supply and adjust to deflect the beam. The low voltage DC power supply controls the Helmholtz coil. Do not go over 2A @ 6VDC.
  3. When finished, turn off both power supplies.