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The National MagLab is funded by the National Science Foundation and the State of Florida.

Cathode Ray Electromagnetic Deflection Basics

Discover how cathode rays behave in a magnetic field.

Experiments with cathode rays in the late 19th and early 20th centuries lead to the discovery of the electron. Reaching this understanding required many small but important experimental steps, for instance, whether cathode rays travel in straight lines, carry energy or, as explored in this tutorial, are affected by magnetic fields.


  1. Take a look at the simple set up of a cathode ray tube. Note the negative electrode (Cathode) at one end and a positive electrode (Anode) at the other.
  2. As you see, the tube is connected to a high voltage source, which induces the cathode to emit electrons – essentially an electrical current.
  3. These electrons, or cathode rays, travel in a straight line toward the anode, passing through afluorescent screenwhich allows you to see the path of the electrons.
  4. Use the slider to lower the magnet toward the cathode ray. You’ll see how the electrons are deflected by the magnetic field. The deflection is a consequence of the Lorentz Force, which is explained by the left hand rule. That rule describes how a charged particle moving in a magnetic field will be deflected by that field at a right angle to both the field and to the direction of the particle.
  5. Click the Flip Magnet button, and observe how the beam of electrons is deflected in the opposite direction when the magnetic field is reversed.