Electromagnetic Deflection in a Cathode Ray Tube, I

Discovering how cathode rays behave in a magnetic field was a big step forward for scientists trying to understand the mysterious phenomenon.

Cathode rays were a great mystery throughout the latter half of the nineteenth century. Heinrich Geissler, Julius Plücker, William Crookes, Karl Ferdinand Braun, Johann Wilhelm Hittorf, Heinrich Hertz and J. J. Thomson are just a small sample of the many great minds who contributed to the modern understanding of cathode rays. By the dawn of the twentieth century, their work on the enigmatic rays proved that they are a stream of electrons. Reaching this understanding required many small but important experimental steps to determine, for instance, whether cathode rays travel in straight lines, carry energy or, as explored in this tutorial, are affected by magnetic fields.

Scientists used special vacuum tubes, such as the Crookes tube and the cathode ray tube, to study this phenomenon. The tube illustrated in the tutorial contains a negative electrode (Cathode) at one end and a positive electrode (Anode) at the other. A high voltage is transmitted to the cathode ray tube, inducing the cathode to emit electrons – essentially an electrical current. These electrons, or cathode rays, are passed through a small opening near the cathode and then travel in a straight line toward the anode, passing through a fluorescent screen positioned between the cathodes that allows you to see the path of the electrons.

Observe the effect of a magnetic field on cathode rays by using the Magnet Position slider to move a horseshoe magnet (its north pole facing you) so that its poles straddle the cathode ray tube. William Crookes experimented with cathode rays and magnets in a similar manner, and his observations on the deflection of the rays by magnetic fields led him to conclude that they were composed of negatively charged molecules. Years later J. J. Thomson would determine that the molecules hypothesized by Crookes were actually negatively charged subatomic particles that he called corpuscles, but which were eventually named electrons.

What happens in the tube is a consequence of the Lorentz Force, which is explained by the left hand rule. That rule describes how a charged particle (our electron) 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. (As you apply that rule, remember that the electrons in the cathode ray are travelling opposite the flow of conventional current.) Try flipping the magnet by checking the Flip Magnet box, and observe how the beam then deflects in the opposite direction.

In addition to experimenting with magnets, scientists also experimented to see what would happen if charged plates were positioned near the tube. The results showed electrostatic deflection (as opposed to the electromagnetic deflection described above). The electrons in the cathode rays would deflect toward the positively charged plates, and away from the negatively charged plates.

Many thanks to MagLab physicist William Coniglio, the science advisor for this page, for his time and expertise.

Last modified on 28 July 2020
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