Cathode Rays are Beams of Electrons

The effect of a magnetic field on a cathode ray shows it is a beam of rapidly moving electrons.

Discussion

When a Tesla coil is held near the cathode of the Crookes tube and turned on, a narrow band of blue light appears on the phosphor coating of the metal strip to the left of the barrier. When cathode ray tubes were first invented, the nature of the beam or ray responsible for this effect was not known, hence the term "cathode ray," since it was clear the cathode was the source of these rays.

The Tesla coil creates a very large electric field at the cathode of the Crookes tube. This electric field is strong enough to eject electrons from the cathode. These rapidly moving electrons travel from the cathode toward the anode, but only those electrons that pass through the narrow horizontal slit in the metal barrier continue their journey past this barrier. They do so as a narrow band or ray of electrons, which strikes the phosphor coated metal strip that angles across their path. When electrons collide with the phosphor, their kinetic energy is absorbed by electrons in the phosphor, exciting them into higher energy electronic states. When these excited electrons return to their ground, or lowest energy level, they release the energy they have absorbed from the electron beam as light, resulting in the blue line of light along the metal strip that you see in this movie.

The blue glow you see at each end of the Crookes tube is emitted by gas molecules in the tube which are ionized by collisions with cathode ray electrons.

To demonstrate that the cathode ray that is produced by the Tesla coil is consistent with it being a beam of electrons, we use a horseshoe magnet to impose a magnetic field at right angles to the "cathode ray." A beam of electrically neutral photons would not be affected by a magnetic field; a beam of positively or negatively charged particles would be deflected, as is the cathode ray in this Crookes tube.

If you apply the right hand rule of electricity and magnetism to what you observe with the two orientations of the magnet in this movie, you will discover that the effect of the magnetic field on the cathode ray is expected if the cathode ray is composed of rapidly moving negatively charged particles. Other, more complex experiments prove that these negatively charged particles are electrons.