The dawn of Quantum Mechanics– Cathode Ray Tube Experiment

Intishar Alam Misbahul
3 min readDec 11, 2020
Cathode Ray Tube

Electrons. The quantum particle that makes up electricity and is a part of atom. We use it everywhere, from our phones, to appliances, to vehicles, to rockets to satellites and many more to come.

But have you ever wondered how it was discovered? What kind of experiment did the discoverer did?

Cathode Ray Tube Experiment By J.J. Thompson

In the 19th century, physicists constructed a glass tube with wires inserted in both ends. After pumping out as much of the air as they could, an electric charge passed was across the tube from the wires would create a fluorescent glow caused by a faint ray from the cathode(the cathode ray tube also became known as an ‘electron gun’).

Experiments showed that the fluorescent glow was in the positive end of the tube. William Crookes, a coworker, discovered that by coating the positive end with a fluorescing material, a ‘dot’ could be seen when rays from the electron gun hit it.

Further research showed that the ‘cathode rays’ emitted from the cathode could not move around solid matter and so travelled in straight lines, property known to belong to waves. While other researchers, specially Crookes, debated that the behaviour of the beam meant it was a stream of particles.

Since the beam was negatively charged, some scientists were thinking of ways to separate the charges from the wave, as the beam displayed the properties of both waves and particles. Which should mean it is a mix of both.

While others kept on debating, Thompson was more interested to experiment with the beam. He had an ‘gut feeling’ that the charges were, in fact, inseparable.

He decided to pass the ray through a metal cylinder with two holes revealing the electrometers, which could measured minuscule levels of electric charges.

After applying a magnetic field across the tube, no readings were displayed nor were there any traces of the wave. Meaning the ray never reached the electrometers. So what could have happened? If the charges was separable, at least the wave should leave a bit of trace of its existence? The answer was simple. The ‘wave’ and ‘particles’ were actually inseparable and intertwined, just as he foresaw.

This led to a second experiment. Earlier experiments were not enough to back the theory that the ray can be deflected. Since both magnetic field and electric field deflects charged particles, he used a tube with near perfect vacuum and fluorescent coating in the positive end. This time, the tube had a cathode and anode outside the tube midway the path of the ray. As expected, it did deflect.

A simplified cathode ray tube diagram

The third experiment involved performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis.

He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths(similar to how mass spectrometry is done).

Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion.

He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.

This finding led to the plum-pudding model of atoms.

Electrons are on a field of positive charge(like plums on a plum pudding)

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Intishar Alam Misbahul

Currently 17. Loves to learn. Doing AS level. Using medium to write easy-to-understand essays/stories regarding science and study tips.