For decades, researchers had observed mysterious “cathode rays” emanating from a negative electrode inside a partial vacuum tube. The scientific community was divided over whether these rays were a form of electromagnetic wave or a stream of previously unknown particles. The Cathode Ray Tube (CRT) became the apparatus for a series of elegant experiments designed to resolve this wave-versus-particle debate. This investigation was driven by the desire to understand if the atom, long considered the smallest, indivisible unit of matter, actually contained smaller constituents.
How the Cathode Ray Tube Worked
The Cathode Ray Tube is essentially a sealed glass cylinder from which most of the air has been evacuated. Inside this highly rarefied environment, two metal plates, the cathode (negative electrode) and the anode (positive electrode), are positioned. A high voltage is applied across these electrodes, causing a beam of radiation—the cathode rays—to shoot from the cathode toward the anode. These rays were known to travel in straight lines and caused the glass walls of the tube to glow, a phenomenon called fluorescence, when they struck them.
The rays were initially thought to be a form of light or “aether waves” by some scientists because they traveled straight and could pass through thin sheets of metal foil. However, the rays were also known to be associated with a negative electrical charge. The apparatus provided a controlled environment to subject this beam to external electric and magnetic forces to reveal its true composition.
Thomson’s Pivotal Observations
J.J. Thomson designed a series of experiments using a modified CRT to settle the debate by subjecting the cathode rays to controlled external forces. In one setup, he placed a pair of electrically charged metal plates, one positive and one negative, on either side of the cathode ray beam. He observed that the beam consistently bent away from the negative plate and was attracted toward the positive plate. This deflection provided conclusive evidence that the cathode rays carried a negative electrical charge.
Another manipulation involved applying a magnetic field perpendicular to the path of the beam. The magnetic field also caused the beam to deflect, confirming the rays were composed of discrete entities with mass and charge. The deflection tests were repeated using different metals for the cathode and various gases within the tube, yet the properties of the rays remained unchanged. This consistency suggested the particles were a universal component of all matter, not just a byproduct of the cathode material.
The Scientific Demonstration
The most revolutionary part of Thomson’s work was converting these deflections into a quantitative measurement. By precisely measuring the amount of deflection caused by the known strengths of the electric and magnetic fields, Thomson was able to calculate the ratio of the particle’s electrical charge (\(e\)) to its mass (\(m\)). He achieved this by balancing the electric and magnetic forces so that the cathode ray beam traveled straight, allowing him to determine the velocity of the particles. Then, using only the magnetic field, he calculated the curvature of the beam’s path to find the charge-to-mass ratio (\(e/m\)).
The calculated charge-to-mass ratio for the cathode ray particles was extraordinarily large, approximately \(1.76 \times 10^{11}\) Coulombs per kilogram. This value was thousands of times greater than the ratio for the lightest known ion, the hydrogen atom. Thomson reasoned that for the ratio to be so large, the particles must possess a minuscule mass. He concluded that the mass of these “corpuscles,” as he initially called them, was about 1/1800th the mass of a hydrogen atom, proving the existence of a subatomic particle that was a constituent of all atoms.
Impact on Atomic Structure
The discovery of a particle much smaller than the atom shattered the theory that atoms were indivisible and structureless. The experimental evidence proved that atoms were not the smallest unit of matter but contained smaller, negatively charged components.
This paradigm shift necessitated the creation of new models to describe the internal structure of the atom. Thomson proposed the “Plum Pudding” model, suggesting the atom was a sphere of uniformly distributed positive charge with the newly discovered negative particles—the electrons—embedded within it. Although this model was later replaced, the Cathode Ray Tube experiment provided the first concrete evidence of a subatomic particle, launching the field of modern atomic physics and paving the way for future discoveries about the atom.