Sir Joseph John Thomson, an English physicist, directed the Cavendish Laboratory at the University of Cambridge when the fundamental nature of matter was poorly understood. Prevailing scientific thought held that the atom was the smallest, indivisible unit of matter, a concept that had stood for millennia. Thomson’s innovative experiments, conducted in 1897, dramatically challenged this belief. His work demonstrated the existence of a component smaller than the atom, leading to the identification of the electron and fundamentally altering the course of physics and chemistry.
The Mystery of Cathode Rays
The scientific community was captivated by cathode rays, which were beams observed in partially evacuated glass tubes when a high voltage was applied. These devices, sometimes called Crookes tubes, produced a luminous stream originating from the negatively charged electrode, the cathode. Scientists knew the rays traveled in straight lines and carried energy, but their true nature was intensely debated.
Two competing theories sought to explain the rays: one proposed they were electromagnetic waves, similar to light, while the other suggested they were streams of charged particles. Earlier attempts to deflect the rays using an electric field had failed, which seemed to support the wave theory. Thomson recognized that resolving this debate was necessary to understand the basic composition of matter.
Experimental Design and Methodology
Thomson designed a series of three experiments using a modified cathode ray tube that allowed for precise control and measurement. In his first experiment, he connected two metal cylinders with slits to an electrometer to measure the electric charge deposited by the rays. He found that the electrometer registered a negative charge only when the rays were directed into the slits, demonstrating that the negative charge and the ray were inseparable.
Previous researchers failed to deflect the rays with an electric field due to poor vacuum quality. Residual gas inside the tube became ionized, creating a conductive path that shielded the rays. By significantly improving the vacuum, Thomson successfully performed his second experiment, showing the beam deflected toward a positively charged plate. This deflection confirmed that the cathode rays consisted of negatively charged particles.
For the third experiment, Thomson applied both an electric field and a magnetic field simultaneously, arranging them so their forces opposed each other. He adjusted the magnetic field strength until the particle beam was perfectly straight, meaning the electric and magnetic forces were precisely balanced. This setup allowed him to calculate the velocity of the particles traveling through the tube.
Measuring the Invisible: The Charge-to-Mass Ratio
The crucial step involved using the data from the balanced-field experiment to calculate a fundamental property of the particle: the ratio of its electric charge (\(e\)) to its mass (\(m\)). The balanced forces provided the particle’s velocity. Once the velocity was known, Thomson could turn off the magnetic field and measure the deflection caused only by the electric field.
The amount of deflection is proportional to the force applied by the electric field and inversely proportional to the mass of the particle. By substituting the known velocity into the resulting equations, Thomson calculated the value of \(e/m\). He repeated this process multiple times, using different gases within the tube and different metals for the electrodes.
In every trial, regardless of the source material, the calculated value for the charge-to-mass ratio was consistent. This constancy suggested that the negatively charged particles were a universal constituent of all matter. The value Thomson calculated for this ratio was approximately \(1.7 \times 10^{11}\) Coulombs per kilogram.
The Revolutionary Conclusion
Thomson’s calculation of the \(e/m\) ratio led to an extraordinary realization about the nature of these particles. He compared the ratio for the cathode ray particle to the known ratio for the lightest charged particle, the hydrogen ion, and found a massive difference. The cathode ray particle’s charge-to-mass ratio was nearly 2,000 times larger than that of the hydrogen ion.
This difference could only be explained in two ways: the particle carried an unimaginably large electric charge, or it possessed an incredibly small mass. Thomson concluded the latter, proposing that the particles, which he initially called “corpuscles,” had a mass approximately \(1/1800\)th the mass of the lightest atom, hydrogen. This meant these corpuscles were subatomic fragments existing within the atom. The discovery of a particle smaller than the atom—later renamed the electron—shattered the paradigm of the atom as the indivisible unit of matter and launched modern particle physics.