Final answer:
Thomson's cathode-ray experiment provided evidence that the rays consisted of particles because they were deflected by magnetic and electric fields, suggesting they carried a negative charge. His calculations of the mass-to-charge ratio of these particles, which we now know are electrons, led to the discovery that electrons are fundamental, subatomic constituents of all atoms.
Step-by-step explanation:
In Thomson's cathode-ray experiment, the evidence that led him to propose that the ray consisted of particles came from the fact that cathode rays were deflected by magnetic and electric fields. Because magnetic fields were known to deflect electric currents, which are made of moving charges, this behavior suggested that cathode rays also consisted of particles with an electric charge. Thomson observed that the rays were repelled by a negatively charged plate and attracted to a positively charged plate, a characteristic of negatively charged particles.
Moreover, by measuring the extent of deflection of cathode rays under different strengths of magnetic and electric fields, Thomson calculated the mass-to-charge ratio of these particles with the conclusion that they had a net negative charge, due to their repulsion from the negative terminal. This experimentation led him to identify these particles as fundamental, subatomic constituents of all atoms, later named electrons.
He concluded that since these particles were the same, regardless of the cathode material or the nature of the gas in the tube, they must be fundamental components inherent to all atoms. Thomson's experiments collectively supported the notion that the cathode ray consisted of negatively charged particles, a groundbreaking discovery that altered the understanding of atomic structure.