Final answer:
Cathode rays are deflected by a magnetic field because they are composed of negatively charged particles, which J. J. Thomson identified as electrons.
Step-by-step explanation:
Experiments with cathode rays being deflected by a magnetic field show that cathode rays are composed of particles that are negatively charged. This was demonstrated by J. J. Thomson's experiments, where cathode rays were deflected away from a negatively charged electrical field and toward a positively charged field, which is characteristic behavior of negatively charged particles. Thomson's calculations of the mass-to-charge ratio from the deflection extent further supported the conclusion that these particles had a net negative charge and were indeed electrons.
Thomson's experiments also highlighted the fact that the mass-to-charge ratio was independent of the metal electrodes or the gas within the cathode ray tube, suggesting that electrons are fundamental components of all atoms. His discoveries marked a significant milestone in physics and chemistry as they challenged the then current notion that atoms were the smallest components of matter.