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J. J. Thomson conducted a series of experiments with cathode ray tubes. Cathode rays are produced when a high voltage is applied across two electrodes within a low-pressure gas-filled tube, causing electrons to flow from the cathode (negative electrode) to the anode (positive electrode). These experiments were designed to study the nature of the particles that form these rays.

Thomson carried out an experiment on cathode rays by passing them through an electric field. He observed that the path of the cathode rays was deflected towards the positive plate, suggesting they were composed of negatively charged particles. This observation demonstrated that the particles making up cathode rays have an electrical charge and are negatively charged because they are attracted to the positive electric field.

J.J. Thomson also examined the behavior of cathode rays in the presence of magnetic fields. When a magnetic field was applied perpendicular to the path of the rays, Thomson observed that the rays were deflected in a direction that followed the interaction between a negatively charged particle and a magnetic field, again suggesting that cathode ray particles possess a negative charge.

Thomson was able to measure the charge-to-mass ratio of the particles in the cathode rays by adjusting both the electric and magnetic fields so that they cancelled each other out and the cathode rays followed a straight path. He found that the charge-to-mass ratio was constant regardless of the type of gas inside the tube or the material of the electrodes. This result provided evidence that the negatively charged particles in the cathode rays were of the same nature and were much lighter than the atoms of any element known at the time. In conclusion, J.J. Thomson's experiments on cathode rays led him to the discovery of negatively charged particles, which were later named electrons. The main evidence for this conclusion was the deflection of cathode rays in both electric and magnetic fields, as well as the constant charge-to-mass ratio regardless of the gas or electrode material used.