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The first step is to understand the cathode ray experiment conducted by J.J. Thomson. He used a cathode ray tube (CRT) which generates a beam of cathode rays. The beam is produced by an electrical discharge between two electrodes (cathode and anode) inside the CRT containing low-pressure gas. When a high voltage is applied across the electrodes, electrons are emitted from the cathode and accelerate towards the anode. On their way, they collide with gas molecules and create a visible beam of light known as cathode rays.

Thomson placed the CRT in an electric field by placing two parallel metal plates on either side of the CRT, one charged positively and the other negatively. He observed that when an electric field is applied perpendicular to the path of the cathode rays, the rays bend towards the positively charged plate. He then placed the CRT in a magnetic field by placing a magnet near the CRT. He observed that when a magnetic field is applied perpendicular to the path of the cathode rays, the rays move in a circular path. These experiments showed that the cathode rays are influenced by both electric and magnetic fields, indicating that they are charged particles.

To further understand the nature of these particles, Thomson conducted an experiment to determine their charge-to-mass ratio. He used the deflection of cathode rays under the influence of electric and magnetic fields to determine this ratio. When the perpendicular electric and magnetic fields were applied simultaneously, Thomson adjusted the strength of the magnetic field until the cathode rays returned to their original path. At this point, the forces exerted by the electric and magnetic fields on the cathode rays were balanced. Using the known strengths of the electric and magnetic fields and the amount of deflection of the cathode rays, he was able to calculate the charge-to-mass ratio of the particles.

Thomson compared the charge-to-mass ratio of the particles in the cathode rays with other charged particles known at the time, including hydrogen ions. The charge-to-mass ratio of the cathode ray particles was found to be much greater than that of hydrogen ions, indicating that they were much lighter and therefore not hydrogen ions or any other known atom. Thomson concluded that the cathode rays were composed of a new type of negatively charged particle, which he called an electron. In summary, the key evidence used by J.J. Thomson to argue that cathode rays consist of negatively charged particles were: 1. Cathode rays are influenced by electric and magnetic fields, indicating that they are charged particles. 2. The charge-to-mass ratio of cathode ray particles is significantly greater than that of hydrogen ions or any other known atoms. 3. The discovery of a new type of negatively charged particle: the electron.