Thomson’s Third Cathode Ray Experiment
Thomson then third to determine the properties of the particles. The negatively charged particles he found in his second experiment were too small for Thomson to measure the mass or electric charge directly. So, he measured the deflection of cathode rays by a magnetic field and compared this to the deflection of cathode rays by electric field. He also varied the strength of the electric field between the aluminium plates to see whether that would change the deflection of the cathode rays. He then deduced the mass and electric charge from the extent of bending of the rays.