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Cathode Ray Tubes
;The first experiment was done in order to see if the charge was separate from the cathode rays. Thomson used a magnet to bend the cathode rays away from the electrometers. When he did this, he discovered that the electrometers stopped measuring electric charge. From this he deduced that the electric charge and the cathode rays must be combined together and not separate entities. Next, J.J. Thomson set out to determine if the charge carried by the cathode rays was negative or positive. To do this he would use an electric field. He put a negatively charged metal plate on one side of the cathode rays let out past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the CRT he now had a fluorescent coating that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent toward the positive plate and away from the negative plate. This showed that the charge carried by the cathode rays was negative
If your teacher said that Thomson discovered the electron disregard this message. Michael Farraday discovered the electron. Thomson discovered the charge to mass ratio