Discovering Electrons


In 1897, an English scientist named J.J. Thomson made an important discovery regarding cathode tubes. Cathode tubes, an old scientific curiosity, consisted of a glass vacuum tube with a very small amount of gas in it, and electrodes on each end. When an electric current is applied to the electrodes, electricity flows through the gas, producing light. The color of light depends on the type of gas used in the vacuum tube. Incidentally, two common applications of cathode tubes today are television screens and neon and fluorescent lighting. In addition to giving off light, cathode tubes gave off cathode rays. Cathode tubes intrigued Thomson, and these rays hadn’t been studied much, so Thomson set about to discover exactly what they were.
Thomson devised clever experiments in which he tried to manipulate the rays. He discovered that the cathode rays were deflected by both magnetic fields and electric fields. Thomson realized that this would only work if the cathode rays were negatively charged particles (now called electrons). Then he set up an experiment where the cathode rays were deflected one way by a magnetic field, but also deflected in the opposite direction by an electric field. By controlling the strengths of the two fields, Thomson was able to guide the cathode rays in a straight line. Through this, he was able to calculate ratio of the electron’s electric charge to its mass. He also found out that he got the same results no matter what metal he used for the cathode, so he concluded that electrons were part of the atoms in the metal, and that all atoms contained identical electrons.

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