The Discovery of the Electron (Julius Plücker)
In 1858 Julius Plücker noted that when the residual pressure of the gas inside the cathode-ray tube is very small, the glass at one end of the tube emits light. He also found that he could change the position of the patch of glass that glowed by bringing a magnet close to the tube, as shown in the figure below. Plucker interpreted the effect of the magnetic field as evidence that whatever produced this glow is electrically charged.
The rays given off by the cathode can be deflected by a magnetic field in a direction which suggests that these cathode rays are negatively charged.
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