Question
Explain how the electron was historically discovered.
✓
Answer
→0.001 mm Hg pressure (a pressure equal to a column of mercury 0.001 mm high) is applied to the discharge tube. Now applying a sufficient electric field between the two electrodes produces an electric discharge in the tube.
→A fluorescent glow is produced on the glass opposite to cathode.
→The colour of glow of the lamp depends on the type of glass, for example a yellowish-green colour appears on soda glass.
→The reason of this fluorescence was attributed to the radiation which comes from the cathode, called cathode rays. It was discovered by William Crookes.
→He suggested that cathode rays are made up of fast moving negatively charged particles.
• J. J. Thomson's work
→J. J. Thomson calculated the particle speed and specific charge (charge to mass ratio) $\frac{e}{m}$ of these cathode ray by applying mutually perpendicular electric and magnetic fields to the discharge tube.
→According to this calculation the speed of these particles is 0.1 to 0.2 times the speed of light in vacuum and the value of $\frac{e}{m}$ is $1.76 \times 10^{11} \frac{ C }{ kg }$.
→The value of $\frac{e}{m}$ does not depend on the metal/ material used as the cathode or on the gas contained in the discharge tube.
→When some metals irradiated by ultraviolet light, they emitted negatively charged particles having less speeds. In addition when some metals heated to a high temperature also produce similar particles.
→The particles produced in each of these cases have the same value of $\frac{e}{m}\left(1.76 \times 10^{11} \frac{ C }{ kg }\right)$.
→Thus, J. J. Thomson stated that the particles produced in each of these events are fundamental (elementry) particles, which he named electrons.
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