Discovery of Electron - Study of Cathode Rays

The electrical nature of matter had been indicated in very early experiments on the production of frictional electricity.  This was further confirmed by the experiments on the electrolysis of substances by Davy, Stoney and Faraday in the first half of the nineteenth century. However, the discovery of electrons came as a result of the study of conduction of electricity through gases as explained :-

William Crookes, in 1879, study the conduction of electricity through gases at low pressure. For this purpose, he took a discharge tube which is a long glass tube , about 60cm long, sealed at both the ends and fitted with the two metal electrodes. It has a side tube fitted with a stop cock.
The tube is connected to a vacuum pump and the pressure inside the discharge tube is reduced to at low as 0.01 mm. Now, when a high voltage is applied between the electrodes, it is found that some invisible rays are emitted from the cathode. The presence of these rays is detected from the fact that the glass wall of the discharge tube opposite to the cathode begins to glow with a faint greenish light. Obviously, this must be due to the bombardment of the wall by some rays emitted from the cathode. These rays were cathode rays.

Properties of Cathode Rays :- From the various experiments carried out by J.J. Thomson (1897) and others, the cathode rays have been found to posses the following properties :-

1. They produce a sharp shadow of the solid object placed in their path. This shows that cathode rays travel in straight lines.

2. If a light paddle wheel mounted on the axle is placed in their path, the wheels begins to rotate. This shows that cathode rays are made up of material particles.

3. When an electric field is applied on the cathode rays , they are deflected towards the positive plate of the electric field. This shows that cathode rays carry negative charge.

4. When cathode rays strike a metal foil , the latter becomes hot. This indicates that cathode rays produce heating effect.

5. They cause ionization of the gas through which they pass.

6. They produce X-rays when they strike against the surface of hard metals like tungsten, molybdenum etc.

7. They produce green fluorescence on the glass walls of the discharge tube as well as on certain other substances such as zinc sulphide.

8. They affect the photographic plates. 

9. They possess penetrating effect. They are stopped only by metal foils of greater thickness.

Hence, the electron may be defined as :-  An electron is that fundamental particle which carries one unit negative charge and has a mass nearly equal to 1/1837 th of that of hydrogen atom.

Origin of Cathode Rays :- The cathode rays are first produce from the material of the cathode and then from the gas inside the discharge tube due to bombardment of the gas molecules by the high speed electrons emitted first from the cathode.