Discovery of Cathode rays

                          Discovery of Cathode rays

Experiment

A gas discharge tube is fitted with two metallic electrodes acting as cathode and anode. The tube is fitted with a gas ,air or vapours of a substance at any desired pressure. The electrodes are connected to a source of high voltage. The exact voltage required depends upon the length of the tube and the pressure inside the tube. The tube is attached to a vaccum pump by means of a small side tube so that the conduction of electricity may be studied at any value of low pressure. It is observed that current does not flow through the gas at ordinary pressure even at high voltage of 5000 volts.when the pressure inside the tube is reduced and a high voltage of 5000_10000 volts is applied, then an electric discharge takes place through the gas producing a uniform glow inside the tube. When the pressure is reduced further to about 0.01 torr, the original glow disappear. Some rays are produced which create fluorescence on the glass wall opposite to the cathode. These rays are called cathode rays. The color of the glow or the fluorescence produced on the walls of the glass tube, depends upon the composition of glass. 

                          Properties of cathode rays 

 1: Cathode rays are negatively charged.

 In 1895 J,perrin showed that when the cathode rays passed between the poles of the magnet , the path of the negatively     

charged particles was curved download to point 2 by the magnetic field.
                         In 1897 , J . Thomson established their electric charge by the application of electric field , the cathode rays were deflected upward ( positive ) to point 3 . Thomson found that by carefully controlling the charge on a plate when the plate and the magnet were both around the tube , he could make cathode rays strike the tube at point 1 .

 2: Production of green florecence 

 They producrme a greenish florecence on striking the wall of the tube . These rays also produce florecence in rare earth and minerals. 

3: Travel on a straight path

 Cathode rays cast a shadow when an opaque object is placed in their path. This proves that cathode rays travel on a,straight line perpendicular to the cathode. 4: Production of X rays Cathode rays can produce X rays when they strike an anode particularly with large atomic mass. 

 5: Production of heat or temperature

 Cathode rays lroduce heat when they falk ob matter e.g when cathode rays from a concave cathode are focussed on a,platinum foil, it being to glow. 6: Cathode rays can ionize gas.

7: They can cause a chemical change, because they have a,reducing effect.

 8: Cathode rays can pass through a thin metal foil like aluminium or gold foil.

 10: The e/m value of cathode rays show that they are,simply electron.

 Conclusion

 J.J Thomson conclude from his experiment that cathode rays consist of a stream of negative charged particles. Stoney named these particles as electron. Thomson also determined the charge to mass ratio (e/m) of electron. He found that e/m value remained the same no matter which gas was used in a discharge tube. He also conclude the all the atom contained electrons.