The Discovery of Electrons

The credit of the discovery of electrons goes to the British physicist J.J. Thomson in 1897. He used a glass tube called the discharge tube for his experiment. The tube is fitted with two metal electrodes and a vacuum pump.The two electrodes are connected to a high voltage source. The tube is evacuated. On increasing a high voltage across the tube, a beam of bluish light emerges from the negative electrode, the cathode. These rays travel to anode in straight lines. These rays were called cathode rays. These rays were deflected to the positive plate in electric and magnetic fields. From this Thomson concluded that the cathode rays were made up of very small negatively charged particles, which he named electrons. Whatever the nature of electrodes or gas may be, electrons are always produced. It proves that electrons are the fundamental particles of all atoms.

Properties of Cathode Rays:

• They produce shadows if an opaque object comes in their way. It proves that they travel in straight lines.
• They can revolve a light paddle wheel. It shows that they are material particles.
• On striking the walls of the tube, they glow or produce fluorescence.
• They deflect towards positive pole in the electric and magnetic fields. It shows that they are negatively charged.
• They can produce mechanical pressure. It shows that they have kinetic energy.
• They raise the temperature of the body on which they fall.
• Their charge to mass (e/m) ration is constant. Its value is fixed which is the same for all electrons.
• Cathode rays are emitted irrespective of the nature of electrodes or the gas present in the discharge tube. It shows they are the essential constituents of all matter.