DISCOVERY OF AN ELECTRON

The electron was discovered during experiments on passing electric current through gases in the discharge tube. Highly evacuated discharge tube becomes dark and a diffused spot is observed on the screen in front of the cathode. It indicated emission of invisible rays from the cathode which were named cathode rays.

In 1895, Jean Perin observed that under the influence of an electric field, the diffused spot on the screen shifted in the direction opposite to the field indicating that the cathode rays are the stream of negatively charged particles. As the charge can be possessed by mass, it was concluded that the cathode rays must possess mass. It also indicated that these charged particles existed as discrete entity. In 1874, G. Johnstone Stoney experimentally demonstrated the existence of such charged particles. In 1891, he named this particle as ‘electron’.

J. J. Thomson conducted experiments of passing charge through the discharge tubes filled with gases at low pressure. On reducing the pressure of gas in the discharge tube, Faraday dark space and Crookes dark space were observed. Hertz, who experimentally proved the existence of electromagnetic rays believed that electromagnetic rays are produced from cathode. Julius Plucker discovered that magnetic field affects the cathode rays. Thomson showed that the cathode rays in Hertz experiment were affected by electric field. This showed that the cathode rays cannot be electromagnetic rays.

J. J. Thomson experimentally determined the ratio of charge to mass of an electron and found it to be
1.8 * 10exp(11) C / kg. Earlier, the ratio of charge to mass was determined for different ions with the experiments of electrolysis. This ratio for hydrogen ion was 9.6 ´ 10exp( 7 ) C / kg. Hence, Thomson concluded that the mass of electron should be 10- 3 to 10- 4 times the mass of hydrogen ion. Thomson called these particles ( electrons ) as primordial particles.

In 1909, Millikan estimated the charge of electron with the help of a series of his famous ‘oil drop’ experiments as - 1.602 * 10exp( - 19) C. Combining the results of Thomson and Millikan, mass of the electron was known. As the atom as a whole was known to be electrically neutral, scientists concluded that it must have some particles carrying positive charge of the same amount as that of negative charge of electrons.