Work of Sir J. J. Thomson
Between 1894 and 1900 a series of experiments was carried out by Sir J. J. Thomson in the Cavendish Laboratory at Cambridge to determine the mass and charge of the electrons as well as the speed with which they travelled between the electrodes. He
found that the electrons were all alike and always possessed the same properties whatever the nature of the gas in the tube or the material of the electrodes.
He first of all designed a special tube in which cathode rays were deflected by both a magnetic and an electric field. From this he was able to calculate their velocity (about 29000 kmjs) and also the ratio of the charge to the mass (e/me2) for an electron. In a later experiment he obtained a rough value for the electronic charge (e). From the the results of these two experiments he was able to calculate the mass (me) of an electron and found it to be approximately 20100 of the mass of a hydrogen atom. Subsequently a more accurate determination of the electronic charge (e) was made by the American physicist Robert Millikan.