The mass and charge of an electron were determined by J.J. Thomson in the late 19th and early 20th centuries. Thomson's groundbreaking work led to the discovery of the electron itself and provided crucial insights into the nature of atomic structure.
To determine the charge-to-mass ratio of an electron, Thomson conducted experiments using cathode ray tubes. These tubes contained a partially evacuated glass chamber with two metal electrodes: the cathode (negative electrode) and the anode (positive electrode). When a high voltage was applied across the electrodes, a stream of particles called cathode rays was emitted from the cathode and traveled toward the anode.
Thomson observed that cathode rays were deflected by both electric and magnetic fields. By measuring the extent of deflection, he was able to determine the charge-to-mass ratio of the particles constituting the cathode rays. Thomson concluded that these particles were fundamental constituents of atoms, which he named "corpuscles" (later known as electrons).
Thomson's most famous experiment, known as the "plum pudding model," involved studying the deflection of cathode rays in a magnetic field. By varying the strength of the magnetic field and measuring the corresponding deflections, Thomson calculated the charge-to-mass ratio of the particles. He found that the ratio was constant, suggesting that the particles were the same regardless of the material used to generate the cathode rays.
Thomson's work on cathode rays and electrons led him to propose the "plum pudding" model of the atom, in which electrons were embedded in a positively charged cloud. This model was later refined by Ernest Rutherford's gold foil experiment, which revealed the existence of a compact, positively charged atomic nucleus.
The precise values of the electron's mass and charge were subsequently refined by other scientists using more sophisticated experimental techniques. The electron's charge was determined by Robert Millikan through his oil drop experiment in 1909, while the electron's mass was measured more accurately by Arthur Compton in 1923 using X-ray scattering experiments.
Overall, J.J. Thomson's pioneering work laid the foundation for our understanding of electrons and their fundamental properties, marking a significant milestone in the development of modern physics.