The mass of an electron was first measured by the British scientist J.J. Thomson in the late 19th century. In 1897, while conducting experiments with cathode rays, Thomson made a groundbreaking discovery that led to the identification of the electron as a subatomic particle.
Thomson’s experiments involved passing electrical currents through gases at low pressures. He observed that cathode rays were deflected by magnetic and electric fields, indicating that they carried a negative charge. This led him to propose that cathode rays were composed of particles much smaller than atoms, which he named ‘corpuscles’; we now know them as electrons.
Although Thomson initially did not determine the exact mass of the electron, his work laid the foundation for further research. Later, in 1909, Robert Millikan conducted his famous oil drop experiment, which provided the first precise measurement of the electron’s charge. Combined with Thomson’s findings, this eventually led to a determination of the electron’s mass, which is approximately 9.11 x 10-31 kg.