Final answer:
The magnitude of the charge on an electron was determined in the Millikan oil drop experiment conducted by Robert A. Millikan in 1909.
Step-by-step explanation:
The magnitude of the charge on an electron was determined in the Millikan oil drop experiment. In 1909, American physicist Robert A. Millikan designed an experiment using electrically charged microscopic oil droplets to calculate the charge of a single electron. By balancing the gravitational force with an electric field and carefully measuring the movement of the droplets, Millikan was able to deduce the charge on individual oil drops.
While J. J. Thomson discovered the electron and its charge-to-mass ratio using the cathode ray tube, he did not measure the magnitude of its charge. The Rutherford gold foil experiment was instrumental in the discovery of the nucleus and showed that atoms have a small, dense, positively charged center, but did not determine the charge of an electron. Thomson's atomic theory of matter, which proposed the 'plum pudding' model, was a precursor to the nuclear model of the atom but also did not measure the charge of the electron.