As has been the case with many developments in science, the discovery of the electron and the recognition of its important role in the structure of matter evolved over a period of almost 100 years. As early as 1838, English physicist Michael Faraday found that when a charge of several thousand volts was applied between metal electrodes in an evacuated glass tube, an electric current flowed between the electrodes. It was found that this current was made up of negatively charged particles by observing their deflection in an electric field. Credit for the discovery of the electron is usually given to the English physicist J. J. Thomson. He was able to make quantitative measurements of the deflection of these particles in electric and magnetic fields and measure e/m, the ratio of their charge to mass.
Later, similar measurements were made on the negatively charged particles emitted by different cathode materials and the same value of e/m was obtained. When the same value of e/m was also obtained for "electrons" emitted by hot filaments (called thermionic emission) and for photoelectrons emitted when light hits certain surfaces, it became clear that these were all the same type of particle, and the fundamental nature of the electron began to emerge. From these and other measurements it soon became known that the charge on the electron was roughly 1.6 × 10-19 coulombs. But the definitive experiment, which indicated that the charge on the electron was the fundamental unit of charge in nature, was carried out by Robert A. Millikan at the University of Chicago between 1907 and 1913. A schematic diagram of this famous "oil drop" experiment is shown in Figure 1. Charged oil drops, produced by an atomizer, were sprayed into the electric field maintained between two parallel metal plates. By measuring the terminal velocity of individual drops as they fell under gravity and again as they rose under an applied electric field, Millikan was able to measure the charge on the drops. He measured the charge on thousands of drops and was able to follow some drops for long periods of time and to observe changes in the charge on these drops produced by ionizing x rays. He observed many drops with only a single electronic charge and never observed a charge that was not an integral multiple of this fundamental unit. Millikan's original measurements gave a value of 1.591 × 10-19 coulombs. These results do not prove that nonintegral charges do not exist, but because many other different experiments later confirmed Millikan's result, he is generally credited with discovering the fundamental nature of the charge on the electron, a discovery for which he received the Nobel Prize in physics in 1923.
Robert L. Stearns