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The value of the charge on the electron has been determined in different ways, notably by the American physicist R A Millikan (1912), who used the following very direct method.
Two metal plates, separated by a distance of about 1 mm., were charged positively and negatively, respectively, by attaching them to the poles of a battery. Into the air above the plates a fine dust of pulverised oil was blown by a spray. The oil drops, which settled very slowly on account of their small size, were electrically charged. A particular drop was focussed in the field of a microscope with a scale in the eyepiece, as shown diagrammatically in.
 | Fig: Millikans determination of the electronic charge |
By varying the potential difference between the plates, the charged drop could be made to move upwards or downwards with any desired velocity, or kept suspended. From the ratio of the velocities with and without the potential difference, the charge on the drop could be calculated.
It was found that this charge was not constant, but varied during an experiment. The important thing, however, was that these variations were not continuous, but took place in jumps. Each sudden change was assumed to correspond with the gain or loss of one or more electrons by the drop, and it was found that the charge varied in small multiples of 1.59 x 10-19 coulombs. Thus, the charge on the electron is 1.59 x 10-19 coulombs.
The value of the charge can be determined in other ways. The value determined by Rutherford and Geiger by counting the α-particles emitted from radium, was 1.55 x 10-19 cmb.
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