Principle and Working:
Compact apparatus for demonstrating the discrete nature (quantisation) of electric charge and for determining the elementary charge of an electron.
With an atomiser, spray oil drops in a capacitor made of two light alloy plates. As they pass through two small holes drilled in the plastic holder of the capacitor, some drops will be charged by electrostatic friction. Once they have penetrated between the two flat capacitor plates, these oil drops are observed through a sighting device.
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The latter is equipped with a long focus lens to single out the oil drops falling along the capacitor axis and standing out as shiny dots against a dark background. Its eyepiece is engraved to a tenth of a millimetre. A camera supplied with a dedicated software, directly placed behind the sighting device, allows real-time display of the phenomenon on the computer.
Oil drop electric charge is determined by calculating its fall velocity. We observe that, according to oil drop size and velocity, this is always a multiple of the electric charge e, the elementary electron charge.