What Was The Method Of Measurement Of Charge By R.A. Millikan Method?

The experimental measurement of charge on one electron was made by R.A. Millikan, in 1909. The principle of experiment made by R.A. Millikan was that two metallic plates were set horizontally and parallel to each other. These were separated by a short distance. The upper plate had a small hole, at its center, and variable voltage was applied across these two plates. One plat was positively charged. These plates were set inside a container to avoid any disturbance.

The plates were set a specific distance. As a potential difference was applied across the plates an electric field was produced in the space between the plates. An atomizer was used to spray a non volatile oil from its nozzle, which had taken the form of small droplets. These were electrically charged due to friction of nozzle when these came out of nozzle of atomizer, at large speed.

The oil drops were very small in size and formed mist. Some of these drops passed through the hole, in the upper plate and entered into the region between the two parallel plates. Here these were illuminated by light placed in front of a window. If the drop was found to be suspended between the plates then was due to the fact that the electric force was equal to the weight of droplet. So by equating electric force equal to weight of the electron the charge on electron was calculated.