Measurement of Electron Charge
Question: How the Charge of an Electron is Measured?
In 1909, Millikan determined the charge on an electron by oil drop method.
(i) Instrumentatio. Millikan constructed a box which consisted of two chambers.
- The upper chamber was filled with air whose pressure was adjusted by a vacuum pump. There were installed two electrodes A and A1.
- The electrodes were attached with electricity to generate an electric field in the space between the electrodes. The upper electrode had a hole in it.
- A fine spray of oil droplets was created by an atomizer. Few droplets entered the hole. The droplet fell under the force of gravity. Then the hole was closed.
- An arc lamp was used to illuminate the space between the electrodes
(ii) Working -. –
The velocity (v1) of the droplet was determined depending upon its weight.
V1 α mg ————(i)
Where, rn is mass of particle and g is acceleration due to gravity.
After that the air between the electrodes was ionized by X-rays. The droplet under observation took up an electron and got charged. Then A and A1 were connected to a battery which generated an electric field having a strength H. The droplet moved upwards against gravitational force with a velocity v2.
i.e. V2 α Ee – mg ————-(ii)
Where E is strength of electric field and e is charge on electron.
Dividing Eq (i) by (ii)
If v1, v2, g and E are known, mass of an electron can be determined by varying the electric field in such a way that the droplet is suspended in the chamber. Hence “e” can be calculated which is 1.6022 x 1023 Coulombs. Millikan further confirmed the charge of an electron.