Millikan used his famous oil drop technique to determine the charge on an electron. In this experiment, an air containing chamber is injected with an oil spray consisting of microscopic droplets produced by an atomizer. The air is exposed to X-rays and ionized. This ionization of air results in electrons which get attached with the oil droplets and the droplets are charged. These charged droplets are made to fall under the effect of gravity and after the force of gravity in downward direction becomes equal to the upward viscous force, it acquires a constant velocity U1 .
Then these charged droplets are made to fall through a region between two parallel plates held at some distance. These plates act as electrodes and apply a field of strength X. This filed is applied such that it exerts a force Xe on the charged electrons in upward direction. Now, the net force on charged electrons becomes (Mg-Xe).
M is the mass of each charged droplet.
The new constant velocity becomes U2.
So, U1/U2=Mg/(Mg-Xe).
Velocities are determined with the help of traveling microscope.
When the droplets are moving under the effect of gravity only, then its velocity is given by Strokes law:-
U1=2gr2r/9h……………(1), where r is radius and r is density of the droplet, h is the coefficient of viscosity of air.
Mass of the droplet is calculated by M= (4/3)pr3r, considering it to be spherical. Its radius is given by above equation and density is same as that of oil.
Knowing the above values, the value of e can be calculated from (1).
He changed the value of X many times but the value of e remained just a multiple of 1.6022X10-19 coulomb. Thus each oil droplet had a charge equal to 1.6022X10-19. So in this way the charge on an electron was calculated.
