How the experiment was performed ?

(1) X-rays are irradiated at the air molecules (mostly, N2 and O2) in the top part of the chamber.
     Electrons are knocked off from the air molecules.
(2) Tiny spherical oil drops are produced from the sprayer ("Atomizer").
     These oil drops pick up electrons (freed from the air molecules):
            they can pick up any numbers(integers) of electrons.
(3) Mass of the spherical oil drop is calculated from its volume (obtained from
     a measurement of diameter of the drop with a microscope)  and the known density of the oil.
(4) Several droplets falls through the hole in the upper plate which is positively charged.
      When a voltage between the two plates is increased, the negatively charged drop falls
      more slowly because it is repelled by negatively charged lower plate.
(5) At one particular voltage, the upward electrical force and downward gravitational force
     on the drop are exactly balanced to suspend the drop in the mid air.

                        qE  =  mg                        g = 9.8 m/s2
                upward     downward

(6) Knowing this voltage and the mass of the drop,
     we can calculate the electrical charge on many of the drop.

                         q = (mg)/E

(7) Then the electronic charge is the the greatest common factor
      of each charges on the oil drops. Millikan reported the value to be
                        e = 4.917 x10-10 esu (electrostatic unit)
                           = 1.64x10-19 Coulomb

                          cf.  1.60217733x10-19 Coulomb (True Value)

(8) Mass of an electron, then, can be calculated from a value of the e/m ratio
     (=1.76 x 108 C/g) which was reported by J.J. Thomson:

      m= e/(e/m) =  1.6x10-19 C / (1.76 x 108 C/g)
                        =  9.1x10-28 g


Note: 1 Coulomb = 2.998x109esu