The Millikan's experience and the electron charge

e of the electron apart from the mass.
As a matter of fact, the charge of the electron e was measured for the first time in 1909 by R.A. Millikan. The main features of his experiment are illustrated below.

Many tiny mineral-oil droplets are sprayed between two charged metal plates, they fall down under the gravity force Mg (where M is the mass of each droplet and g the gravity acceleration). By irradiating the droplets with X-rays, some of them become negatively charged: X-rays extract electrons from air molecules and one or more of these electrons can be caught by the oil droplets. If a droplet takes a negative charge (named Q), it experiences an upward force due to the electric field between the plates. The magnitude of this force is E ·Q where E is the field strength and Q the total charge of the droplet.
It's possible to stop the falling down of the droplet by adjusting the potential difference between the plates, in such a condition the gravity force Mg is balanced by the electric force E ·Q and, since E and g are known, Q can be deduced once the mass of the droplet M was known too.

EQ=Mg ® Q=  Mg E

M can be measured by multipling the volume of each droplet V=[ 4/3]pr³ with the oil density r (pronounce rho).

M=  4 3 pr3r

The droplet radius r can be deduced from fluidodinamic considerations (details omitted).
Millikan observed that, although the charge Q on each oil droplet might change, the measured values were always an integer multiple, usually small, of 1,6 ×10^-19 Coulomb and correctly he assumed this value as the charge of a single electron:

e=-1,6 ×10-19C

By coupling this value with the charge/mass ratio deduced by Thompson, (1,76 ×10¹¹ C/Kg) the mass of the electron can be computed too:

e m =1,76 ×1011C/Kg ® m=  e 1,76 ×1011C/Kg =  1,6 ×10-19C 1,76 ×1011C/Kg =0,9 ×10-30Kg @ 10-30Kg