The mass of an electron moving at a velocity much smaller than the velocity of light is known as rest mass. It is calculated by the relation:-
M=e/ (e/m) =9.1091X10-31
where e is the charge on an electron
e/m is the ratio calculated by Millikan.
When the electron is moving at velocities comparable to the velocity of light, its mass is given by:-
m=m0/ (1-v2/c2)1/2
where v is the velocity of electron
c is the velocity of light
m0 is the rest mass of the electron.