Earlier discovered cathode rays turn out to be electrons. As a high potential difference is applied between cathode and anode, this field ionises gas atoms, creating ions with cause large number of electrons to be knocked off cathode's surface which in turn accelerate electrons to anode and collide with gas atoms, creating visible light.
There were 2 methods that could determine the specific charge of an electron - the Cross Fields Method by balancing an electric and magnetic field and the Fine Beam Tube Method by equating the centripetal force to the magnetic force on the electron.
Millikan determined the calculated how much the large of an electron is using his oil drop experiment. When no electric field is applied to the oil drop, he resolves the weight and the viscous drag using Stoke's Law when the oil drop is at terminal speed. When electric field is turned on and the oil drop remains in equilibrium, Millikan resolved the weight and the electric force by the field.