This was an extremely important experiment because it discovered the numerical charge of an electron and set up the idea of a quantized nature of electric charge.

Millikan set up two parallel horizontal metal plates and allowed oil droplets charged by the friction in an atomizer to fall between the two plates. While a light beam (making the oil bright on a black background) is played on the oil droplets, magnification is used to watch the oil droplets fall in-between the plates.

Now here is a working generalization, while gravity still is acting on the droplet, the difference in potential between the plates (hooked up so some form of battery) holds the droplets airborne.

Example, lets say there is a negative drop of oil with mass of m and charge of q between the two uncharged plates. First the drop falls as gravity acts on it and its drag is holding it slightly airborne. When the plates are charged with a electric difference (explained later, as of now imagine that the top plate is positive, and that the oil drop is attracted to it). Then the oil drop moves upward with the force of qE and gravity and its drag holds it down. By turning the battery hooked up to the plates on and off the oil drop can be pulled upward or let fall or can be made to hang in the air. Thus allowing Millikan to judge the strength of the electric force acting on the particle.

After measuring hundreds upon thousands of droplets of oil an exact value of an electron can be determined. E was found to be 1.60 x 10^-19 C.

With a metal pail, a positive ball was lowered inside the pail, When a positive ball was dropped inside a metal pail, a positive charge was detected on the outside of the pail. Thus it was determined that the positive ball was attracting all the electrons towards the inside of the pail and the outside was left with a positive charge.