A fine mist of oil was sprayed from an atomizer. Most of the droplets get negatively charged as they picked up some small, unknown number of electrons as they passed through its nozzle. Some of the drops then fell through a hole in the top plate and drifted into the region between the two parallel plates between which a variable electric field had been established.

 

 

Lit from the side by an intense light, these drops glistened when the region was viewed through a telescope. Once a drop was located, the voltage was varied controlling the electric field, E, to slow down the drop's descent. When the drop reached terminal velocity (mg = qE) it was tracked through the remainder of its fall and a ratio of mass per unit charge was recorded.

 

 

Once the mass of the drop could be determined then the drop's electric charge could be calculated from the recorded electric field strengths, (q = mg/E). By timing the drop's motion, its terminal velocity was calculated (s = vt). Using equations from a detailed theory of air resistance, Millikan was able to determine each drop's radius. From the radius, he was able to calculate each drop's volume. Using the density of oil and the volume, he was able to determine the mass of each drop.

 

Once the mass of each drop was determined, Millikan, with the help of his graduate student H. Fletcher, showed that the charges of the droplets always carried a whole number multiple of a basic charge, q_e = 1.592 x 10^-19 C. Today, the accepted value for the fundamental unit of charge is e = 1.602 x 10^-19 C.