R. A. Milikan's Oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron.
His experiment involved an atomizer, which helps to spray tiny droplets. By means of a short focal distance telescope, the droplets can be viewed. There are two plates, one positive and the other negative above and below the bottom chamber. DC supply is attached to the plates. Some of the oil drops fall through the hole in the upper plate.
Using X-rays the bottom chamber is illuminated causing the air to ionize. As the droplets traverses through the air, electrons accumulate over the droplets and negative charge is acquired. With the help of dc supply a voltage is applied. Speed of its motion can be controlled by altering the voltage applied on the plates. By adjusting the voltage applied, drop can be suspended in air. Millikan observed one drop after another, varying the voltage and noting the effect. After many repetitions he concluded that charge could assume only certain fixed values.
He repeated the experiment for many droplets and confirmed that the charges were all multiples of some fundamental value and calculated it to be 1.5924(17) ×10−19 C, within one percent of the currently accepted value of 1.602176487(40) ×10−19 C. He proposed that this was the charge of a single electron.