From 1909 to 1913, Robert Andrews Millikan (1868-1953) performed a brilliant set of experiements in which he measured the elementary charge of the electron (e) and demonstrated the quatized nature of the electrical charge. The apparatus he used is shown in the figure 1 on the left. The apparatus contains two parallel metal plates with a hole in the upper plate through which the oil-droplets passes. A horizontal light beam is used to illuminate the oil droplets so the droplets then appear as shinning stars against a dark background when viewed through the telescope and the rate of fall of individual drops can be then determined. Assume a single drop has a mass m and carries a charge q and its negative. When this drop passes through the plates if the electric field between the plates is turned off then the only forces acting on the drop are the force of gravity and the drag force as shown in the figure 2 below . When the two forces are balanced, the drop reaches terminal velocity therefore D=mg. Now suppose we turn on the electric field between the plates by a battery connected so the upper plate is positively charged. In this case, there is a third force in addition to the first two I mentioned earlier, the electric force qE. Because drop is negatively charged, and the electric field (E) is acting downwards as it goes from positive to negative, the electric force on the drop will be upwards as shown in the figure 3 on the left. If the electric force is great enough, then the drop starts to move upwards and therefore the drag force acts downward. When the drop reaches terminal velocity, the electric force (acting upward) will balance the sum of the force of gravity and the drag force (acting downwards). After making measurements on thousands of droplets, Millikan found that, to withing about 1% precision, every drop had a charge equal to some positive or negative integer multiple of the elementary charge e (where e= 1.60x10^-19 C). It was later established that positive integer multiples of e would arise when an oil droplet has lost one or more electrons. Likewise, negative integer multiple of e would arise when a drop had gained one or more electrons. Gains or losses in integral number provide conclusive evidence that charge is quantized. In 1923 he was awarded Nobel Prize for this work.
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