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In the Millikan experiment we measure the charge of one oil drop. But how can I measure charge of one electron when I’m not sure how many electrons are contained within one oil drop?

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I think the key was that when you measure enough oil drops, you will notice that the charge is always a discrete multiple of some number. So it shows that charge is quantized. With enough data points, you'd see that the smallest charge difference possible is about $1.6\times 10^{-19}$ coulombs.

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Strong peaks on a graph are expected if charge is packaged in discrete units, but not if charge is infinitely divisible.

One constructs a histogram of the charge measured, and accumulates a few thousand data points, with very fine drops in a relatively high field. The intent was to only observe oil drops with small charge, such as would be held stationary in gravity with a high E field if the droplet was of low mass.

So, if the entire range of charges measured is 1 to 20 electrons, and the resolution of the experimental charge/mass and radius-of-droplet measurements is sufficient, a few thousand measurements in a histogram would show strong peaks. The distance between the peaks is the increment of charge corresponding to addition of one electron to a droplet.

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