# How to deduce the charge of an electron from the charge of an oil drop in the Millikan experiment?

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?

## 2 Answers

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.

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.