The Millikan Oil Drop Experiment to Determine the Charge of an Electron

Photo: Rhett Allain

The charge of an electron (e) is often called the elementary electric charge. This is because everything object has some integer value of this charge since the electron has a charge of -e (e = 1.6 x 10^-19 Coulombs) and the proton is +e.

Because this quantity is so small, measuring the charge on just one electron can be quite tricky. But this is exactly what Robert Millikan and Harvey Fletcher did in 1909. It’s such a famous experiment that it’s just called the Millikan oil drop experiment. Oh, you can even do this as an undergraduate physics lab.

Let’s go over all the physics you need for this experiment.

Drops of Oil in a Capacitor

Here’s the basic idea. Suppose you take a super tiny drop of oil and put it in a parallel plate capacitor oriented with the electric field in the downward direction. Something like this.

Different sized oil drops are injected into the region between the plates with a constant electric field. In the process of launching these oil drops, they accumulate a negative charge. Since the drops have an excess electric…