The Millikan Oil Drop Experiment measures the charge of the electron. Designed and first performed by Robert Andrews Millikan of Caltech in 1909, the experiment shows that the oil droplets are charged in integer multiples of -1.6*10^-19 Coulombs. This proved that charge was quantized, and showed the magnitude of that quantum. Millikan won the Nobel Prize in Physics in 1923 for this work.
The experiment works as follows:
Millikan put oil in an atomizer, much like a perfume bottle, and put it over a chamber with two parallel plates, top and bottom, and a hole in the top one. There was a microscope looking into the side of the chamber. First, he measured the terminal velocity of the oil drops without any charge, which allowed him to calculate the mass of the oil droplets.
Then, by applying X-rays before the droplets enter the chamber, he was able to add unknown amounts of charge to each drop. He would then vary the voltage on the two parallel plates until a given droplet hung still in the air. He knew the forces worked out thusly:
^
| (charge of the drop * magnitude of the electric field)
|
Droplet
|
| (Mass of the droplet * the acceleration of gravity on Earth, 9.8 meters/second^2)
V
Since those two forces had to balance each other out, and mass, gravity, and the electric field are known, it is a trivial matter to calculate charge. From there, one just has to calculate the Least Common Multiple, and that is the value of charge for e, the electron.
The website below has a simulator for the experiment, which is neat. Requires a Java Virtual Machine.
Sources: high school AP Physics course, and http://www68.pair.com/willisb/millikan/experiment.html for a refresher.