My chemistry teacher recently explained the Millikan oil drop experiment to us. However, he rushes his lectures to the point where it is almost impossible to ask him questions. What I was wondering in regards to this experiment is why is the positive plate used in the experiment? I understand that the negative plate at the bottom is used to repel the negative electrons that were bound to the oil drops. However, my teacher's description of the experiment did not define the purpose of the positive plate above where the oil drops were suspended. Can someone please explain to me the purpose of the positive plate and its impact on the test results? His explanation was confusing, because it seemed like the intent of the experiment was to quantize the negative charge, and it would seem like the repulsion from the negative plate would accomplish the task and require less calculation than the addition of the positive plate. I would just like to gain a better understanding of all the details of the experiment, and I haven't been able to find a good website that explained it either.