Physics307L F07:People/Lancaster/Notebook/071119

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--Michael T. Lancaster 17:21, 19 November 2007 (CST)

Milikan Oil Drop Experiment

The background and purpose for this experiment can be found on the wiki ( or in the apparatus manual(

Here are some highlights from the procedure

The apparatus Robert Millikan’s design is just a uniform electric field, which is a pair of parallel plates that lie horizontal with large potential difference. Then the oil drops are dropped in to the plates and the drops are suspended between the plates. By changing the voltage you can make the oil drops rise and fall. A ring of insulating material is used to hold the plates together. The plates have four holes cut into it and three have a bright light shining through them, and the other has a microscope placed through it.

The oil is a type that is usually used in vacuum apparatus. This is because this type of oil has an extremely low vapour pressure. Ordinary oil would evaporate away under the heat of the light source, so the mass of the oil drop would not remain constant over the course of the experiment. Some oil drops will pick up a charge through friction with the nozzle as they are sprayed, but more can be charged by including an ionising radiation source (such as an X-ray tube).


At first it was difficult to make measurements with only myself looking through the apparatus, but I finally discovered a way to reduce the time to write down the measurements and was able to obtained consistent data.