Sunday, January 23, 2011

Recreating the Oil-Drop Experiment

Creating a full-scale reproduction of Millikan's apparatus is not feasible on our budget. For now, at least, we will be using a commercially available PASCO apparatus intended for instructional use in undergraduate physics labs. We will be borrowing the PASCO apparatus from the Physics & Astronomy department, which apparently used to use it for undergraduate labs but no longer does.

The fact that we will be using a commercially available device raises at least two questions. First, why bother to recreate the experiment when thousands of undergrads have already done so? Second, won't differences between the PASCO device and Millikan's apparatus limit what we can learn from the re-production?

The answer to the first question is that we will be using the PASCO device to address questions that it is not used to address in undergraduate physics labs. We are not re-visiting Millikan's experiments because it would be fun to try to replicate his results; we are trying to gain insight into historical questions, such as questions about why Millikan handled his data the way that he did, by learning from firsthand experience what Millikan saw when he looked into the oil-drop chamber.

The answer to the second question is a bit more complicated. The PASCO apparatus is essentially a miniaturized version of Millikan's apparatus. All of the physics is there--including a source of ionization radiation--but some things will be different. For instance, one of the advantages of the larger size of Millikan's apparatus is that the metal plates across which a charge is placed more closely approximate the idealized infinite parallel-plate capacitor for a longer range of fall distances. As a result, Millikan's electric field was likely more uniform than ours will be. Another issue will be that the phenomena won't look quite the same in our experiment as they did in Millikan's. The inside walls of the oil drop chamber are made of a different material, the light source that illuminates the drops will be different, the viewing telescope will be different, and so on. In short, we will have to be cautious about extrapolating from what we see to what Millikan would have seen. Ideally, we would like to find textual support in Millikan's writings for any claims we make about what he would have seen based on what we see.

There are special video cameras that would allow us to record the experiment. We are hoping to take videos and post them online, as Paolo has done for his Galileo and Coulomb experiments (here). However, we do not yet know whether Physics & Astronomy has the right kind of camera and, if so, whether they would be willing to lend it to us.

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