Today I started a new lab cycle on Charges and Fields. There are two experiments in it— Millikan’s famous oil drop experiment to measure the fundamental charge of the electron and measuring the Earth’s magnetic field. Both of them are quite simple ideas (observing how oil drops are deflected in a capacitor and watching the deflection of a current carrying wire respectively) but they are both difficult to carry out accurately and get good results. Also they are done in the dark.
I am doing the floating magic oil drop one. I have a vaporiser to squeeze in a spray of oil droplets between the capacitor plates, a power supply to adjust their motion and a microscope to squint in at the tiny tiny little floating dots. The trick of the experiment (as far as I can tell) is to quickly spot a drop that is moving up in the electric field (so guaranteeing it is charged) and then speedily timing it moving between two plates. You have to take two measurements of its speed- in a known electric field and in free fall, so the difficult bit—especially for me with no partner—is memorising which oil drop is your oil drop when you write down the times. Or if it is even an oil drop and not just a trick of your imagination that you’ve been frantically timing.
This means that there are significant experimental uncertainties in the measurement—you have to guess when it has reached terminal velocity, and often it is hard to have the droplet and the background scale thing in focus at the same time, as well as reaction times in the timing. Due to this, it is advised that we take over a hundred readings as well as about thirty for a preliminary set.
So far I have… sixteen. With all the setting up, these probably only took me an hour but ahh. Tomorrow’s lab isn’t going to be thrilling, even with my pretty spreadsheet that plots things as I go.
This isn’t helped by the fact that I am not a fan of Robert Millikan in the first place. He won a Nobel Prize for this experiment (though I may add that it was easier than our version as apparently he had a larger capacitor and a system so that a single drop rather than a cloud could be isolated.)
However, his experiment was later met with controversy as he claimed in a paper that he had included an analysis of a representative sample of drops, not a selected few, whereas his notebooks revealed that he had rejected a lot of results for no apparent reason. These rejected results do not change his overall calculation of the fundamental charge, but would have provided him with 2% uncertainty not just the 0.5% he gave which meant that his result was not compatible with the currently accepted value.
There are also rumours that he talked a graduate student out of co-authoring the Nobel Prize winning paper with him even though the student may have been the first to perform the experiment, the first to measure single droplets and who suggested oil.
Oh Mr. Millikan (if that is indeed even his name.)
Anyway, once the data has been collected, analysing the results looks like it could be interesting. Estimating an accurate error on the final result is also going to be tricky. I feel like this is going to be one of those labs where if my data ends up alright and my analysis goes well then I will look back on it as a good experience, but if it doesn’t this might be a right-up-to-the-night-before-error-analysis-lab-report-hell.
In other news the spacecraft Rosetta that has been sling-shotting around the solar system since 2004 came out its hibernation to conserve energy today in preparation to approach and send out a lander to a comet this November! Exciting times in Physics.
: )
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