Physics307L F09:People/Gonzalez/Millikan Oil Drop Summary

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Millikan Oil Drop


Robert Millikan along with his grad student Harvey Fletcher were the first to conduct this experiment, in order to determine the charge of an electron. The theory is that the electron can be measured if weighed against electric and gravity. In his time Robert Millikan came closer than anyone ever had to measuring the charge of an electron through this experiment. In time he perfected his experiment and was able to come within half a percent error on his measurements.

More on Millikan can be found here, in much greater detail. wikipedia article

Overview of procedure

This is a picture of the grid inside the eyepiece
This is a picture of the grid inside the eyepiece
  • First off the apparatus needed to be balanced and cleaned, soap and water work well for cleaning, and there is a level to help with making sure that the apparatus is flat. Since the elevation platform is missing, books were used to gain a good height for the apparatus.
  • Next was connecting all of the power and making sure it all worked correctly.
  • We Brought both the grid inside the eyepiece and the focusing wire, into focus. This is important, make sure that everything is in perfect focus now, since it will only slow you down later if it's not.
  • After the lights were turned off we sprayed the oil into the drum, one needs to be careful not to spray too much or else the chamber will be overfilled with oil making it very hard to measure anything.
  • Once the oil was sprayed we measured the fall of an oil drop, using the major grid lines as indicators for distance (1 grid line is .5mm).
  • Oil drops are measured repeatedly, and several drops should be measured as well, try taking as many measurements as you can, the only thing that limited our measurements was eye fatigue.
  • After the oil is measured simply analyze the data and show the results.
The picture shows what the oil drops will look like when seen through the eyepiece, you can also see the grid as well, its important that both are in focus.
The picture shows what the oil drops will look like when seen through the eyepiece, you can also see the grid as well, its important that both are in focus.


I wrestled with the data on this experiment for a long time, mostly because I failed to make appropriate unit conversions, my final results came to be:
q = 4.76 * 10 − 10e.s.u. for my average, and best guess.
My range using standard deviation divided by the square root of my trails was:
Range: q = 4.86 * 10 − 10e.s.u. and q = 4.66 * 10 − 9e.s.u.

My SEM was at 1.01e-11 e.s.u.

This is a lot better than my original calculations, however, these numbers were found by using the accepted value of e. the numbers are much higher when not using the accepted value, I gained this value by using the smallest measured q found and subtracted it into all other values, I then took the smallest value from that calculation and divided it into all of my measured values for q. I made the assumption that this was my N value, I then used the relation q=n*e and divided q by its related n value, from this I got the following values:

5.28(19)*10^-10 e.s.u.
The accepted value is: 4.083*10^-10 e.s.u.

Unfortunately I had trouble getting a graph from Microsoft excel up here. The graph simply showed my results as compared to the accepted value.


  • What I learned:
  • This lab was not too hard, but unfortunately I made some very bad mistakes with my units, thus my initial calculations were very far off. I think if anything I learned to take a step back and re-evaluated the situation.
  • As far as the experiment goes, I have a deep appreciation for Millikan, he was absolutely brilliant to come up with this experiment. I was unaware of the electrostaic units, and through the experiment I was introduced to the Dyne.
  • I am far more comfortable with error propagation, and will show it far clearer in future experiments. I'm also trying not to jam my summary up with everything from my notebook, hopefully my summaries will become easier to read from now on.

Other Links

My Lab Notebook
My lab partner, John Callow He had far better success at finding e, his data analysis is amazing.

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