# Millikan Oil drop experiment

by jenny777
Tags: experiment, millikan
 P: 18 For drop 1 and 3 I get 10.8/6.4=1.6875=16875/10000=27/16.
 P: 28 but that value doesn't make too much sense to me since, drop 2 has less charge than drop 3, but has 45 charges..... I have no idea why the author published such answers/methods on a textbook......
P: 18
 Quote by Vanadium 50 DarthMatter's advice is stunningly bad. If you were to follow it in a class that I was teaching, I would flunk you. 1) The point of a lab is not to see if you can find the "correct" value in a book somewhere and write it down. 2) Converting N measured values into N^2 pairs of values does not add information. It just adds complexity to the analysis.
Hi,

1) You can consider the literature to find out what the correct value should be, and discuss why you came to another conclusion. At least that is how I did it im many lab reports at my non-US university.
2) I was not converting N values into N^2 values (or just accidentally), just taking the differences of the smallest charge-multiples with the measured charges. This was similarly also considered by others. Most of these values can be thrown away since their absolute value is too big to be considered an elementary charge.
 Mentor P: 16,182 To add - most people are adopting the strategy "guess at the number of electrons on the sphere with the least charge, and see how well that agrees with the other measurements". This strategy fails if that sphere is mismeasured, even if all the others are perfect. A better way, especially if you can use Excel, is to pick a candidate elementary charge, vary it, and find the charge that gives you the minimum total deviation from integer charges of the ensemble. If you do this, the charge you get will not be the book value, but it is a lot more honest.
 P: 18 I was actually going to start another rant about how the number of usable values by my method was actually not squared and how it is pure rhetorics to claim it was, even if it seems mathematically right. But I consider your method with the total (standard?) deviation to be sophisticated and cool, so maybe you could elaborate a little more so I don't have to start to babble again.
 P: 28 I tried to find a charge that when divided into each one of the measured charges gives a result within an error bar or two-three of an integer for each and every charge... but there was none in my 10 data sets and by the way, after getting those 100 values, how should I plot it? what would my x-axis be? Thank you :)!!
 Mentor P: 16,182 Jenny, please look at what other people are saying. The "100 values" technique will not help you. More importantly, you need more data. The data you have is not good enough for you to measure anything. You don't quote uncertainties, but you probably have at best about a 5% measurement of the charge of each sphere. That means that the only data points that will help you constrain e are those from spheres with fewer than 10 electrons on them: a 5% uncertainty means you cannot distinguish a 10 electron sphere that was measured high from an 11 electron sphere that was measured low. DarthMatter, the way the method works is as follows. Assume the smallest charge is q. For each measurement q, find the smallest non-integral part of the number Q/q. If Q=11.5 and q = 2, Q/q = 5.75, so write down 0.25. (6-5.75 is 0.25). That's the minimum error on each measurement, assuming q = e. Now total all those up. For some value of q, this is at a minimum: that's the q for which the values of Q/q look most like integers, and is the best estimator of the fudamental charge. This can be made more complicated in multiple ways, but the idea is to answer the question "What value of q makes my measured values of Q/q look most like integers?"