User:Daniel T Young/Notebook/Junior Lab 307L/2008/11/12

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Milken Oil Drop

SJK Incomplete Feedback Notice
Incomplete Feedback Notice
My feedback is incomplete on this page for two reasons. First, the value of the feedback to the students is low, given that the course is over. Second, I'm running out of time to finish grading!


  • First we removed the apparatus and the pieces from the box and set them on books in order to elevate the eyepiece to a place that would be comfortable for us to through throughout the experiment.
  • Using the individual legs and the bubble level we adjusted the apparatus so it was completely level.
  • We then disassembled the observation chamber so that we could measure the plate separation by measuring the plastic separation width. Plastic separator distance.=7.64mm
  • Then we put the focusing wire into the center and put our light in.
  • Now we need a powerful potential difference that will make a noticeable effect on our charged droplets. We run a power source to our plates making sure to never exceed 500.0 V DC.
  • Then using the focusing wire we focused the eyepiece and calibrated the halogen light.
  • Now we may remove the focusing wire and we are now ready to begin our experiment.


  • Voltage Source: 150V - 500V TEL-Atomic Supply
  • Multimeter: True RMS WaveTek Meterman
  • Millikan Oil Drop Apparatus: Pasco Scientific AP-8210
  • Stopwatch: Digital Stop Watch (Radio Shack)


We have a plate charging switch which can...

  • give the top plate a negative charge
  • give the top plate a positive charge
  • ground both plates.

Also there is a Ionization source lever which has the functions ...

  • off which blocks alpha particles from reaching the drop chamber
  • on which allows alpha particles to pass
  • Spray Droplet position which allows air to pass through the chamber.

Plate Voltage=499.5±.1V Thermal Resistance = 2.025±.001 MΩ

Add the Oil

  • We added the oil to the apparatus and now the light reflects off the oil so that we can see them in the eyepiece as the slowly fall due to gravity.
  • By creating a potential difference we can move the charged oil drops up and down.
  • Now we are going to take measurements with a stop watch to measure the time it takes the oil to travel in various Scenarios including...
  • Gravity only
  • Top plate positive Potential difference and Gravity
  • Top plate negative Potential difference and Gravity.


Drop Velocity

  • All rise and fall times were taken of the distance between major reticle lines which is .5mm

No potential Difference

Droplet Droplet 1 Droplet 2 Droplet 3 Droplet 4
time 1 11.25 34.95 13.08 18.25
time 2 11.12 30.56 18.08 20.68
time 3 13.17 30.43 16.25 21.95
time 4 17.56 33.39 17.30 20.83
time 5 16.06 44.02 17.92 24.93
time 6 17.2 47.35 23.4
time 7 25.1
time 8 24.65
time 9 22.81
time 10 28.25

Rise Velocity

Droplet Droplet 1 Droplet 2 Droplet 3 Droplet 4
time 1 9.46 3.96 12.28 3.37
time 2 9.55 2.56 11.50 3.27
time 3 4.95 4.95 12.71 2.83
time 4 13.28 4.36 11.77 2.71
time 5 13.18 2.22 2.28
time 6 13.14 2.3 2.52
time 7 2.05
time 8 2.12
time 9 2.00
time 10 2.92


We had multiple problems over the course of the two days that we collected data for this experiment preventing us from collecting any data up until the very last hour. When we finally got to where we were getting data, we had little time to think about our technique, we were just endeavoring to get something we could perform some analysis on.

Day one:

On the first day we set everything up as Daniel outlined in the above sections on setup, we took a voltage reading and then set our multimeter to measure resistance (which we could convert to temperature using a table). We reviewed the procedure for how to introduce oil into the viewing chamber (at this point we were not using the droplet hole cover, it seemed to me that it would entirely block oil from entering the chamber) and proceeded. Our first attempts seemed relatively successful, after a couple squirts with the atomizer a cloud of golden sparkles became visible to the person looking in to the viewing chamber and the other person moved the ionization lever from the spay droplet to the off position. We then messed around with the plate charging switch turning it from grounded to top or bottom plate charged to see if and how our droplets moved. Once we were feeling comfortable with this we attempted to take some data. I was the one that was viewing the drops at the time, and while it seemed easy to watch a drop move up and down, it soon became apparent that it was alot harder to focus in on one drop and track its course over time. I would find a drop that seemed to fall under the force of gravity and rise when exposed to an electric field, but once I started trying to measure these velocities I found my chosen drop had virtually ceased to fall under gravity, or when I introduced an electric field it would fail to rise. After a few futile attempts to take some data Dan and I decided to try to introduce some more oil and get a fresh set of drops. We squirted in some more oil but did not get many more drops, and none that seemed promising so we decided to clean the apparatus and start over.

We cleaned everything up, put it back together, and since I was feeling bad, we decided that Daniel would focus the apparatus and be the one to collect the actual data. He focused the lens's using the focusing wire, and we proceeded, but we couldn't get any drops to come into view. Later we realized that we should have known something was off right from the beginning, because when Dan looked at the focusing wire it appeared at a slant in the viewing frame. At the time he didn't think anything of it, and I didn't check the focus because our eye sight is different and we were setting up so that Dan could do the viewing without his glasses. I am writing this somewhat after the fact, so I don't remember all the different things we tried, but the end of the story is we could not manage to see any oil drops after multiple attempts, and eventually called in assistance in the form of Dr Koch. Dr Koch noted the problem that had slipped by Daniel, that when the focusing wire was in place, it appeared to be slanted. The wire its self was not bent so the problem had to be in some way connected to the focus, the lenses, ....? Something was distorting our view. We tinkered with everything and eventually discovered a large drop of water or oil on the clear plastic that allows for a view from the lens into the viewing chamber. We hypothesized that the curvature of the drop was effecting our line of sight because after it was wiped away visibility returned to expected. Now we encountered a new problem, we still could not see any drops in the viewing chamber. Eventually we found that our atomizer was not really spraying, in the mean time we discovered that the purpose of the droplet hole cover is to reduce air currents in the viewing chamber, and it does not block the oil drops from getting into the chamber. Dr Koch also claimed he found a way to focus the light so that the viewing chamber appeared brighter. While he may have done so, I could not get the chamber noticeably brighter on day two, even though both myself and Dan spent quite a bit more time playing around with the light on the second day. To conclude day one, we did eventually find our drops again, (once we got the atomizer to really spray) just in time to wrap things up for the day.

As seems to have become a pattern for us, we got next nothing done at least in the planed direction. We did, once again find ourselves very familiar with the equipment, and armed with many ideas about what not to do, and what to check if things went wrong on day two. What I learned is that nothing we are doing here is a mystery, if something is not working there is a reason, and most likely its a simple one since none of the equipment is very complicated. The best tool here is persistence and curiosity, and the willingness not to give up. SJK 02:24, 18 December 2008 (EST)
02:24, 18 December 2008 (EST)
This kind of stuff makes me very glad we use two lab periods for experiments. This is how research really goes: it takes time to figure out what the hell you're doing. No amount of reading ahead of time can really eliminate this, in my opinion, so it's just going to take time. Sometimes, you can even discover something really interesting and unexpected during these fumbling around periods. Or at least, you learn a lot, even if you don't get to the point of acquiring data as soon as you'd like!

Day two:

Of course, the persistence and good attitude are hard to keep when things continue to go wrong. We started out day two by cleaning the equipment, and going over what we hoped to accomplish. The plan was to get the viewing chamber in the best possible light and focus and get down some data on a couple of drops, and then go on to that last of the data collection having to do with changing the charge on a drop. Keeping in mind the pit falls from the previous lab, we got set up, got a few drops in the chamber and I tried to take some data, but immediately ran into the same problems I outlined in day one. Thinking we could do better as far a creating an environment in the chamber where we had a few more drops to select from, we broke everything down again. This time when we set back up we, once again had lost our drops. We had oil coming out of the atomizer this time and we tracked our problem down (we thought) to a clog in the hole leading into the viewing chamber. We cleaned out the passage, but still no drops, or maybe there were. I saw a large blob kind of hit the lens at one point, but no specks of light (what the drops look like). Cleaning again, and then Dan hypothesized that our focus was off, he thought the drops were there, but we had messed around with the focus too much and now couldn't see them. Dan tried everything, would have a moment of success, but then we would add more oil and nothing. Finally Dan went for Dr Koch again, while I decided to try my hand again. I added some oil, and there were the drops, it wasn't great, but at this point we needed some data. Dan came back and we went to work. He started, and followed a drop as long as he could, but our drops were drifting left (even though the apparatus was level, according to its leveling bubble) and also tended to cease to respond to the electric field over time. After about 6 trials (up and down) Dan lost his drop and it was my turn.

Both of us when selecting a drop, made sure that it did fall under the force of gravity and rise upwards in an induced electric field, but a drop that seemed to be a likely candidate at the outset soon lost its appealing properties. I made about 6 trials on my drop before I lost it. It was drifting off screen, and was barely falling in gravity. Our time measurements, made by stopwatch by which ever one of us was not actually watching the drop seemed approximate at best. Dan did another short trial, but was less successful at keeping his drop on his second try. I did the final secession and I really looked for a drop that fell in gravity (I thought I found one, and it was not as slow as my previous one, but it was still pretty slow), but it also had a very rapid upward velocity that made taking data even less accurate and worried me given what the manual said about the amount of charge on a drop vs its rise time. The speed of this drops rise time indicated that it was not the best candidate, still at this point we were not in the position to be picky. I managed 10 trials out of this drop, which was the greatest amount of data that we were able to collect. I made one change before taking this last data set, I put the droplet hole cover back on, we had taken it off when we were having trouble getting drops into the chamber. Replacing the droplet hole cover had the effect I had hoped for, it seemed to eliminate the leftward drift we had been experiencing, so in this one small area I was able to slightly improve our technique.

Day two ended with us trying to get our data into the computer, we had been taking is by hand to reduce the amount of time so that the person doing the viewing would not have to stay plastered to the apparatus for longer than absolutely necessary. As Daniel entered data I took notes on the equipment, and checked the temperature, and voltage again. I wrote this information down and put it in my folder to enter it later, helped Dan with the last bit of data and we ran out the door a little after 5. Luckily we got all the data into the computer, because I misplaced my folder, it may still be in the lab, I guess I will find out Wed, but for now I will have to do the analysis using the temp and voltage data taken on day one. This is far from ideal, the analysis on this lab is really going to be more an exercise in practicing data analysis techniques since with conditions from a week prior (and pressure from a week later, I didn't realize we need pressure in the analysis until yesterday, and I have yet to be able to find Albuquerque's pressure for Nov 19 on the web) our data is fundamentally flawed, even if by chance our calculated value is close to the accepted one.


Using the data we took over the course of this experiment we should now be able to calculate the charge on the drops we observed, ideally we should be able to show that the charge is some integral multiple of some smallest charge, if we can show this it will be a good indication that electricity is atomic in nature. We might be able to do this with the data we collected, but because we did not change the charge on any of the drops we were observing we will not be able to actually prove that electricity is atomic in nature.

Calculating the charge on a drop involves multiple parts:

  • for each drop we will need to calculate its radius (a) (use eqn # 7, page 2 in manual)

for this calculation we will need...

  • the average fall time, vf, (cm/s)
  • n, the viscosity of air (dyne s/cm^2) (our value (day1) for resistance was2.025MΩ which corresponds to 24.5°C which gives aprox 1.8450*10^ -5 Ns/m^2 which will need to be converted, one N = 10^5 dyne)
  • the constant b = 6.17*10^-4 cm Hg
  • (found pressure for Albuquerque on Nov 19, using the weather underground) 30.45 in or 1031.0hPa (will need to be converted)

p = 77.343cm Hg

  • solve for q, (use eqn # 10, page 2)

this equation has three separate terms;

the fist term only need to be calculated once, it uses

  • d (separation of plates in the condenser in cm) 7.64 mm
  • g (gravity in cm/s^2)
  • ρ density of oil = 886kg/m^3, convert to g/cm^3 (manual gives density of Squibb mineral oil and our brand is the brand that used to be Squibb)
  • n (viscosity of air)

the second termmust be calculated for each drop, it uses

  • a the radius of the drop
  • b pressure constant
  • p the pressure = 77.343 cm Hg

the third term must also be calculated for each trial, it uses

  • average vf in cm/s
  • average vr in cm/s
  • V (potential difference across the plates) 499.5V (I think on day 2 our voltage was slightly lower, if I find that data I will substitute it in here)
  • Eqn 10 (page 2) will give use the charge q on our drops in e.s.u. (electrostatic units), the accepted value (in e.s.u.) for e is

e = 4.803 * 10^ -10 e.s.u.


I retrieved my folder out of lab the Wednesday that this was due and on the day we took the data presented here...

  • Voltage was 483.1 V
  • Temp was 1.843 MΩ ≈ 29.5°C ≈ 1.8680 E-5 Ns/m^2 (viscosity)

I will add a picture of my correct data to my summary, but the change in Voltage and viscosity will have no effect on the final outcome, since I still can not do qualitative data analysis given the deficiencies in our data collection.

Arianna's data analysis

I used both Excel and MatLab to manipulate my data for this lab. Excel for finding the average values and the SEM of my velocities, and MatLab for crunching the lengthy equations and plotting my values of the charge on the oil drops. Both files are attached below.