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--Linh N Le 01:38, 11 September 2007 (EDT)

Millikan Oil Drop Experiment

Used Millikan Oil Drop Apparatus by PASCO Scientific Model AP-8210


see comment

Steven J. Koch 01:59, 2 October 2007 (EDT):Model numbers and brands of instruments would be important to record...

The apparatus came pretty much ready to go. We made sure to level the machine and calibrate it. For more details see Operations Maunal

  • Calibration dealt with inserting a thin metal "needle" into the capacitor and adjusting the light

levels and focusing the microscope. (Steven J. Koch 01:52, 13 September 2007 (EDT):To me, it seemed that the needle was more centered for Nik and Bradley, even though they didn't adjust anything, to my knowledge.)

  • A 500V power source was required to charge the capacitor, so we hooked one up and a digital multimeter

to keep thing accurate

  • We had to measure the thikness of the spacer between the capacitor plates using a micrometer. Taking

several measurements we got: (in mm) 8.11 , 8.10 , 8.09 , 8.105 , 8.095

  • Keeping track of the internal temperature was also crucial, so we attached another multimeter to a

thermistor to keep track of the resistance and temp (resistance falls as e^(-1/T))

  • We got readings of 2M(ohms) which was about 25C. As the experiment went on, the resistance dropped to about 1.96M(ohms).
  • Using Squibb's Mineral Oil (density 886kg/m^3)

The Experiment

  • The capacitor has a small hole on the top of it, allowing us in insert oil droplets. With the housing and lid over the

capacitor, spraying in the oil with an atomizer while keeping a very small hole on the bottom plate of the capacitor open, we should be able to get the air flowing out the bottom of the capacitor and the oil along with it

  • After getting some oil into the capacitor, we switch on the ionizer to get electrons into the oil droplets and then apply voltage to the plates of the capacitor.
    • We should see the oil droplets float around and we can switch the polarity of the plates to make the drops rise or fall

see comment

Steven J. Koch 16:56, 11 September 2007 (EDT):Excellent write-up! I think your ideas are all very good, and it shows the knack of good experimentalist to be able to make a plan of attack like that. I will share this notebook with tomorrow's students and hope it helps them get it working. If not, my hunch is that the apparatus will work when you two have more time on Monday. Another idea: did you try googling this experiment at all to see if there is any advice out there on how to get it working?


The instructions tell us to look for a spray of oil in the microscope as we pump the oil into the capacitor but we never saw any oil go in. Oil is entering into the capacitor, since we have found oil residue when we open it up. After attempting to insert oil into the capacitor, we went ahead and continued the experiment by ionizing the oil and then turning on the plates. Unfortuantly, no oil droplets were seen.

  • Possible Solutions:
    • The microscope was not centered with respect to the focus rod we use to calibrate the optics, so, perhaps our field of vision is skewed. Realigning the microscope may allow us to see the drops.
    • If too much oil is added, they will clump up rather than make fine droplets for us to see. I noticed that when we did see oil residue inside the capacitor, the TA pumped the apparatus with plenty of oil.
    • If any of the holes get clogged, it will make it very difficult to get air flowing, and oil into the apparatus. So cleaning the holes may be an option.
    • The plastic spacer has a small "window" in which the microscope sees through. That window could get dirty fairly easily, making it hard for us to see clearly.

Hopefully the Wednesday lab gets it working. I'll be back in there next Monday trying to work it out. (Steven J. Koch 01:56, 13 September 2007 (EDT):Indeed, the Wednesday peeps (Bradley and Nik) got the experiment working so that tiny droplets were clearly visible. It looks pretty cool, and once you get it, it's obvious (so there are no problems with fuzzy windows, etc. I'm not sure exactly what was fixed -- Bradley thinks the spraying technique may be the kicker, and he showed me how to do it, so hopefully I can pass on the wisdom.)