Physics307L:People/Cochran/Millikan oil drop

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The purpose of this lab was to calculate the charge of an electron by measuring the charge of an oil drop, and to compare it to the accepted value. Cristhian was my lab partner for this lab. We used a EL-Atomic 500V power supply, two Wavetek 85XT multimeters, a Pasco scientific Model AP-8210 (mineral oil, atomizer, halogen lamp, thermistor, Thorium-232, viewing scope, spacer and capacitors), banana plug connectors and a stopwatch. My procedure for this lab is described in my lab notebook.


We injected tiny oil drops into a viewing chamber lit by the halogen lamp and visible through the viewing scope. The direction of these drops was controlled by a voltage across them. We recorded the time of flight with and without voltage applied for different oil droplets across a distance of 0.1 mm, and using voltage, air pressure, viscosity, and a formula found in the Pasco kit manual, we calculated the charge on each drop. We found these to be:

  • q1 = 1.0897 x 10-20 ± 3.6766 x 10-20 C
  • q2 = 6.2910 x 10-21 ± 3.7180 x 10-21 C
  • q3 = 4.8342 x 10-20 ± 6.9860 x 10-21 C
  • q3 with Th = 7.0758 x 10-20 ± 1.0487 x 10-20 C
  • q4 = 2.0642 x 10-17 ± 3.0899 x 10-18 C
  • q4 with Th = 4.1899 x 10-17 ± 2.8041 x 10-18 C
  • q5 = 4.6468 x 10-17 ± 1.1432 x 10-17 C
  • q6 = 4.6110 x 10-17 ± 1.2955 x 10-17 C


Half of our data points were not even possible values, and these reflect the difficulties involved in obtaining a measurement - it is difficult to see the reticle and the drop at the same time, and the necessity of having someone timing what they cannot see causes the rise and fall times to be more inaccurate than they already were. It was odd that our measured charge increased after the Thorium bombardment. If our usable drops (4-6) contained 100 to 200 electrons, then our values for the charge of the electron were satisfactorily close to the accepted value of 1.602176487(40)×10-19 C. If I were to do this experiment over again, I would measure the rise and fall times over a larger distance (we used 0.1 mm), find a way to have a single person both observe and record a flight, and display the drops against a brighter background, possibly by projecting them against a screen. A white reticle would be easier to see at the level of light we were using.SJK 22:34, 21 December 2010 (EST)
22:34, 21 December 2010 (EST)
Yes, definitely, over a distance of 0.5 mm (one large division) is better. I just think there must be a calculation error somewhere for you to obtain these results.