User:Ginevra Cochran/Notebook/Physics 307L/Millikan oil drop lab

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Steve Koch 22:37, 21 December 2010 (EST):This is a good notebook. There is a flaw somewhere. One idea is that you used only one small box on the grid, instead of one large box -- this may have thrown everything off.


The purpose of this lab is to calculate the charge on a single electron based on measurements of the charge of various drops, and to compare our result to the accepted value.


The most dangerous component of this lab is the 500V power supply, which could cause serious bodily harm. The thermistor connectors should not have direct voltage applied across them, and the halogen lamp's effect on the drop viewing chamber should be monitored to prevent overheating.


  • TEL-Atomic 500V power supply
  • 2 Wavetek 85XT multimeters
  • Pasco scientific Model AP-8210 (mineral oil, atomizer, halogen lamp, thermistor, Thorium-232, viewing scope, spacer and capacitors)
  • banana plug connectors
  • stopwatch


The setup for this lab is mainly described in the Pasco kit manual - we added two Wavetek 85XT multimeters. I worked with Cristhian Carrillo.

Millikan setup

We rinsed and dried the capacitor plates and used the focusing wire to calibrate the viewing scope, first adjusting the grid and then attempting to bring the wire into better focus, with less success than we would have liked. The focusing wire seemed strangely tilted, but it wasn't very secure in the drop chamber.Tyler explained why we needed to use the droplet hole cover to decrease horizontal drop drift. We set our power supply to 500.5 V,

Millikan power supply

sprayed the atomizer into the viewing chamber, and chose a drop of medium weight, timing its fall and rise times for a variety of distances. We recorded the thermistor resistance at each new set of times. We measured the spacer width to be 7.57 mm according to a SMIEC micrometer.


{{#widget:Google Spreadsheet |key=0AqzpS6URre5adFpiZjZPcEt3b3lBaVBDRXo5Y2ROc3c |width=750 |height=250 }}


We did not measure the barometric pressure directly, but calculated it based on Albuquerque's elevation (5312 ft according to Wikipedia). According to The Engineering Toolbox, the air pressure at a given altitude is p = 101325 (1 - 2.2557 * 10-5 * h) 5.25588, where h is the elevation in meters. By this equation, our barometric pressure was ~83329 Pa. The ρ of our mineral oil was equal to 886 [math]\displaystyle{ kg/m^3 }[/math]. Using the equations described in the the Pasco kit manual and a Google spreadsheet, we calculated our charge per drop:

  • 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

These values are in the form average q ± q(SEM).

  • According to Wikipedia, the charge of an electron is 1.602176487(40)×10−19 C.


There is obviously a problem with points 1, 2, and 3 - we should have been able to measure fractional charge, especially on a drop that contained so many electrons. This error is probably due to how difficult it was to see the drops once they reached the left side of the screen, and how dim the reticle had to be in order to even see the drops in the first place. There was also error involved in having one person watching the drop and the other doing the timing, as there is inevitable reaction time from being told to begin timing and actually beginning to time. This might be reduced by having one person perform both tasks, but recording the time would be tricky. The conversion from resistance to viscosity was also a little iffy, since we were reading values off a graph and not plugging anything into an equation. Considering drops 4, 5, and 6, and assuming 100 to 200 electrons per drop, our values for q are reasonable.


Cristhian Carrillo was my lab partner for this lab. Jeremy Spates (not in this class) helped me take data for points 9-11. Katie, Tyler, Dan, Professor Koch, and Emran helped us actually see drops in the viewing scope. I referenced the Pasco kit manual, the Wikipedia article on elementary charge and Tom Mahony's lab notebook.