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I am preparing a presentation on the famous “oil drop” experiment, directed by Robert Millikan in 1909. Specifically, I need to identify the placement of the devices at the ports of the oil can.

Here is a picture of it. I have placed six labels on it for identification:

It is clear that the device at [A] is the eyepiece of the microscope where oil drops were observed. And [B] appears to be the oil atomizer.

The port at [C] looks to be the connector for electrodes which charged the plates, which provided the electric force opposite to that of gravity when acting on the oil drop.

I went to Millikan’s manuscript on the experiment (“On the Elementary Electrical Charge and the Avogadro Constant”, Phys. Rev., Vol II, Series II, p. 109-143). On page 123 of this paper, it states (if I understand correctly) that the observation port is offset 18 degrees from the X-ray port, in the same horizontal plane. I take this to mean that [D] is the port where the X-ray device was attached.

My questions are:

  1. If [D] really is the X-ray port, why is it so close to the observation port?
  2. What is the port at [E]?
  3. What is [F]?
  4. Where is the barometer connected? Figure 1 on page 122 of Millikan’s paper shows a barometer, but I don’t see one on the picture above.
  5. Where is the port for a light source? On the other side?
  6. Is there anything on the other side of the can that is important to the experiment, that I missed?

Many thanks in advance for your input.

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  • $\begingroup$ It appears that D is the X-ray port (18 degrees). It appears E is used to evacuate the air (the barometer would be connected to the pipe that creates the vacuum). F could be a lever for a flap covering the hole that lets the oil drops through. Most likely, the light port is directly opposite the lens. $\endgroup$ – Guill Dec 27 '17 at 7:38
  • $\begingroup$ Thanks for your answer. It would explain a few things. I need to make some changes... $\endgroup$ – Michael Roberts Dec 28 '17 at 16:06

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