Before long, I was able to figure out how to use the ionizing radiation source to change the charges on the oil drops. I had forgotten that the drops generally don't pick up ions while the electric field is on, because the electric field causes the ions to disperse rapidly. It was easy, though, to get the drops to pick up ions when the field was off. Several times I picked a drop and turned on the field and watched that drop move up; then turned off the field and let the drop fall for a few seconds; then turned back on the field and found that it either had no effect on the droplet or had the opposite effect of causing the droplet to fall more rapidly. These changes are quite clear and easy to produce.
I did take one (very sloppy) measurement just to see how it would go and to practice doing the calculations to find the charge on a drop and got something on the right order of magnitude (2.94 * 10-19 C, whereas a recent value for the charge on a single electron is 1.60 * 10-19 C. I hope I was measuring a drop with two net charges, but with the sloppiness of my procedure there’s really no telling).
One thing that has me puzzled right now is that in some of the pages of his notebook Millikan records "Red Drop" or "Blue Drop." Here's an example:
Similarly Harvey Fletcher, a graduate student who worked with Millikan on the oil-drop experiment, reported seeing " Why did they see a variety of colors, while I only say orangish white? It seems especially strange to me that color is apparently a consistent characteristic of individual drops, which indicates that it isn’t due to prismatic effects and that it isn’t a simple matter of different colored lighting sources. Evidently Millikan decided that drop color is not important for his purposes, because I don’t believe that he mentioned it in any of his papers. It’s probably not an important issue, but it is a bit of a puzzle.