Charge polarity in the Millikan oil drop experiment

[sera97]

In the Millikan oil drop experiment, was the oil particles positively or negatively charged. and what were the charges of the plates. My textbook and what my gr.10 notes say is that the oil droplets and the bottom plate were positive, but every source online says that they were negative and that makes sense to me. Does it work either way? or have I been taught completely wrong?

 

[eigenperson]

The drops investigated could be either positive or negative, and the charge on the bottom plate could be either positive or negative, depending on which way he decided to put the battery.

In fact, Millikan writes, "On account of the slightly greater mobility of the negative ion, and also on account of the somewhat greater numbers in which they occur, we found, in general, a greater tendency of the drops to take up negative than positive charges. In view, therefore, of the greater ease with which negative drops could be held for long intervals without being lost to the plates most of the drops studied have been of negative sign." But his data includes drops with both types of charge.

 

[sera97]

Thank you!

 

[sophiecentaur]

I think it should be pointed out that it is the Field between the plates that you need to know and not the "charges". And PD divided by the spacing would give you that.