In this picture(all others on google are the same), the positive plate is at the top and the negative plate is on the bottom. This means for this experiment to make any sense, the oil drops must be negatively charged. If this is the case, then why are X-rays used to cause the oil droplet to be charged? Surely since X-rays are ionising, they would remove electrons, making the oil droplets positively charged.
Once you have an X-ray source you have electrons present in your setup. Be it by ionising the electrodes or the oil itself. But where do these ionised electrons go? They are present in the bulk of the system.
The ones that stick around the oil droplet are the ones that need to be observed. Because they’re the ones that’ll be balanced under gravity-electric equilibrium.
The positively charged ones don’t enter the chamber between the electrodes anyway and if they do, they won’t be under equilibrium as gravity and electric force acts in the same direction and thus won’t be considered for observation.
The X-rays do not remove ALL of the electrons. So the drop is still negatively charged. The total charge has changed by a whole number multiple of the elementary charge. This allowed Millikan to deduce the charge on one electron.
