Final answer:
Each ionized oil droplet in the Millikan oil drop experiment lost electrons, resulting in the droplet being electrically charged, and Robert A. Millikan discovered that the charge on any oil droplet was a multiple of the elementary charge.
Step-by-step explanation:
When oil droplets were ionized by x-rays in the Millikan oil drop experiment, each oil droplet lost electrons. The process involved ionizing the air with x-rays, which led to the detachment of electrons that were then captured by the oil droplets. As a result, the oil droplets became electrically charged. By observing their behavior in an electric field and calculating the drop's mass, Robert A. Millikan was able to determine that the charge on an oil droplet was always a multiple of the elementary charge, which is the charge of a single electron, approximately 1.6 × 10-19 C.