Question

Why do oil droplets lose an electron when they drop in Millikan's oil drop experiment?

Answer 1

Robert A. Millikan and Harvey Fletcher conducted the oil drop experiment in $1909$ to determine the elementary electric charge (the charge of an electron). The Ryerson Physical Laboratory, which is located at the University of Chicago, was used for this experiment.

Complete answer:
In the experiment, Millikan permitted charged tiny oil droplets to pass through a hole into an electric field. The charge over an oil droplet is determined using the variable strength of the electric field, and it always comes as a fundamental value of '$e$.'
The electrically charged oil droplets penetrate the electrical field and, by modifying the field, are balanced between two plates. The gravitational and electric forces on the charged drops were equal when they fell at a steady velocity.
The weight of an oil drop might lead it to fall. An electric field can be used to suspend a drop if it is given a charge. The electrostatic force balances the weight of each drop at this point. The amount of the electrostatic force is then fully determined by the decrease. As soon as Millikan understood the weight, he should have figured out the charge.
Millikan let the drop fall into the air in order to determine its weight. It swiftly approaches its terminal velocity. The weight is balanced at this point by the air's viscous drag. The Stokes' Law, which Millikan used to compute the weight, may be used to calculate drag.

Note:
Millikan carried out the same experiment for over $150$ oil droplets and chose $58$ of the findings to discover the highest common factor. It refers to the one unit of charge that might be multiplied to get the total charge he measured across all of his oil drops.