Millikan oil drop apparatus.

Millikan's apparatus.

Millikan's oil drop apparatus is another apparatus from early modern physics that epitomizes the ideal subject for an MCAT physical sciences passage. Like J.J. Thompson's cathode ray apparatus, free-body diagrams are key to understanding the events of the experiment. In Millikan's apparatus, the force equilibrium on an oil droplet in the upper chamber exists between air resistance and gravitational force and in the lower chamber between electrostatic force and gravitational force. From the expressions derived from the free body diagrams of these two situations, Millikan was able to determine the charge of the electron.