The main body of the experimental apparatus were a pair of capacitor plates. They were separated by three "spacer balls", whose diameter determined the separation of the capacitor plates. The capacitors were kept in a chamber, so that no external source affected the motion of the droplets. A light source made it possible to look at the motion of the droplets through a microscope which was placed in the chamber. In the lens of the microscope there were two lines, whose separation was measured on beforehand. The velocity of the droplets could then be found by timing the droplets as they traveled the predetermined distance between the lines.
Through a small hole in the upper capacitor oil droplets were sprayed in between the plates. Oil droplets with varying velocities (and hence radii) soon became visible in the microscope. The practical part of the experiment then consisted of "trapping" droplets of suitable radius and charge between the two lines of the microscope. A mechanism for turning the field on and off quickly made this possible. Individual droplets were then followed as they made several complete cycles up and down. Timing the motion, and recording the potential (which was typically around 3000V), gave a sufficient number of parameters to determine r' and e'.