This is a beta version of the classic Millikan experiment. It is not, however, as complex as the actual experiment. The user sees one drop at a time and the assumption is that it falls in a vacuum. By adjusting the voltage between the plates, the student attempts to have the drop reach terminal velocity. From this the charge on the drop can be deduced.
Follow this link to a complete suite on Millikan which includes a demonstration of air resistance effects and a link to a much more realistic JAVA applet simulating the Millikan experiment.
If running this applet on a Mac, follow this link.