Oil and Water

Between 1896 and 1906, J. J. Thomson and his students performed a series of experiments that led to the "cloud method" for measuring the charge on a gaseous ion.  Around 1908, Millikan began working to improve upon the cloud method.  He first discovered that increasing the voltage he used allowed him to experiment on single droplets rather than an entire cloud, using what he called the "balanced water-drop" method.  Depending on whose account you believe, either Millikan or his graduate student Harvey Fletcher thought to use watch oil instead of water because watch oil would evaporate only very slowly.

Today I learned why water drops were so unsatisfactory.  It is impossible to experiment on them with an atomizer!  I spent several hours trying to figure out why I wasn't getting any water drops in the chamber only to realize that I was getting some, but they disappeared as soon as they arrived.  I knew that evaporation was an issue, but I didn't know that it happened so fast.