One of standard measurements that we take for granted these days is the charge of a single electron. This is a vivid example of how scientific knowledge has exploded during the past century. In 1870, there was no concept of an electron, and atoms were just very crudely becoming accepted.
After the turn of the previous century, it was generally accepted that there were atoms, and that electrons were likely a part of them, but no one knew much about them. We still have not all of the answers, but Robert Milliken elucidated the intrinsic charge of the electron in xxxx.
Not to long before, in xxxx, xxxx had determined the charge to mass ratio of the electron. For several technical reasons, that was somewhat easier to do. But that did not give either the fundamental charge, not the fundamental mass, of the electron. It merely said that something that has a mass of such and such grams has a charge of such and such coulombs. Progress, but not a lot of help in a fundamental way.
Robert Anders Millikan finally thought of a way to determine both. Since the ratio was know, if he could nail down either of the two, the other would be easily calculated (just one equation is required when one of two variables is known). Millikan decided that it would be easier to determine the unit charge of the electron rather than the unit mass of it, and devised one of the most clever, simple, and difficult to carry out experiments ever.
His general idea was that most things in our ambient environment are electrically neutral, or otherwise big old static sparks would be shooting everywhere. With that thesis, he tried a few things, then thought of a very simple concept: watch a neutral particle fall by gravity, then charge it by knocking off an electron, then see what electrical field would make it suspended. Remember, uncharged objects are not affected by electrical fields very much, but charged ones are. The Newtonian laws of gravity were well known then, and the newer Coulombic laws of electrical attraction and repulsion were also well known. Millikan had a come to Jesus moment.
So, how to make something that is little but can be measured (the electron has a tiny charge, so the carrier needs to be little). He decided to use clock oil, which evaporates extremely slowly (if the drop evaporates very fast, everything else is thrown off). It turns out that sperm whale oil was the bee’s knees at the time, but if we were to do it now we would use a silicone oil. When I was in junior college we did the experiment, but used latex spheres that were sorted to be very uniform, but that was for a three hour laboratory demonstration.
Next, he had to decide how to get really little bitty drops. He borrowed one of his spouse’s perfume atomizers and used it to get the very small drops. Next he had to figure how to get just a relative few into the chamber, and that chamber was a wonder into itself. We shall come back to that.