Robert Millikan (1868-1953), another Nobel Prize winning scientist, didn’t develop an interest in physics until after graduating college, and then only because he was assigned to teach it! His first passions were Greek, mathematics, and tennis. After earning a doctorate in physics from Columbia University, he went on to make numerous discoveries in the field.
The most important of these to us as students of chemistry was his determination of the electrical charge (q) of a single electron. Through careful repeated measurements using an ingenious methodology, he surmised that the charge on a single electron was 1.6 x 10-19 Coulombs. Or, put another way, 0.00000000000000000016 Coulombs. Tiny!
How did he figure that out? He started with a metal chamber with two horizontal plates inside. He connected the plates to an electrical circuit so that the bottom plate was given a negative (-) charge and the top plate was given a positive charge (+).
He then used an atomizer, something your great-great grandmother might have used for perfume, to fill the top of the chamber with a fine mist of oil. Some of those droplets of oil would pass through a tiny hole in the top metal plate as gravity pulled downward on the mist.
He then bombarded the falling droplets with x-rays. Because x-rays are very high energy radiation, they would cause electrons to be ejected from the falling droplets.
Electrons are not very stable by themselves, so these free electrons would immediately attach themselves to the next falling droplet of oil, giving it a negative charge.
He also had an eyepiece built into the chamber so that he was able to visualize the falling droplets.
Remember that like charges repel and opposite charges attract. Also remember that the bottom plate had a negative charge, and thus would push up on any negatively charged falling droplet.
With much patience and practice, Milliken was able to adjust the electric field on the horizontal plates so that, through his viewfinder, he could actually see droplets suspended in mid-air. Since he had perfectly balanced the droplet between the force of gravity pulling it down…
Forcedownward = mg (mass of droplet x acceleration of gravity)
…and the electric field pushing it up…
Fupward = qE (charge on the droplet x electric field)
Thus, mg = qE
Millikan could solve for the charge (q) on the droplet because he knew g (from Galileo), he knew E (he was the one adjusting the electric field), he knew m (using the density of the oil and the volume of the droplet).
qdroplet = (mg)/E
The only problem was that each droplet held a random number of electrons, not a single electron. To find the charge of a single electron, he had to repeat his experiment hundreds of times. Imagine the patience!
