Shortly after Thomson confirmed the existence of the electron with a charge-to-mass ratio, Robert Millikan designed and performed an experiment to determine the charge on an electron.  His well-known oil-drop experiment involved spraying tiny oil droplets into a vertical chamber with two metal plates on either end.  The oil droplets became charged in the spraying process and when they entered the chamber, they began to fall under the influence of gravity.  Millikan could then stop the free-falling droplets and reverse their direction of motion by applying a voltage across the two metal plates.  Using a microscope and a timer, he measured the velocity of a single oil droplet in the electric field in order to determine the electrical force acting on it.  This allowed him to determine the charge on the oil droplet, because . 

Note that the charge on the oil droplet was likely due to the presence of many, many extra electrons.  Therefore, the charge of an oil droplet will be an integer multiple of the charge on a single electron.  Do not confuse the charge of the oil droplet with the elementary charge of a single electron.

How could Millikan determine the charge of a single electron using the charge on multiple oil droplets?  By measuring the charge of many droplets and comparing them, he reasoned that the smallest difference in charge among all the droplets would be due to the presence of one extra electron.  That small difference in charge would then be equal to the charge of a single electron or the elementary charge.  Using the charge on numerous oil droplets and through careful analysis, Millikan discovered that the charges of the oil droplets were always integer multiples of 1.602 × 10-19 C.

How could Millikan determine the charge of a single electron using the charge on multiple oil droplets?  By measuring the charge of many droplets and comparing them, he reasoned that the smallest difference in charge among all the droplets would be due to the presence of one extra electron.  That small difference in charge would then be equal to the charge of a single electron or the elementary charge.  Using the charge on numerous oil droplets and through careful analysis, Millikan discovered that the charges of the oil droplets were always integer multiples of 1.602 × 10-19 C.

He reasoned this must be the charge of a single electron, a value that is referred to as the elementary unit of charge.


Elementary unit of charge: the charge of an electron or a proton 
1.60 × 10-19 C



Watch This
Watch the video " The Millikan Experiment: Part 2" from 5:29 to see Millikan's experimental design and apparatus.

 

Although time-consuming, Millikan's experiment was instrumental in establishing not only the value of the elementary charge, but also the quantized nature of electric charge.  This follows Planck's earlier discovery that energy is also quantized.

Building on Thomson's work with the charge-to-mass ratio of an electron, it was then possible to determine the mass of an electron as 9.11 × 10-31 kg.

From J. J. Thomson:

 

From R. A. Millikan:

 

Combined:

 

Due to rounding and significant digits, the value is extremely close to the accepted mass of the electron 9.11 × 10-31 kg.

By accelerating hydrogen ions (protons) through a potential difference and determining their charge-to-mass ratio, it was found that their mass was 1.67 × 10-27 kg.  Together, Thomson and Millikan had essentially weighed and measured the charge of some of the smallest fundamental particles that make up matter.  They also confirmed the idea that the atom was not the smallest form of matter and that it had divisible parts.