Electron Charge
The Charge of the electron was first measured by J.J. Thomson and two of his co-workers, J.S.E Townsend and H.A.Wilson, starting in 1897. Each of these men used a slightly different method.The following is some of Townsend's work:
His work depended on the fact that drops of water will grow around ions in humid air. Under the influence of gravity, the drop would fall, accelerating until it hit a constant speed.
Several items were measured in his experiment:
1. The mass of a water droplet.
2. The total electric charge carried on all the droplets.
3. The velocity of the droplet.
4. The total mass of all water droplets.
He determined the e/m ration off the droplets, then multiplied by the mass of one droplet to the value of e .
Thomson, Townsend, and Wilson each obtained roughly the same value for the charge on positive and negative ions. It was about 1 x 10 to the -19 power coulombs. This work continued until about 1901.
Robert A. Millikan's Definitive Measurement
Robert A. Millikan began his work on electron charge in 1906 and continued for seven years. His 1913 article announcing the determination of the electron's charge is a classic and Milikan received great recognition by receiving the Nobel Prize for his efforts.
He conducted an experiment on electron charge. It was a huge advance in his modern day science world.
Milikan's Improvements over Thomson
1. Oil evaporated much slower than water, so that the drops stayed essentially constant in mass.
2. Millikan could study one drop at a time, rather than a whole cloud.
3. In following the oil drop over many ascents and descents, he could measure the drop as it lost or gained electrons, sometimes for only one at a time. Every time these drops gained or lost charge, it always did so in a whole number multiple of the exact same charge.
The value in 1991 is 1.60217733 (49) x 10 to the -19 power coulombs. The is less than 1% higher than the value obtained by Millikan in 1913. The 49 in parenthesis shows the plus/minus range of the last two digits. It is unlikely that there will be much more improvement to the accuracy of the model in the years to come.