If the elements constituting the periodic table are fundamental to chemistry, then the electron, a subatomic particle with a negative elementary electric charge, is equally so to physics. While it was J. J. Thomson who discovered the electron in 1897 and measured its charge to mass ratio, it was Robert Andrews Millikan who is credited with determining the electron’s charge separately.
Millikan was born in 1868. He was the second son of Silas Franklin Millikan, a Congregational preacher, and Mary Jane Andrews. His childhood had little or no science in it as he enjoyed a rural existence in the American middle west - fishing, farming and fooling around.
Turning to physics
He entered the Oberlin College in 1886 after a stint as a court reporter and his favourite subjects were mathematics and Greek. It was then that his life veered towards physics. After graduation he took a teaching post in elementary physics for two years, developing a strong interest in the subject.
After obtaining his mastership in physics and completing his PhD from Columbia University, Millikan made his mark as an educator and a successful textbook writer. What he lacked, however, was research of any significance.
Thomson, who had discovered the electron, tried to measure its charge by using clouds of charged water and observing how fast or slow they fell under the influence of gravity and an electric field. Millikan believed he could improve on these measurements and realised that trying to measure the charge on individual droplets might work better when compared to a cloud of water.
The trouble with water droplets was that it evaporated too quickly to make any accurate measurements. With the help of his graduate student Harvey Fletcher, he zeroed in on oil droplets to be used instead of water.
The oil-drop experiment
Oil drops injected into an air-filled chamber were allowed to pick up charge from the ionized air. Under the combined influence of gravity, viscosity of air and the electric field, which could be controlled, the drops had a tendency to rise or fall. By watching these drops through a special telescope and timing them, observations were made.
By repeatedly timing the rise and fall of the drop, Millikan could calculate the charge on them. He published these results in 1910, showing that the charge on these drops were integral multiples of a fundamental unit of charge.
Striving for more accuracy, Millikan decided to improve on his experiment by collecting more data. On February 13, 1912, he gathered his observations from the first of 58 drops that were to make their way to the results he published in August 1913.
Millikan stated that these results had only 0.2 uncertainty and reported a value of 1.592 x 10-19 coulombs for the elementary charge, only slightly lower than the currently accepted value of 1.602 x 10-19 C. He went on to win the Nobel Prize in Physics, in part for this work (and for determining the value of Planck’s constant in 1916) in 1923.
