The Contributions of Milliken to Electricity Theory

Robert Andrews Millikan (22 March 1868 – 19 December 1953) was a high profile American experimental physicist. He was a Professor of Physics in Chicago then moved to Caltech. At Caltech he served as president from 1921 to 1945. His contributions to physics derive from three areas. He developed a famous experiment which determined the charge on the electron, made contributions to the photoelectric effect and investigated cosmic rays. In 1923 he received a Nobel prize for the famous Millikan oil drop experiment and for his work on the photoelectric effect.

In 1909 physics was an exciting field. Millikan was in a department headed by A.A.Michelson whose famous Michelson Morley experiment provided that the speed of light was a constant. In 1896 J.J, Thompson discovered the electron. In 1905 Albert Einstein published landmark papers on relativity and the photo-electric effect. Max Planck’s work on black body radiation was inching towards quantum theory and Perrin’s work on Brownian motion proved without doubt that matter was made of atoms.

At the age of 40 Millikan felt left out and saw that the opportunities to male major discoveries were closing rapidly around him. Anxious to make a name for himself he seized on a major outstanding problem. In 1896 Thompson showed that electrons were electrically charged and carried a constant ratio of electric charge to mass. When finding the actual charge on the electron eluded Thompson Millikan was determined to refine experimental techniques until the elusive charge could be determined.

Both Millikan and Thompson realized that the key to the experiment would involve balancing electrical and gravitational forces in some way. The problem lay in observation. Both knew that if they could observe a particle which appeared to balance in mid air within their apparatus they would be able to determine the charge upon it. Eventually, after many observations this would enable them to look for a common denominator in their results which could be attributed to the charge on a single electron.

Thompson had a clumsy apparatus. He used cathode rays to ionize water droplets in a cloud chamber. He assumed that the upper edge of the cloud must contain singly charged particles. In practice it was difficult to determine the upper edge of the cloud and he could only get rough estimates . Millikan’s first clue that there was a better method came when he and his graduate student, Louis Begeman repeated the work of Thompson using a higher electrical charge They found that a few water drops could be stopped in the field of view and there was no need to measure the average charge carried by particles at the edge of the cloud.

One major problem remained. The water droplets evaporated too quickly for accurate measurements to be made. In 1909 Millikan took on a new graduate student called Harvey Fletcher to investigate if the experiment could be repeated with substances other than water. Harvey Fletcher discovered that oil droplets were sufficiently stable to allow the experiment to give meaningful results.

When the work of Fletcher produced two results, it determined the value of the electronic charge and also gave insights into Brownian motion, Milliken arranged a deal. Fletcher could receive a PhD on the basis of a published paper on Brownian motion provided Millikan was allowed to write under sole authorship a paper on the oil drop experiment. Millikan published in 1910.

The 1910 paper produced a response from an Austrian physicist called Felix Ehrenhaft had conducted an experiment similar to that of Milliken and arrived at an electronic charge much lower than that of the Chicago professor. Millikan quickly repeated his experiments and dismissed Ehrenhaft’s claims in a paper published in 1913.

The 1913 paper lead to Millikan receiving a Nobel prize in 1923. Fletcher was naturally disappointed but remained good friends with Millikan throughout his life and only divulged the story after his death.

A much later allegation was published by Sigma Xi in 1984 in a book called Honor in Science. Sigma Xi claimed that Millikan selectively used his experimental results in the 1913 paper. Unfortunately the claim is born out through inspection of his notebooks. The case is very difficult to unravel. Millikan makes legitimate reasons for rejecting results. Drops might be too small giving too much Brownian motion. They might be too large and falling too quickly. They might be asymmetric drops, or subject to convection currents or non uniform fields. All this comments reflect the problem with the oil drop experiment. It is a very fiddly experiment which requires strong experimental skill

When scientists repeated Millkan’s experiment they consistently found higher results as though Millikan;s result was a lower bound for the charge on an electron. Whereas Millkan in fact determined the charge on an electron to be 1.592 × 10-19 the modern accepted value is 1.602 176 53(14) x 10-19 . The difference arises because Millikan used an inaccurate value for the viscosity of air. He assumed that a simple correction would be adequate when he applied Stokes law at the molecular level.

In other areas of physics Milliken proved to be a brilliant experimentalist who grappled with modern theory. Although Milliken refused to accept Einstein’s interpretation of the photoelectric effect he spent a decade producing ultra clean metal surfaces with replicated the predicted effect n every detail. Milliken also held on to the theory of the ether until the last moment. He referred to it in a text book that he wrote in 1927 and relegated the special theory of relativity to a footnote. During the 1930s Milliken engaged in a debate with Arthur Compton over the nature of cosmic rays. Milliken thought that they were photons. Compton who was proved right thought that they were charged particles.

Milliken died of a heart attack at his home in San Marino, California in 1953 aged 85 years.