edVirtus Reference

Who Was Robert Millikan?

Robert Millikan (1868–1953): The Experimentalist Who Measured Elementary Charge

Robert Millikan measured the elementary electric charge with the oil-drop experiment and carried out a stringent experimental test of Albert Einstein's photoelectric equation. The two programmes helped establish that electric charge occurs in discrete units and that light transfers energy to electrons according to frequency. Millikan also became an influential organiser of American science and a central builder of the California Institute of Technology.

His reputation requires more than a heroic summary. Harvey Fletcher made an important contribution to developing the oil-drop method, and Millikan's notebooks later raised a continuing debate about selection and description of the published data. The measured quantisation of charge has survived; the history remains valuable because it exposes judgement, hierarchy, and rhetoric inside a celebrated experiment.

Education and the Chicago Laboratory

Robert Andrews Millikan was born in Morrison, Illinois, on 22 March 1868. He studied at Oberlin College and earned a doctorate at Columbia University before joining the University of Chicago. He became a successful teacher and textbook author while seeking an experiment that could address a central question in the emerging electron theory.

J. J. Thomson had measured the electron's charge-to-mass ratio, but the charge itself was not yet known precisely and some researchers disputed whether charge had a smallest unit. Millikan's laboratory began with charged water droplets, then turned to oil because it evaporated far more slowly.

How the Oil-Drop Method Worked

A fine spray introduced tiny oil droplets between horizontal metal plates. Some droplets acquired electric charge. Observed through a microscope, a droplet fell under gravity and air resistance; when voltage was applied, the electric force could slow, stop, or reverse its motion. Timing its rise and fall allowed the charge to be inferred from a force model.

The method depended on more than balancing two arrows in a textbook diagram. The researchers needed estimates of droplet size, air viscosity, buoyancy, temperature, Brownian motion, plate spacing, and electric field. Millikan introduced corrections for the motion of very small spheres through a gas and repeatedly improved the apparatus.

Charge Comes in Units

The inferred droplet charges clustered near integer multiples of a smallest value, now written e. A droplet could gain or lose electrons, changing its total charge by discrete steps. Millikan's 1913 result was close to the modern value once the air-viscosity data used in the calculation were corrected.

Measuring e allowed other quantities to be separated. Combined with electrochemical measurements of charge per mole, it yielded Avogadro's number and therefore a quantitative bridge between laboratory-scale matter and atoms. The experiment made the electron a metrological object rather than only a component of a theory.

Harvey Fletcher and Scientific Credit

Graduate student Harvey Fletcher played a major role in developing the use of oil droplets and in the experimental work. Millikan and Fletcher arranged that Fletcher would be sole author of a Brownian-motion paper usable as his doctoral thesis, while Millikan would be sole author of the elementary-charge paper. The arrangement reflected academic rules and unequal authority, and it also concentrated public credit for the charge measurement on Millikan.

Later recollections do not agree on every detail, but the collaboration is now an essential part of the history. Calling the work simply Millikan's experiment is convenient; it should not erase the students and technicians whose ideas and labour made the measurement possible.

The Data-Selection Debate

Millikan's 1913 paper described 58 drops and stated that they represented the complete drops observed over a period. His laboratory notebooks contain additional trials, marginal assessments, and incomplete observations. Historians have therefore debated whether he reported only measurements meeting defensible quality criteria or presented a selected set more absolutely than the record justified.

Including the omitted observations does not remove the evidence for charge quantisation or materially reverse the result. The issue concerns transparency and the distinction between rejecting a technically invalid run and selecting data that support an expectation. It remains a useful case because real experiments require judgement, while scientific trust requires that the basis of that judgement be stated accurately.

Testing the Photoelectric Equation

Philipp Lenard had shown that the maximum energy of photoelectrons depends on light frequency rather than simply on intensity. Einstein explained the result by proposing light quanta with energy hf. Millikan initially resisted this interpretation and designed careful measurements intended to test the quantitative relation.

His experiments confirmed that stopping voltage, and therefore maximum electron energy, varied linearly with frequency. The slope supplied a precise value of Planck's constant, while the intercept represented the material's work function. Millikan could remain sceptical about photons while producing some of the strongest evidence for Einstein's equation—a useful example of experiment constraining belief.

Caltech and Cosmic Rays

Millikan moved to Pasadena in 1921 and helped transform the institution that became Caltech into a major research university. He recruited scientists, attracted private support, promoted links between fundamental research and engineering, and used public communication to build institutional standing.

He also studied penetrating atmospheric radiation and popularised the name cosmic rays. Millikan argued that the radiation was primarily high-energy photons, whereas later evidence showed that much of the primary radiation consists of charged particles. The episode demonstrates both his ability to mobilise ambitious measurement and his capacity to defend an interpretation after competing evidence had grown.

Nobel Prize and Legacy

Millikan received the 1923 Nobel Prize in Physics for work on elementary charge and the photoelectric effect. He died in San Marino, California, on 19 December 1953. His experimental methods helped establish the numerical foundations of electron and photon physics.

Millikan's legacy is mixed but durable. He measured a quantum of charge, confirmed a relation he had doubted, built Caltech, and left disputes showing that scientific authority must remain answerable to evidence.