The Weight of a Number: Robert Millikan and the Charge of Scientific Integrity

In the pantheon of great scientific experiments, Robert Millikan’s oil drop experiment stands as a monumental achievement. First published in its seminal form in 1913, it was an act of supreme elegance and precision. For this work, which gave the world the first accurate measure of the charge of a single electron, Millikan was awarded the 1923 Nobel Prize in Physics. He became a titan of science, his result printed in every textbook, his method lauded as a model of empirical genius.

But the pristine story taught to undergraduates conceals a powerful and cautionary tale. A closer look at Millikan’s private laboratory notebooks reveals a more complex narrative, one that explores the subtle line between careful judgment and improper bias. It is a story that illustrates how even a Nobel laureate can fall victim to the very human desire to be right, and how the shadow of a great authority can slow the progress of science for a generation.

The Experiment and the Seeds of Error

The genius of Millikan’s experiment was its simplicity. By observing tiny, electrically charged oil droplets suspended in an electric field, he demonstrated that the charge on any droplet was always a whole number multiple of a fundamental value, e, the elementary charge of a single electron.

The controversy arises from two key issues. First, Millikan’s private notebooks reveal that he excluded about one-third of his measured droplets from the final analysis in his landmark 1913 paper, deeming them “poor” or “wrong.” Second, and more critically, the value he used for the viscosity of air was incorrect. This systematic error meant that his final, celebrated result for e was about 0.6% too low.

The Slow Crawl to the Truth

Millikan’s immense prestige established his value for the electron’s charge as scientific dogma. For nearly two decades, the global scientific community was anchored to his flawed number. As the data shows, new measurements, including those using entirely different methods like X-ray diffraction, miraculously produced results that hovered right around Millikan’s value of 1.59 x 10−19 C.

Table 1: Historical Measurements of the Elementary Charge (e)

Data compiled from sources listed and adapted from Christian Hill .

The scientific community remained anchored to Millikan’s value for over a decade. By the early 1930s, however, a troubling discrepancy had emerged: values for the electron’s charge derived from X-ray diffraction experiments were consistently higher than those from oil drop experiments. This forced physicists to question whether the X-ray method was flawed or if a systematic error was hiding within the oil drop calculations.

The prime suspect became the accepted value for the viscosity of air, a key parameter in Millikan’s equations. The puzzle began to unravel in the mid-1930s when several researchers conducted new, precise measurements of this property. A crucial piece of evidence came from G. Kellström in 1936, who reported a viscosity value significantly higher than the one Millikan had used. This discovery provided a physical explanation for the discrepancy. When influential reviewers like Raymond T. Birge applied this corrected viscosity value to the older oil drop data, they found that the re-calculated results now agreed with the higher values from the X-ray experiments. The dam had broken. With a clear reason to distrust the old constant, the community could finally embrace the higher values, leading to the rapid convergence on a more accurate number after 1936.

In a now famous 1974 commencement address at Caltech, Richard Feynman perfectly diagnosed this phenomenon:

We have learned a lot from experience about how to handle some of the ways we fool ourselves… It’s interesting to look at the history of measurements of the charge of an electron, after Millikan. If you plot them as a function of time, you find that one is a little bit bigger than Millikan’s, and the next one’s a little bit bigger than that… until finally they settle down to a number which is higher.

Why didn’t they discover the new number was higher right away? It’s a thing that scientists are ashamed of—this history—because it’s apparent that people did things like this: When they got a number that was too high above Millikan’s, they thought something must be wrong… When they got a number close to Millikan’s value they didn’t look so hard.

A Legacy of Truth and Caution

Millikan’s story is not one of a simple fraud. He was a brilliant experimentalist who made a genuine breakthrough. His failing was one of confirmation bias, a trap that awaits any researcher. The ultimate legacy of the oil drop experiment is therefore twofold. It remains a triumph of experimental physics that first pinned down one of nature’s most fundamental constants. But it also serves as one of science’s most enduring cautionary tales. It teaches us that the greatest obstacle to scientific truth is not a lack of genius, but the very human tendency to want to be right. It reminds us that scientific integrity demands we report all the data, especially the pieces that seem to tell us we are wrong, because those are often the most important observations of all.



References

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