The test meant to measure the electron might be one of the world's most perfect experiments, or it might be the most successful case of scientific misconduct in history. It all depends on how you interpret the scandal surrounding Robert Millikan and the oil-drop experiment.
The Charge of the Electron
Around the turn of the century, while Einstein was figuring out relativity and forever changing theoretical science, Robert Millikan was forever changing experimental science. He measured the charge of the smallest thing in the known universe, the electron. And he did it with a box, a microscope, a set of plates, and a perfume spritzer.
The box housed the experiment. The metal plates were set up inside the box, both horizontal and parallel to each other. Apply an electric potential difference, or voltage, across the plate and the space between the plates is now permeated with an electric field. Millikan was able to measure and adjust the voltage, and the field, between the plates very precisely.
On one side of the two plates, Millikan placed a microscope. On the other, a perfume spritzer - otherwise known as an atomizer. He spritzed drops of oil through the atomizer, and as they moved through the nozzle they picked up a negative charge. The droplets were affected by gravity, just like anything else, and fell downwards toward the bottom plate. But if Millikan adjusted the field just so, he could make the drops hover, or even rise. Millikan knew that different oil drops contained different amounts of spare electrons. He had no way to measure the amount of the electrons on the drop, but that didn't matter. The amount of force required to keep a drop in the air was always a multiple of a certain number, and that number was the charge of the electron.
The Controversy
This was a brilliant experiment, and Millikan knew it. He said, "One who has seen this experiment has seen the electron," and the rest of the scientific community agreed. It was no surprise when he won the Nobel Prize for physics in 1923. Not only had he overcome technical problems - he'd had to find the precise kind of oil that would hang together in rather extreme conditions - but his findings were accurate. The "Oil Drop Experiment," provided a measurement for the charge of an electron that is comparable to today's estimates. The major difference between the values comes from our understanding of the viscosity of air, through which the drops moved. In 60 days, with 58 drops and "no single drop being omitted," Millikan had measured the charge of the electron.
When people went over his records, decades later, they got an unwelcome surprise. It seemed that a few drops had been omitted. In fact 115 drops had been omitted. In Millikan's notes, certain specific drops were omitted, and considered obvious errors, because they veered away from Millikan's expected value for the charge of the electron and towards the value that Millikan's scientific rival, Felix Ehrenhaft, predicted for the electron.
Millikan's experiment has, since the publication of these adjustments, come into disrepute. While no one can dispute that Millikan's predictions were accurate, and that the experiment works, the concealed conflicting data has turned the Oil Drop experiment into a famous case of misconduct. Some people even call it fraud.
Did Millikan Simply Overstate His Case?
The trouble with scientific misconduct isn't just that it's dishonest - it's that it creates erroneous data. If you go about things the wrong way, you get the wrong result. So if Millikan didn't know what he was doing, and was motivated entirely by ego, the fact that his results were so accurate is an amazing coincidence. With so many ways to go wrong, it's rare to blunder into being right.
Taking a closer look at the missing drops shows us why Millikan was right about the value of the charge. Some of the drops were measured when Millikan was still not sure of the distance between the microscope and the drop. He was using his estimates to make sure the apparatus was working "right," but when it was clear something was "very wrong," he threw out measurements that were close to his calculation, as well as drops that were close to his rival's calculation. Of the hundred drops leftover, about 25 were released during problem runs of the experiment. Something had gone wrong and no measurement could be gotten from them. That left about 75 drops - only 17 more than he reported - of which some were affected by air flow or Brownian motion.
Millikan hardly lucked into fame and honor. He was trying to do an accurate experiment. While he can be faulted for omitting any of the 75 drops, his biggest error was the claim that "no drop" was omitted. This, in the end, is what baffles most people. He didn't have to pretend that he counted every drop, and yet he did. And a lot of scandal ensued.
[Via The Millikan Oil Drop Experiment, Flirting With Fraud, Robert Millikan Reports His Oil Drop Results, In the Case of Robert Andrews Millikan.]